The Weeds on Your Farm

Common name: Downy brome

Scientific name: Bromus tectorum L.

Life Cycle: Winter annual (may also germinate in spring).

Also known as cheatgrass, cheatgrass brome, downy bromegrass, wild oats, and cheat.

Identification: Barnyardgrass distribution ranges through much of the U.S. and is a weed of many agronomic crops, nurseries, landscape, and turf but is especially problematic in rice in the southern states. Barnyardgrass is a summer annual with thick stems that may reach 5 feet in height.

Seedlings are without hairs (glabrous), auricles, and ligules, and the leaf sheaths are often tinted red or maroon at the base (Figure 9). Leaves are rolled in the shoot, smooth and without ligules or auricles and have a distinct white midvein that becomes keeled toward the basal potions of the leaf (Figure 10) A few short hairs may occur at the leaf bases of mature plants. Stems are usually erect, thick, without hairs (glabrous), often branched at the lower nodes, and may be tinted red to maroon at the base (Figure 11). Seedheads are a terminal panicle ranging from 4 to 16 inches in length (Figure 12). Panicles may be green to purple in color and are comprised of individual spikelets that may develop a 2 to 10 mm long terminal awn.

Similar Species: Fall panicum (Panicum dichotomiflorum) and barnyardgrass are often confused prior to seedhead formation due to their similar growth habit and appearance. However, the characteristic absent ligule of barnyardgrass helps to distinguish this weed from most other grasses in both the seedling and mature stage of growth.

Management: Herbicide-resistant barnyardgrass is widespread in rice-growing regions. Resistance to Group 7 (Photosystem II inhibitors), Group 1 (ACCase inhibitors), and Group 4 (synthetic auxins) herbicides have been well documented in southern states. Many resistant populations are known to have multiple resistance.

Glyphosate reliably controls barnyardgrass. Furthermore, in areas where Palmer amaranth is a problem, barnyardgrass is often controlled as a result of chemistries utilized to manage glyphosate-resistant Palmer amaranth. Rotating crops and herbicide sites of action as well as utilizing residual herbicides are all excellent resistance management techniques.

Sources:

Bond, J. and Golden, B. March 29, 2016. Barnyardgrass management in Mississippi Delta. Mississippi Crop Situation. Mississippi State University Extension. http://mississippi-crops.com

Common name: Large crabgrass

Scientific name: Digitaria sanguinalis

Life cycle: Annual

Identification: Large crabgrass is an annual weed with a prolific tillering or branching habit. It reproduces by seed and is problematic not only in row crops but also in pastures and home lawn environments.

Large crabgrass emerges later than most annual grass weeds and grows well under hot, dry conditions (Figure 13). Large crabgrass has a prostrate to ascending growth habit and is capable of rooting at the nodes. Both leaf surfaces and leaf sheath are densely hairy (Figure 14) The ligule is jagged and membranous. The seed head is a terminal panicle that consists of slender, finger-like branches arranged in a whorl (Figure 15).

Similar Species: Smooth crabgrass (Digitaria ischaemum) is a similar species to large crabgrass but differs by having hairless to sparsely hairy leaves and sheath, a tuft of long hairs at the collar region, and stems that do not root at the nodes.

Management: Due to late emergence patterns, preemergence herbicides don’t always provide season-long control. In corn, emerged crabgrass can be difficult to control. Glyphosate provides excellent control in glyphosate-tolerant corn and soybean products. In soybean, postemergence products also include foliar grass herbicides. Resistance to Group 1 (ACCase inhibitors) herbicides have been identified in some populations.

Identification: Fall panicum is a summer annual found throughout the United States. The seedling stage of fall panicum has hairs on lower leaf surfaces. Seedlings and mature plants have a hairy ligule that is up to ¼ inch long (Figure 16). As fall panicum matures, leaves become hairless or sparsely hairy with a prominent midvein (Figure 17). Fall panicum seed heads are wide, spreading panicles that develop a purplish tint when mature and yield small, straw-colored, oval seeds (Figure 18).

Similar Species: Fall panicum is most often mistaken for barnyardgrass and Johnsongrass prior to seedhead formation. Barnyardgrass also has a similar growth habit but it lacks a ligule. Johnsongrass has a similar growth habit but is a rhizomatous perennial, and its mature ligule is topped with a fringe of hair.

Management: Fall panicum is easily controlled in soybean. Earlier planting dates may improve crop canopy development to help suppress fall panicum infestations. Fall panicum seedlings are also readily controlled with tillage. Postemergence control of fall panicum in corn is difficult and must be timely; herbicide applications must be made to small plants to be effective.

Sources:

Spaunhorst, D. and Bradley, K. September 1, 2011. Weed of the month: fall panicum. Integrated Pest Management. University of Missouri. http://ipm.missouri.edu

Identification: Giant foxtail is a summer annual that is problematic throughout most of the United States, especially in the northern and eastern corn growing region. It has a round stem, hairy ligule, and distinctive fuzzy seed head with a nodding appearance (Figure 19). Yellow foxtail and green foxtail closely resemble giant foxtail in general characteristics – ligule, growth habit, habitat, and reproductive characteristics. Giant foxtail is the largest of the three species and its seedlings and mature plants have numerous short hairs on the upper surface of the blades and on the margin of the sheaths.

Giant foxtail begins germinating in the spring. The emergence period for giant foxtail is lengthy and its seeds can emerge from shallow soils (Figure 20).

Management: Giant foxtail has developed resistance to Group 1 (ACCase inhibitors), Group 2 (ALS inhibitors), and Group 5 (PSII inhibitors) sites of action. Using cultural practices along with chemical control diversifies management and reduces the risk of developing herbicide resistance.

To manage giant foxtail, start with tillage or burndown herbicides. Apply postemergence herbicides early when plants are young (4 inches or less). Use multiple sites of action for both pre-and postemergence herbicide applications to help manage resistance development.

