Johnsongrass in the early stages of growth. It is often mistaken for fall panicum or barnyard grass before seedhead formation.
Grasses can have a significant economic impact on crops. They compete for water, light, space and nutrients, all factors which can reduce yields. In order to manage their farms for maximum profitability, farmers must be able to first identify and understand the biology of common grasses and then design effective integrated weed management programs to control them.
Tricky Grass Identification
Many common grasses share similar characteristics that make them difficult to identify. One example is the group of similarities shared by fall panicum, Johnsongrass and barnyard grass. Fall panicum is often mistaken for Johnsongrass or barnyard grass prior to the formation of a seedhead. Johnsongrass, however, has a thin membranous growth from the blade, known as a ligule. Unlike fall panicum, Johnsongrass seedlings lack hairs on their lower leaf surfaces. Additionally, barnyard grass does not have any ligules, and barnyard grass seedlings sometimes only have hairs near the leaf base.
Fall panicum, a summer annual that can reach 7 feet in height, is found throughout most of the United States. Its zigzag growth pattern, which it exhibits by bending at the nodes of its large, round, smooth sheaths, is a primary identifying characteristic. In addition to being mistaken for barnyard grass and johnsongrass at the seedling stage, fall panicum can easily be confused with green foxtail until the differences become more evident in fully grown plants.
Barnyardgrass is a summer annual with thick stems that can grow up to 5 feet in height. Found throughout the United States, Canada and Mexico, it is one of the few grass weeds that lacks ligules, a feature that helps distinguish it from most other grasses in both the seedling and mature stages of growth.
Johnsongrass, which originally made its debut in the United States as a forage crop, is now one of the most prevalent and troublesome weeds of crop fields, hay fields, pastures, roadsides and ditches. This perennial weed has a thick mass of jointed white rhizomes that can reach 9 feet in height. It is capable of rapidly colonizing in various environments because of the large amount of seed and mass of rhizomes it produces. To identify Johnsongrass, look for leaf blades without hairs on both sides; however, some hairs might appear at the base of the leaf blade. The ligules are 3 to 4 millimeters in length, membranous and often toothed at the end. This membranous ligule is distinct from fall panicum or barnyardgrass. Seedheads are large, open and often have a purplish hue. The seedlings and mature plants of Johnsongrass also share some of the same characteristics as shattercane, but shattercane lacks the mass of tough rhizomes characteristic of Johnsongrass.
Shattercane is an annual weed that looks and grows like Johnsongrass and grain sorghum. It has even been known to hybridize with cultivated Johnsongrass and sorghum. This grass is usually without hairs and has very long, flat leaves that are 1 to 2 feet long and 1 to 2 inches wide. The leaves have a distinct midvein. The membranous ligule is 1 to 2 millimeters long, and the seedhead is quite compact, unlike Johnsongrass.
Several varieties of foxtail – yellow, giant and green – can cause problems in agronomic fields. All foxtail weeds are clump-forming summer annuals with seedheads that resemble a fox’s tail. They can be found throughout the United States, particularly in areas with fertile soil. Long, silky hairs appear at the base of the leaf blades, which can reach 12 inches in length and 7 to 12 millimeters in width. There are no auricles (outgrowths on the leaf collar), and the ligule has a fringe of hairs reaching 2 millimeters in length. Stems are erect, hairless and often flattened with a reddish tint at the base.
To identify yellow foxtail, look for the characteristic foxtail-like seedhead, which yellows upon maturity, and leaves with long, silky hairs, which appear only at the base. Conversely, the leaf blades of giant foxtail have many short hairs on their upper surfaces, as does green foxtail. Giant foxtail is also typically larger and has a seedhead that droops – or nods – when mature, unlike the other foxtail species.
Pesky Pair: Goosegrass and Crabgrass
Another commonly confused pair of common grass weeds that are typically treated postemergence are goosegrass and crabgrass. Goosegrass is a summer annual with flattened stems that radiate outward from a distinctive white or silver center, even on seedlings, which distinguishes this weed from most other grasses. Leaf sheaths are flat and smooth. Plants often appear compressed or flattened against the soil . On mature plants, leaf blades are 2 to 14 inches long and 3 to 8 millimeters wide, without hairs or only sparsely hairy, and folded along the midvein. This weed might be confused with smooth crabgrass, but the leaves of goosegrass are folded in the bud, while those of smooth crabgrass are rolled in the bud.
Crabgrass has two varieties: smooth and large, which is also referred to as hairy crabgrass. They are summer annual grasses with ascending growth habits, but smooth crabgrass has hairless sheaths and stems that do not root at the node. Leaf blades are 2 to 8 inches long, and seedheads are composed of four to six spikes at the top of the stems. Seedlings or mature plants often have a reddish tint at the base.
Other Common Grass Weeds
Wooly cupgrass is an erect summer annual weed with relatively robust seedlings and wide leaves that are covered with very short hairs. Seedlings often have a red tinge at the base. The leaves have rough margins, no auricles and a small ligule with a fringe of hairs. Prairie cupgrass is similar in appearance, but its flower head is not covered with hairs.
Field sandbur is an annual grass widely distributed across the southern United States, particularly in sandy sections of fields. Its distinctive seedhead, which is approximately 5 to 7 millimeters wide, has many round, spiky, somewhat hairy burs Leaves are rolled in the shoot and do not have auricles. Leaf blades, which are hairy on top and smooth underneath, can grow to 8 inches in length and 6 millimeters in width. The ligule has a fringe of hairs approximately 1 millimeter long. Overall, this weed grows about 20 inches in height.
Best Management Practices (BMPs) to Control Grasses
Farmers can keep common grass weeds from affecting their bottom line by using well-designed best management practices. Herbicide programs should consist of a fall and spring burndown treatment to ensure the field is clean at planting, as well as spring-applied residual herbicides to control weeds after planting. An integrated approach combining different sites and modes of action along with cultural practices, such as cover crops and equipment maintenance, can protect your fields now and prevent resistance in the future.
Crop Science Solutions
Crop Science has a broad portfolio to combat tough-to-control and resistant weeds. A well-thought-out herbicide program, using multiple modes of action, should be implemented to sustainably manage weeds. Before applying any herbicide, please read the entire label for the best possible results and to confirm that the product is effective on the weeds you wish to control. Not every product is suitable for every situation, and use of the correct application technique will ensure the best results.
The following Crop Science solutions are valuable tools to consider for your program.