Identifying Russian Thistle
Russian thistle is a bushy summer annual weed with thin, upward-growing stems that become woody when mature. Seedlings have finely dissected leaves that are about 1 inch long, narrow and a dark green color. Russian thistle seedlings look similar to pine seedlings. As weeds mature, their stems can grow up to 3 feet tall and have red or purple stripes. The roots of a mature Russian thistle plant can grow up to 6 feet deep and 6 feet wide.
Flowers are subtle and develop in the axils of the upper leaves. A pair of spiny, floral bracts accompanies each flower. Each small, one-seeded fruit has winged tips. Russian thistle can emerge throughout the growing season. Typically, seeds germinate in late winter or early spring and usually emerge in early April through late June. Plants typically flower from July through October.
Favorable Conditions for Prolific Weed Seed Production
Russian thistle develops as a small plantlet inside the seed. Under conditions of ideal moisture and temperatures of 52 to 90 degrees, the plantlet begins to uncoil. A young taproot develops in the soil, allowing germination to occur in 12 hours or less, giving Russian thistle a significant advantage over many other plants competing for limited moisture.
One Russian thistle plant can produce approximately 250,000 seeds. After seed maturation, which generally occurs from August through October, the plant stem detaches from the root. The plant is then carried by wind, scattering seeds as it tumbles across fields and other landscapes, similar to kochia.
In spring, months after seeds are deposited into the soil, farmers may see green trails of germinating Russian thistle seedlings resulting from the paths taken by tumbleweeds blown across plowed fields. This requires additional control measures to protect yields.
Potential Damage and Economic Impact
Russian thistle competes with crops throughout the growing season, robbing them of water, sunlight, nutrients and yields. Yield loss increases with density of Russian thistle populations. The first Russian thistles to emerge will be the most competitive, so it’s important to control weeds early before they reach 3-inches-tall.
Another worry for farmers is that Russian thistle can delay grain harvest because of the weed’s high moisture content in the field. It can also increase grain moisture levels because of weed particles mixed with grain after harvest. In some cases, fields have become unharvestable because of moderate to high densities of Russian thistle.
Known Resistance in Russian Thistle
Russian thistle is resistant to ALS-inhibiting herbicides (Group 2). Resistance to triazines (Group 5) is also suspected. Another concern for farmers to consider is that Russian thistle is among a list of weeds posing the greatest concern for glyphosate (Group 9) resistance, according to Kansas State University.
It’s sometimes a challenge for farmers to keep track of the many factors that contribute to the management and proliferation of herbicide-resistant weeds; therefore, the best approach to manage resistance is to use best management practices (BMPs) to keep tough-to-control and resistant weeds from going to seed and entering the soil seedbank, where they can remain viable and cause problems in future crops.
For more information on Russian thistle and other confirmed resistant weeds by state, refer to the International Survey of Herbicide Resistant Weeds
Russian Thistle Control
The most effective management strategies for Russian thistle in cereals, corn, soybeans and cotton should focus on preventing seed production throughout the year. Russian thistle is most susceptible to herbicide application prior to emergence and before weeds exceed 2 to 3 inches in height. The first weeds to emerge will be the most competitive and difficult to control.
- Consider pre- and postharvest spraying where dense populations of Russian thistle exist in your field. Farmers benefit from a preharvest application because it helps speed dry-down of Russian thistle, improves harvest efficiency and lessens residue difficulties during fall cultivation. At postharvest, control Russian thistle with appropriate herbicides or with tillage two weeks after harvest. Tillage eliminates most Russian thistle but will result in less residue and less water/soil moisture retained in the soil through winter than the use of postharvest herbicide control.
- Do not plant into existing stands of Russian thistle. Start weed-free at planting using a burndown tankmixed with a pre-emergence residual herbicide just prior to or at planting.
- Russian thistle-infested fields not treated with a fall herbicide application or not cultivated in the fall can result in early-emerging weeds in the spring. In these situations, Russian thistle should be controlled early to ensure effective burndown. Farmers may want to apply burndown herbicides with some of the residual herbicide in early spring and then apply the remainder of the residual herbicide at planting. Remember to manage resistance by choosing products with different modes of actions from different classes of chemistry.
- Crop competitiveness and crop rotation are two important cultural practices in cereal, corn and soybean crops. Managing crop residue, controlling weeds and planting high-quality, uninfested seed helps establish a vigorous crop to compete more effectively with emerging weeds for water, light, space and nutrients. Depending upon geography, farmers may consider rotating grain crops such as corn, grain sorghum, wheat, soybeans and sunflower.
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.