Identification: Johnsongrass is a noxious weed in many states and can be problematic in all Midsouth states. Johnsongrass is a perennial, warm-season grass that grows from seed and rhizomes. Leaves are bright green, hairless, with a prominent midvein (Figure 25). Leaf sheaths are also hairless. Johnsongrass has a very prominent, jagged, membranous ligule (Figure 26). Plants can reach 2 to 7 feet in height and have a loosely spreading or open, coarse, purplish panicle (Figure 27). Plants emerging from seed are capable of producing rhizomes in 3 to 4 weeks.

Management: Best management practices to control Johnsongrass require efforts to control plants that emerge from seed and rhizomes. Johnsongrass is generally more difficult to control in corn than soybean. Johnsongrass can emerge before planting and new plants are likely to emerge after planting. Those present at planting should be managed with a burndown herbicide application or tillage. Soil residual herbicides can help suppress Johnsongrass emerging from seed but will be ineffective on that emerging from rhizomes. An effective postemergence herbicide should also be applied. Some Johnsongrass populations have developed resistance to glyphosate and Group 1 (ACCase inhibitor) herbicides. To reduce the potential for resistance, it is important to rotate between herbicide sites of action. Crop rotation including corn will also allow the use of Group 2 (ALS inhibitor) site-of-action products.

Similar species: Shattercane (Sorghum bicolor) is a summer annual grass species that can resemble Johnsongrass and corn plants. It has smooth, waxy leaves, a membranous ligule, and reaches 4 to 8 feet in height at maturity. Being an annual species, it does not have rhizomes like Johnsongrass. Shattercane and Johnsongrass seeds are similar: football- to egg-shaped and dark reddish-brown to black. Some shattercane populations have confirmed resistance to Group 2 (ALS inhibitor) herbicides.

Sources:

2018. Shattercane. Sorghum bicolor. Weed ID Guide. Division of Plant Sciences. University of Missouri. http://weedid.missouri.edu.

Identification: Common chickweed is a winter annual that can have several generations per year during cool, wet seasons, and forms prostrate, dense mats. Common chickweed leaves are opposite, light green, rounded to egg-shaped, with pointed tips, smooth margins, and hairless or with a few hairs near the base (Figure 37). Common chickweed has a single straight row of hairs along the stem. Lower stems often root at the nodes.

Small, white flowers are borne in clusters at the end of stems. Flowers have 5 deeply-notched petals, resembling a pair of petals, and though small, are quite noticeable (Figure 38). Fruit are slightly curved, an oval capsule that contains many tiny, brown seeds (Figure 39).

Management: Heavy infestations of common chickweed are competitive with crops, especially in small grains. Tall wheat cultivars and denser plant populations can help suppress chickweed. Tilling in the fall prior to wheat planting can help give wheat a head start. Common chickweed should be controlled prior to planting in no-till systems. A fall herbicide application can effectively reduce weed populations from overwintering and help reduce the number of seeds set in the spring. In addition, chickweed can serve as an alternate host for a range of crop pests including wireworm, black cutworm, and soybean cyst nematode and should be managed.

Similar species: Common chickweed is similar in growth habit and appearance to mouseear chickweed (Cerastum fontanum ssp. vulgare). Mouseear chickweed differs by having a perennial habit with densely hairy, oblongleaves lacking petioles, and dark-green foliage (Figure 40).

Identification: Field pennycress is a member of the mustard family and has either a winter or summer annual growth habit. It can be found in nearly every state, even as far north as Alaska. Seedlings develop into a compact basal rosette with a slender taproot system (Figure 41). Lower leaves are oval to spatula-shaped with wavy margins, rounded tips, and distinct petioles. Upper leaves are alternate with toothed to smooth margins and clasping bases. Basal leaves fall off before maturity.

Stems bolt from the basal rosette to flower (Figure 42). Flowers are terminal clusters with 4 small, white petals. Each flower produces a single seedpod that is bright green, flat, and round to oval in shape with distinct membranous wings that have a notch at the end. (Figure 43). The entire plant turns bright yellow to tan at maturity. All plant parts have a sharp odor and can give a bitter taste to milk if eaten by cows.

Management: Field pennycress can be controlled easily with tillage or burndown herbicide applications. Herbicides work best when plants are in the rosette stage and growth is active, before seeds start to shed.

Sources:

Anderson, B., Bradley, K., and Perez-Hernandez, O. April 19, 2016. Weed of the month: field pennycress. Integrated Pest Management. University of Missouri. https//ipm.missouri.edu.

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Identification: Henbit is a member of the mint family and has square stems. Leaves are opposite, circular to heart-shaped, with a crinkled leaf surface, hairy, and have rounded teeth along the leaf margins (Figure 44). Lower leaves are attached to the stem by long petioles, upper leaves lack petioles and encircle the stem. Flowers are pink to purple in a two-lipped tube form in whorls in upper leaf axils (Figure 45). Henbit is more prevalent in no-till fields and thick patches can make areas appear purplish as plants flower (Figure 46).

Similar Species: Henbit can be confused with ground ivy (Glechoma hederacea) and purple deadnettle (Lamium purpureum). However, purple deadnettle differs by having more triangular-shaped leaves, upper leaves with petioles, and red to purple leaf coloration (Figure 47). Ground ivy differs in that it is a creeping perennial with square stems that root at the nodes.

Management: Henbit and purple deadnettle both emerge mostly in the fall with flowering and seed set in the spring and are primarily weeds of no-till systems. Both are easily controlled with herbicides. Fall and early spring herbicide applications are most effective and may also suppress seed production. There is also some spring emergence, so consider herbicide options with some residual control.

Identification: Marestail, also known as horseweed, emerges in both fall and spring. Seedlings develop into a basal rosette (Figure 48). Fall-emerged plants survive the winter as a basal rosette and bolt in the spring. Marestail leaves are alternate, linear, and simple with entirely or slightly toothed margins (Figure 49). Leaves get progressively smaller toward the top of the plant. When plants are mature, leaves do not have petioles. The main stem bolts, giving the plant the appearance of a horse’s tail. Flowers are arranged in a panicle with numerous white ray flowers (Figure 50). Fruit are small, yellow achenes, each with a white pappus that aids in wind dispersion (Figure 51). Marestail seeds usually germinate in the fall or spring but can germinate midsummer when growing conditions are adequate. Plants can reach up to 6 feet in height. Once plants are over 5 inches tall, they are difficult to control with herbicides alone.

Watchouts: Marestail has increased in prevalence and herbicide-resistant biotypes have developed due to three primary factors: use of reduced or no-tillage, a lack of diversity in crop rotations (production of soybeans in the same field for consecutive years), and limited herbicide diversity. Herbicide-resistant marestail biotypes have been identified in several states for ALS inhibitor, triazine, paraquat, and glyphosate herbicides. Abundant seed production and wind dispersal facilitate the spread of herbicide-resistant marestail biotypes.

Management: Several factors can influence marestail control. The size and stage of growth at the time of herbicide application can have a significant effect on efficacy. Marestail is most susceptible to control when it is small, in the rosette stage of growth, and less than 4 to 6 inches tall. Once marestail bolts it is more difficult to control. The second factor is the extended emergence time of marestail, which can complicate the timing of burndown applications. In many cases, including a herbicide with residual activity can help control later-emerging plants. A third challenging factor to marestail control is the limited number of postemergence herbicide options, particularly in soybean. Because of this limitation it is important to use burndown, preplant, and preemergence herbicides effectively.

There are herbicide options available for fall or spring burndown, preemergence, and postemergence applications for marestail control. Preemergence herbicides with residual activity should be the foundation of a marestail management program. Marestail susceptibility to herbicides declines substantially once the plant bolts in the spring.

 Marestail management tips:

• Fall or early spring herbicide application for emerged plants to reduce heavy populations or seedlings. Plan for an additional herbicide application closer to planting.
• Spring burndown application to marestail less than 4 inches tall to prepare a clean seed bed.  
• Add residual herbicides to the spring burndown application.
• Use full label rates of herbicides for maximum residual activity and postemergence activity on variable-height marestail.
• Herbicide applications with multiple sites of action should be used in areas with ALS-, glyphosate-, or multiple herbicide-resistant biotypes.¹
• Use postemergence herbicides in-crop to control escaped plants.
• Rotate crops and till to reduce heavy marestail infestations.¹
• Consult product labels for use precautions and restrictions to prevent crop injury or carryover.

Sources:

¹Loux, M., Stachler, J., Johnson, B., Nice, G., Davis, V., and Norby, D. 2006. Biology and management of horseweed. GWC-9. https://www.extension.purdue.edu

Lingenfelter, O., Klodd, A., and Curran, N. November 13, 2017. Marestail (horseweed) management. Penn State Extension. http://extension.psu.edu.

Sprague, C. November 2018. Herbicide-resistant horseweed (marestail) in Michigan: keys to management in no-till soybean. Michigan State University Weed Science. http://www.msuweeds.com

Identification: Prickly lettuce, also known as wild lettuce, is a common winter annual or biennial that can grow up to 6 feet tall. Seedlings develop as a rosette and stem elongate later (Figure52). The leaves are alternate, clasp the stem, are deeply lobed with spiny margins (Figure 53). The leaves have a row of spines along the mid-vein of the lower surface which is a distinguishing characteristic of this plant (Figure 54). Leaves become smaller and more lanceolate toward the top of the stem and may lack spines. The stems are hollow, and the stem and leaves will exude a milky sap.

Prickly lettuce produces yellow flower heads with up to 25 flowers resembling dandelion flowers. Prickly lettuce seeds travel on wind currents with the help of their downy, white plumes (Figure 55).

Management: Seedlings and rosettes of prickly lettuce are easily controlled by cultivation. There are several herbicide options available for fall or spring burndown, preemergence, and postemergence applications for prickly lettuce control. Plants become more difficult to control with herbicides once the flowering stems have begun to elongate.

Sources:

Montgomery, G., Bond, J., and Eubank, T. November 28, 2012. Weed of the week: prickly lettuce. Mississippi Crop Situation. Mississippi State University Extension. http://www.mississippi-crops.com

Identification: Shepherd’s purse is a member of the mustard family. Dandelion-like leaves initially develop from a basal rosette (Figure 56). Small stem leaves are alternate with smooth or toothed margins and clasping bases. Stems, up to 2 feet tall, bolt from the basal rosette to flower. White flowers with 4 small petals are found in a terminal cluster. The seed pods are very distinct heart-shaped to triangular pods found on elongated, unbranched stems (Figure 57). Seeds are oblong, grooved and yellowish red to brown (Figure 58). Each plant can produce thousands of long-lived seeds.

Similar species: In the basal rosette stage, shepherd’s purse can be confused with dandelion. Once it starts producing pods in the spring there is very little mistaking of identity.  The arrow-shaped pod is very distinctive.

Management: Although shepherd’s purse is fairly common, it is easy to control in the fall.  The use of glyphosate or 2,4-D in the fall when shepherd’s purse is in the rosette stage will typically control it.  Shepherd’s purse bolts, flowers, and starts to set seed early in the spring, often too early to get into the field.  Once it starts to set seed control can be more inconsistent. 

Identification: Wild carrot, also known as Queen Anne’s lace, is a biennial plant in the parsley family. It germinates and emerges in spring, developing a rosette of delicate, deeply-lobed, feathery or fern-like leaves (Figures 59 and 60). The foliage is very similar to garden carrots and will smell similar when crushed. Some plants will flower and set seed during the first year of growth, but most will overwinter without flowering. When winter comes, these first-year leaves will die back, but the strong taproot overwinters. The second year, plants send up a tall flower stalk and more leaves.

The flower, called an umbel, is a flat-topped cluster of small, white flowers, 2 to 5 inches across (Figure 61). Each plant can have several flowers. The flower heads face upward during the day and bend down at night. After fertilization, the flower head folds up and has a “birds nest” appearance. Fruit have two egg-shaped sections; each section is yellow to grayish brown, flattened on one side, and ridged with barbed prickles to aid in dispersal (Figure 62). Healthy wild carrot plants can produce as many as 1,000 to 40,000 seeds per plant, and seeds can live and germinate over a 7-year period or longer.

Management: Wild carrot disseminates only by seed so any time plants are prevented from going to seed future infestations are reduced. Tillage will control wild carrot. Spring applications of labeled herbicides are effective in controlling wild carrot seedlings in other crops. However, once wild carrot becomes established, fall herbicide applications are more effective.

Identification: Wild mustard is a winter or summer annual and is considered a noxious weed in several states. The lower leaves of wild mustard occur as a basal rosette, petiolated, and are unevenly-lobed with coarsely-toothed margins (Figure 63). Upper leaves are alternate, petioles are absent or short, with coarsely-toothed margins, pointed tips, and gradually become smaller towards the top.

Erect stems bolt from the basal rosette to flower with white hairs that point downward and purple rings at the base of young stems. Bright-yellow flowers with 4 petals are found in terminal clusters (Figure 64). Fruit are cylinder-shaped capsules about an inch long with a beak at the tip (Figure 65).

Management: In a no-till system, timing herbicide applications before bolting is the key to successful control. Some populations (North Dakota) of wild mustard have developed resistance to Group 2 (ALS inhibitor) herbicides.

Diseases and Insect Pests Harbored: Wild mustard is more prevalent in no-till systems and can also serve as an alternate host to soybean cyst nematode and many insect pests.

Sources:

Cahoon, C. 2016. Wild mustard and wild radish. Virginia Tech Extension. http://blogs.ext.vt.edu

Identification: Yellow rocket can be a winter annual or biennial and reproduces by seed. Yellow rocket is most commonly found in no-till fields and contributes to the blanket of yellow covering in fields in the spring.

Yellow rocket is a member of the mustard family and has leaves with several smaller lobes and one large terminal lobe (Figure 66). Seedlings have round to egg shaped cotyledons on long stalks with smooth, shiny, dark green leaves that initially develop from a basal rosette (Figure 67). Leaf stems are arranged alternately and become progressively shorter toward the tip of the plant. Flowering stems are produced during the second year and can reach up to 3 feet in height (Figure 68). Bright-yellow flowers with 4 petals are found in terminal clusters. Fruit are slender, slightly curved, approximately 1-inch long capsules, with a beak at the tip and are almost square in cross-section (Figure 69).

Similar Species: At maturity, the flowers of several mustard species, including wild mustard and wild radish, will resemble those of yellow rocket. However, wild radish has leaves that are covered with stiff hairs unlike wild mustard or yellow rocket. Additionally, the large terminal lobe in the leaves of yellow rocket helps to distinguish this weed from wild mustard.

Management: Herbicides are highly effective at controlling younger, smaller weeds. The use of an effective early burndown to manage winter annual weeds like the mustard species is an important first step for a clean field for crop planting. Including a product with residual activity helps reduce early-season weed competition and helps manage winter annuals that can serve as alternate hosts for insects and soybean cyst nematode.

Common name: Common cocklebur

Scientific name: Xanthium strumarium

Life Cycle: Summer annual

Common name: Common lambsquarters

Scientific name: Chenopodium album

Life Cycle: Summer annual

Identification: Common lambsquarters is an annual weed belonging to the goosefoot family and is one of the most prevalent weed species found in the corn growing region. Like many small-seeded annuals, common lambsquarters germinates at or near the soil surface with the optimum seed depth for emergence being about 1/10 of an inch. Initially, leaves appear opposite in arrangement, later becoming alternate, and may be purplish on the underside with both surfaces covered with white granules or a mealy substance (Figure 78). More mature plants have broad, triangle-shaped leaves with irregular, shallow-toothed margins (Figure 79). The stems are vertically grooved with green or reddish stripes, and hairless. Mature plants generally reach a height of 2 to 6 feet. An average common lambsquarters plant can produce more than 70,000 seeds (Figure 80). Common lambsquarters seed is one of the most persistent in the soil seedbank. On average, it takes 12 years to reduce common lambsquarters seed in the soil seedbank by 50% and 78 years to deplete the seedbank by 99%.¹

Common name: Eastern black nightshade

Scientific name: Solanum ptycanthum

Life Cycle: Summer annual

Common name: Common waterhemp

Scientific name: Amaranthus rudis

Life Cycle: Summer annual

Common name: Redroot pigweed

Scientific name: Amaranthus retroflexus

Life Cycle: Summer annual

Identification: Redroot pigweed is a summer annual that is an abundant seed producer and can be found throughout the U.S. It is important to note that there is often physical variation within pigweed species and that crossing can occur between species, resulting in hybrid plants. Redroot pigweed can be difficult to distinguish from other pigweeds, but identification is important for management. Cotyledons are linear with a prominent midvein and reddish-tinted undersides (Figure 98). Leaves are alternate and egg-shaped with a small notch in the tip (Figure 99). Leaf surfaces are rough with hairs on the underside, most notably, the veins. As seedlings mature, the stem becomes very roughly hairy (Figure 100).

Stems grow up to 6 feet in height, are pale green to reddish, and usually red at the base. Small, greenish flowers grow in dense terminal and auxiliary clusters (Figure 101). Clusters are thick, prickly spikes up to 8 inches long. Flowers have bracts about twice as long as the sepals and yield small, round, shiny, black seeds. A single, large, mature plant of redroot pigweed can produce well over 200,000 seeds.

Similar Species: Redroot pigweed can be confused with smooth pigweed (Amaranthus hybridus), which differs by having hairless leaves, slightly hairy upper stems, and narrower, less dense, less prickly seed heads (Figures 102 and 103). Smooth pigweed flowering stems are the shortest of the pigweeds.

Common name: Morningglory species

Scientific name: Ipomoea

Life Cycle: Summer annual

Morningglory species can be invasive pests of agronomic crops. They are summer annual, trailing or climbing vines that can grow up to 10 feet. Stems twine in whorls coiling clockwise. The 4 species of morningglory that occur most frequently are ivyleaf morningglory, entireleaf morningglory, tall morningglory, and pitted morningglory.

Ivyleaf morningglory (Ipomoea hederacea) has butterfly-shaped cotyledons that are usually narrower and the base (Figure 104). Leaves are alternate and ivy-shaped with erect hairs on both surfaces (Figure 105). The first leaf may be unlobed but all other leaves are three-lobed. Stems are densely hairy with hairs that stick straight out. Flowers are blue to purple or sometimes white petals fused into a funnel (Figure 106). Fruit are capsules shaped like a slightly-flattened ball and contain 4 to 6 dark-brown to black, wedge-shaped fruits (Figure 107).

Entireleaf morningglory (Ipomoea hederacea var. integruiscula) differs from ivyleaf only by having heart-shaped leaves (Figure 108). Leaf hairs are erect on both leaf surfaces.

Tall morningglory (Ipomoea purpurea) differs from ivyleaf by having a butterfly-shaped cotyledon with the tip and base of similar width and heart-shaped leaves with dense hairs that lie flat (Figures 109 and 110).

Pitted morningglory (Ipomoea lacunosa) differs from ivyleaf by having v-shaped cotyledons and heart-shaped leaves with few to no hairs (Figure 111). In addition, white flowers set it apart from tall morningglory, which usually has large, purple, showy flowers (Figure 112)

Management: The large seed size coupled with a hard, impenetrable seed coat can make morningglory more difficult to control with preemergence herbicides. The ability of morningglory species to emerge throughout the growing season adds to the challenge to manage. Its vining, climbing nature makes morningglory difficult to culturally control. Thin crop stands that allow light to penetrate the soil favor emergence of this weed. In addition, corn fields that have lost their tops due to maturity, wind, or insect damage can allow a late-season infestation. Crops wrapped in morningglory can complicate harvest efficiency. There are several herbicides used in corn and soybean to control these weeds.

Common name: Common ragweed

Scientific name: Ambrosia artemisiifolia

Life Cycle: Summer Annual

Common name: Kochia

Scientific name: Kochia scoparia

Life Cycle: Summer annual

Identification: Kochia is one of the most troublesome weeds across the Great Plains and western U.S. It can be found in crop fields, pastures, rangeland, roadsides, ditch banks, and wastelands. Kochia is competitive with crops and problematic in fallow periods between crops. Understanding the biology of kochia helps in developing management strategies for the weed.¹

Kochia is one of the earliest summer annual broadleaf weeds to germinate in the spring, and it will continue to germinate throughout the growing season. Kochia reproduces only by seeds, which will generally survive 1 to 2 years in the soil. Kochia seeds germinate at shallow soil depths.² Seedlings have thick, dull-green leaves, and the undersides of kochia cotyledons on seedlings are usually bright pink or magenta in color. Young plants form a small rosette (Figure 121).³ Kochia is capable of germinating at colder soil temperatures and is quite tolerant of frost. Because kochia germinates early, it can be in advanced stages of growth when other weeds are ready to spray.

Kochia grows rapidly and can reach 6 feet in height when grown in competition with other plants. Without competition, it can grow to be a bushy plant, typically reaching heights of 3 to 4 feet. Kochia plants develop many branches with stems that are often red-tinged (Figure 122). Leaves are lance-shaped, pale green, and covered with hairs (Figure 123). Kochia flowers are inconspicuous, green, and in clusters up to 2 inches long (Figure 124). Clusters occur at the ends of stems and in leaf axils. Kochia flowers and sets seed in late summer and fall, with a single plant capable of producing up to 50,000 seeds that can be spread by wind, water, equipment, and as contaminants in hay.¹ The seed is often dispersed after plants mature and become “tumbleweeds”, dropping seeds as they blow around.

Management: Kochia is tough to control because it is widely adapted, tolerates stress, emerges early with an extended germination period, has narrow leaves which are more difficult to cover with herbicide applications, and produces an abundance of seed that can spread over long distances. Kochia plants are tolerant of drought and saline soils and do well under growing conditions considered poor for most crops. Seeds germinate at shallow soil depths, making no-till a good environment for kochia survival. Kochia plants can self– or cross-pollinate, increasing the likelihood of resistant genes moving across populations.

 Kochia has confirmed herbicide resistance to Group 2 (ALS inhibitor), Group 4 (synthetic auxins), Group 5 (PSII inhibitors), and Group 9 (glyphosate) herbicides. Best management practices should be adopted for the containment and management of resistant weed populations.¹ This includes integration of cultural and mechanical weed management practices, and the use of diversified and effective control tactics with herbicides.

Crop rotation has been shown to be an important factor for managing kochia. A multi-state university study has shown corn and soybean plants to compete well with kochia, suppressing growth and development.⁵ Corn and soybeans have more herbicide options for kochia management than other annual rotational crops. Wheat and fallow were less effective than corn or soybeans, and sugarbeets were found to be ineffective in suppressing kochia growth. Diverse crop rotations can include the use of multiple management tactics and herbicides with different sites of action for more effective control of kochia.

Using both preemergence (soil-residual) and postemergence herbicides with multiple sites of action is also an important management strategy to control kochia populations. Using multiple herbicide application timings (burndown, preplant, preemergence and postemergence, and postharvest) in-crop or fallow are the most effective means to control multiple flushes of kochia during the season. Kochia plants that escape herbicide applications should be controlled using tillage or physical removal before those plants produce seed.

Sources:

¹Jha, P., McVay, K., Varanasi, A., and Kumar, V. 2013. Glyphosate-resistant kochia in Montana - Herbicide recommendations and best management practices for growers. Montana State University Extension Research Bulletin No. 4602.

²Schwinghamer, T.D. and Van Acker, R.C. 2008. Emergence timing and persistence of kochia.

Weed Science 56:37-41.

³Kochia weed gallery. University of California IPM program. http://www.ipm.ucdavis.edu .

⁴Heap, I. The international survey of herbicide resistant weeds. http://www.weedscience.org .

⁵Sandell, L. 2012. Glyphosate-resistant kochia confirmed in Nebraska. UNL CropWatch. http://cropwatch.unl.edu .

Common name: Velvetleaf

Scientific name: Abutilon theophrasti

Life Cycle: Summer annual

Common name: Ladysthumb

Scientific name: Polygonum persicaria

Life Cycle: Summer annual

Common name: Pennsylvania smartweed

Scientific name: Polygonum pensylvanicum

Life Cycle: Summer annual

Identification: Pennsylvania smartweed and ladysthumb are early-emerging annuals. These two species are very similar in appearance and growth habit. Both species can be problematic in agronomic cropping systems since they tolerate a range of soil types and conditions. However, they do not tolerate drought conditions. Cotyledons are narrow, oval to lance-shaped, with rounded tips (Figure 130). Leaves are usually hairless, alternate, and lance-shaped with pointed tips and smooth margins. The center of both leaf surfaces can be marked with a purple blotch resembling a thumbprint (Figure 131). Many feel this purple blotch helps distinguish ladysthumb; however, the blotch can be seen on either species.

Stems of Pennsylvania smartweed and ladysthumb are branched and can be green or reddish and, as with all species in the Polygonaceae family, are swollen and jointed at the nodes. A smooth, membranous sheath (ocrea) surrounds the stem at the base of each petiole (Figure 132). The distinguishing feature between Pennsylvania smartweed and ladysthumb is the pubescence on top of the ocrea (Figure 133). Ladysthumb has a fringe of hairs at the top of the ocrea, whereas Pennsylvania smartweed does not.

Small, pink or white flowers are clustered in terminal spikes at the end of stems (Figure 132). The seed is enclosed in a single-seeded, flat, glossy, black, round fruit with a pointed tip (Figure 134).

Management: Smartweeds are somewhat competitive but low-growing weeds. Soybean yield was reduced 13% by 8 smartweed plants in 30 feet² of field (about 1 plant per 4 feet of row).¹ Tillage in the spring triggers smartweed to germinate. Tilling early and planting later will help reduce smartweed infestations because many will have emerged and been controlled by tillage. Tilling at night (dark tillage) helps reduce smartweed emergence by 30 to 50%.¹ Dicamba-containing products are effective herbicides, especially in a burndown situation. Other herbicides are also effective for smartweed control in field crops.

Sources:

Smartweeds (Pennsylvania smartweed and ladysthumb). Weeds. Department of Plant, Soil, and Microbial Sciences. Michigan State University. http://canr.msu.edu

Common name: Wild buckwheat

Scientific name: Polygonum convolvulus

Life Cycle: Summer annual

Identification: Wild buckwheat is a summer annual that grows from a hard, black, triangular seed about 1/8-inch long (Figure 135). The cotyledons are narrow with a rounded tip and base (Figure 136). The plant has long, slender, creeping stems that trail along the ground or climb any plants or objects they contact. The leaves are alternate, heart- or arrowhead-shaped, pointed at the tip, and have widely-separated lobes at the base (Figure 137). Plants bloom throughout the summer and the flowers are small and have no petals but have 5 greenish or pinkish sepals. Flowers are in short-stemmed clusters in the axils of the leaves or at the end of stems. Wild buckwheat is a member of the smartweed family. Like other smartweeds, wild buckwheat possesses an ocrea around the stem at each node. It can be confused with field bindweed because of its vining growth habit and arrowhead-shaped leaves. Field bindweed, however, is a perennial, has an extensive underground root system, does not possess an ocrea, and has large, funnel-shaped flowers that are white or pink. Also, wild buckwheat’s lower leaves are usually much wider than bindweed leaves.

Wild buckwheat begins germinating very early in the spring during the first prolonged warm period, beginning as early as March in Kansas and April in North Dakota. Most of wild buckwheat seed emergence occurs by mid-June. Wild buckwheat’s climbing habit allows the plant to obtain sunlight while growing in stands of grain or other tall crops. It also can spread horizontally and quickly cover bare ground, enabling it to maximize light-gathering efficiency.

Watchouts and Disease Host: In addition to competing for nutrients, light, and moisture, wild buckwheat causes difficulty in harvesting crops. Furthermore, wild buckwheat is a host for several crop plant diseases, including virus yellows of beets and cucumber, alfalfa mosaic virus, and tobacco rattle virus.

Management: Cultural practices, such as crop rotation, mowing, delayed seeding, post-seeding harrowing, or postharvest cultivating, can help reduce wild buckwheat stands, but have not been effective in control. Delayed crop seeding can help manage wild buckwheat by killing early-germinating seedlings through seedbed preparation or burndown herbicide applications.

Wild buckwheat and related smartweed species have some natural tolerance to glyphosate. Wild buckwheat control from glyphosate is reduced when applications are made at lower than labeled rates, when only one glyphosate application is made, when plants are stressed from adverse environmental conditions, or when runners are more than 3 to 4 inches long. The most effective herbicides on wild buckwheat are atrazine, bromoxynil, clopyralid, dicamba, glufosinate, and some sulfonylurea products. Using these herbicides or mixtures with these ingredients will ensure the most effective wild buckwheat control. Atrazine, clopyralid, dicamba, and some sulfonylurea herbicides may persist in the soil and carry over for more than one growing season, especially in soils with high pH.

Wild buckwheat has become a major weed problem in no-till fields prior to spring planting. All existing weeds (including wild buckwheat) should be controlled prior to planting or emergence of no-till crops with a preplant burndown herbicide treatment. Glyphosate or glyphosate plus 2,4-D is often inadequate for wild buckwheat control. Adding or using another herbicide may be necessary to achieve adequate wild buckwheat control, but proper waiting intervals between applications must be observed. Crops planted before early or mid-May could require an in-crop herbicide application to control late-emerging plants. This strategy will allow earlier postemergence applications to smaller plants.

Sources:

Wild buckwheat. UC IPM. University of California Agriculture and Natural Resources. http://ipm.ucanr.edu

Zollinger, R., Peterson, D., and Moechning, M. 2006. Biology and management of wild buckwheat. GWC-10. Purdue Extension. http://extension.purdue.edu

Common name: Canada thistle

Scientific name: Cirsium arvense

Life Cycle: Perennial

Common name: Curly dock

Scientific name: Rumex crispus

Life Cycle: Perennial

Identification: Curly dock is an erect, top-rooted, simple perennial weed that can thrive in a variety of habitats including agronomic fields and pastures. Curly dock cotyledons are round at the apex and narrow at the base (Figure 144). Young leaves are spatula-shaped and may have red patches. Mature leaves are alternate, long, and narrow with round to pointed tips and wavy margins resembling bacon strips (Figure 145). Leaves have prominent midveins and an ocrea surrounds the stem. The plant forms a basal rosette that bolts prior to flowering, reaching 3 to 4 feet in height. Stems are unbranched, thick, ridged, hairless, and often turn red. Flowers are small, usually greenish, in narrow panicles on the upper stems and become reddish-brown at maturity (Figures 146 and 147). Seeds are three-sided, rust-colored, and shiny. Each plant can produce hundreds to thousands of seeds that can move easily in wind and water. Curly dock can also reproduce vegetatively from sprouts that regenerate from buds on the taproot.

Management: Curly dock seeds can remain dormant in the soil for decades and germinate at various times throughout the year. Curly dock can become problematic in no-till corn and soybean systems but is rarely an issue in tilled soils. It is difficult to achieve complete control of rootstocks with any treatment, but there are a number of herbicide combinations that manage the weed for subsequent corn and soybean crops. Higher rates of glyphosate in combination with dicamba or 2,4-D can provide acceptable control prior to planting. Uprooting, chopping, and burying the taproots through tillage will control curly dock. Spring herbicide applications are effective in controlling curly dock. Once established, fall herbicide applications are more effective. Sequential herbicide applications may be necessary for control.

Common name: Dandelion

Scientific name: Taraxacum officinale

Life Cycle: Perennial

Identification: Dandelion is a simple perennial that forms a basal rosette with yellow flowers and a puff-ball seed head. Dandelion is one of the most common and problematic weeds of turfgrass and lawns and can also be problematic in no-till crop production. Plants emerge from seed and the root becomes a perennial taproot system that persists over the winter. Dandelion captures space in forage crops and in no-till systems. It is not competitive for light but captures soil moisture and nutrients.

Cotyledons are light-green, smooth, and oval to spatulate in shape (Figure 148). Young leaves form a basal rosette and are also oval to spatulate in shape, 2 to 6 inches in length (Figure 149). Leaf margins are noticeably wavy, especially on older leaves. All leaves are basal, ranging from 2 to 16 inches in length depending on the environment. Usually, leaves are more in the range of 2 to 8 inches in length. Leaves are oblong in outline, sometimes sparsely hairy, deeply indented, with lobes that point toward the center of the rosette. Stems are erect, hollow, and approximately 2 to 6 inches in height giving rise to solitary, bright-yellow flowers (Figure 150). Fruit are an achene that is brown, 3 to 5 mm long, with a feathery pappus attached that aids in wind dispersal of seed (Figure 151). Collectively, the achenes form a white seed head that resembles a puff-ball.

Management: Regular soil disturbance is effective at preventing the establishment of dandelion plants. This is why dandelions are seldom a problem in tilled cropping systems. Tillage must be deep enough to cut the root 4 inches below the crown and the soil surface. Both rotary hoeing and cultivation control seedling dandelions but have little effect on those that are established.

Herbicide options are available to suppress dandelion in corn, soybean, and alfalfa. Spring applications of some herbicides control seedling dandelion. For perennial weeds, fall applications of herbicides are usually more effective than spring applications. Dandelion is one of the more difficult weeds to control. Differences in response of dandelion populations to 2,4-D ester and glyphosate have been observed. Where corn or soybeans have emerged and dandelion are established, the strategy should be to suppress the plants to minimize yield loss from dandelion competition. In-season applications will substantially reduce dandelion competition but will likely not greatly reduce dandelion populations. Then a fall application can be planned following harvest with the goal of actually removing the plants.

Sources:

Kells, J., Hillger, D., and Sprague, C. May 4, 2006. Dandelion management in field and forage crops. Department of Crop and Soil Sciences. Michigan State University. http://canr.msu.edu

Common name: Field bindweed

Scientific name: Convolvulus arvensis

Life Cycle: Perennial

Identification: Field bindweed is a creeping, deep-rooted perennial that is considered noxious in many states. It spreads from extensive rootstock and seed. Field bindweed vines grow out in a spiral pattern, allowing them to twist around whatever may be growing near them, up to 6 feet high (Figure 152). The cotyledons on seedling plants are square, usually with a slight indentation on the apex (Figure 153). The first true leaves are arrowhead-shaped, lobed at the base, and have petioles. No cotyledons are present on young plants emerging from established rhizomes. Mature plants also have alternate, arrowhead-shaped leaves up to 2 inches long depending on environmental conditions (Figure 154). Under warm, moist conditions, leaves are larger, and vines are more robust than under drought conditions. Stems are smooth to slightly hairy and trail along the ground or climb vegetation and other objects. Flowers have white to pink petals fused into a funnel shape with two leafy, small bracts one inch below the flower base (Figure 155). Fruit are oval to round capsules containing dull-gray to brown or black seeds with one rounded side and two flattened sides (Figure 156). An average plant produces about 250 seeds. The root system has both deep, vertical and shallow, lateral roots. The vertical roots can reach for 20 feet or more. Most of the root structure, however, is in the top 2 feet of the soil.

Similar species: Hedge bindweed (Calystegia sepium) is often confused with field bindweed but differs by having larger, triangle-shaped leaves with square lobes extending behind and perpendicular to the petiole (Figures 157 and 158.). Flowers of hedge bindweed are larger, and this weed also has large bracts beneath the flowers, unlike field bindweed (Figure 159).

Management: Established field bindweed is difficult to control and should include a combination of management tactics including cultivation, selective herbicides, and crop competition. Timely and intensive cultivation helps control newly-emerged seedlings and manage young field bindweed infestations. Timely cultivation depletes root reserves and helps stimulate dormant seeds to germinate. Intensive cultivation is usually not practical as the sole method of control because tillage must be done every 8 to 14 days throughout the growing season for multiple years and exposes the soil to erosion.

Field bindweed prefers full sunlight, so competitive crops may help discourage field bindweed as it is not competitive under shady conditions. Rotation with crops like winter wheat, corn, alfalfa, and sorghum can reduce field bindweed growth.

Preventive control measures, along with tillage and herbicide applications, are most effective for controlling field bindweed. Closely-drilled, vigorous, competitive crops such as winter wheat or forage sorghum may also aid control. Systemic herbicides like dicamba, 2,4-D, picloram, and glyphosate products alone or in various combinations are registered for suppression or control of field bindweed. Single herbicide applications rarely eliminate established bindweed stands. Applications of 2,4-D and glyphosate products are most effective when spring-applied to vigorously-growing field bindweed in mid to full bloom. However, dicamba applications are most effective when applied in the fall. Most herbicide treatments are least effective when applied in midsummer or when bindweed plants are stressed.

Common name: Hemp dogbane

Scientific name: Apocynum cannabinum

Life Cycle: Perennial

Identification: Hemp dogbane is a perennial weed that secretes a milky sap when broken, reaching 5 to 6 feet in height. In pastures and hay fields, hemp dogbane represents a potential threat both as a weed capable of reducing yields and is also poisonous to cattle, horses, and sheep. Leaves are entire, ovate or elliptic, 2 to 5 inches long, 1/2 to 1 & 1/2 inches wide, and arranged oppositely along the stem (Figures 161 and 162). Stems are without hairs, often have a reddish tint when mature, and undivided at the base but become heavily branched in its upper half (Figure 163). Flowers are small, white to greenish-white, and produced in terminal clusters (Figure 164). The fruit are long (5 inches or more), narrow follicles that are produced in pairs (Figure 165). Hemp dogbane reproduces by seeds (Figure 166), but the primary mode of reproduction of plants in agronomic fields is vegetative creeping roots resulting in hemp dogbane to arise in patches. Seedlings can become perennial 5 to 6 weeks after emergence. The perennial creeping rootstocks persist many years in the soil.

Common name: Common milkweed

Scientific name: Asclepias syriaca

Life Cycle: Perennial

All plant parts contain a white milky latex that is excreted upon injury (Figure 169). The leaves of common milkweed are opposite, oval, oblong to egg-shaped, 3 to 8 inches long, with smooth margins and distinctly-white midveins and veins. The upper leaf surface is dark green and usually smooth; the lower leaf surface is pale green and finely hairy (Figure 170). Milkweed flowers occur in rounded clusters at stem ends and in upper leaf axils and are sweetly scented, and each occurs at the end of a long, slender flower stalk (Figure 171). Although commonly lavender-pink, the flowers can also be shades of yellow or green. The flowers have both male and female parts but must be cross-pollinated by insects to form seeds. A single common milkweed plant can produce an estimated 25 fruit with each fruit containing as many as 450 seeds. The conspicuous seed pods are initially fleshy, pale green, covered with soft-pointed bumps, and shaped like long teardrops (3 to 5 inches long) (Figure 172). With maturity, the follicles dry up and turn brown before splitting open lengthwise, releasing numerous brown, flat seeds, each with a tuft of silky hair (pappus) attached to one end (Figure 173). Seeds, commonly carried off by the wind, reportedly remain viable in the soil for at least 3 years.

Management: Cultivation does not work well because it spreads root fragments and cannot reach the roots of common milkweed that extend far below the plow depth. To be effective, tillage must be done often enough to deplete underground root reserves—every 2 or 3 weeks.  

This weed has an extensive and deep root system and is tolerant to many common herbicides. Multiple herbicide applications are often required. Glyphosate, when applied at the proper rate and timing, will give good control. Late, postemergence applications when plants are in the bloom stage will be most effective in killing roots. There are several herbicides listed for tank mixes with glyphosate for corn and soybeans. 

In small grains, milkweed should be allowed to regrow to a mature stage after harvest or mowing, and then treated. In hay or pasture, milkweed can be spot-treated with glyphosate applied with a wipe-on applicator while the milkweed is taller than the crop, or spot-treated with a hand-sprayer. When these fields are rotated or renovated, that is the time to make your best effort to deal with milkweed aggressively.

Common name: Horsenettle

Scientific name: Solanum carolinense

Life Cycle: Perennial