The Q&A Archives: Russian Thistles

Question: Our garden is plagued with 'Russian Thistles

Answer: Control of Russian thistle is difficult. There have been numerous attempts through the years to import biological control agents, but none have been successful. Cultural control practices such as mowing or destroying young plants can prevent seed production. Avoid discing or loosening the soil in abandoned areas because loose soil is necessary for Russian thistle germination. Burning is sometimes used to destroy accumulated Russian thistle plants. While this may eliminate the accumulated organic debris and some seed, much of the seed will already have been disseminated. Planting competitive, more desirable species can be an effective method of preventing Russian thistle establishment in most noncrop environments. Russian thistle competes poorly in situations with firm, regularly irrigated soil, and it is rarely a problem in managed gardens, turfgrass, or landscapes.

Preemergent herbicides are applied to the soil before the weed seed germinates and are usually incorporated into the soil with irrigation or rainfall. The most effective preemergent herbicides are Aatrex (atrazine), Velpar (hexazinone), Devrinol (napropamide), Telar (chlorsulfuron), Oust (sulfometuron), Princep (simazine) and Hyvar (bromacil). Other preemergent herbicides that are registered but only moderately effective in controlling Russian thistle are Surflan (oryzalin), Treflan (trifluralin), Prowl (pendimethalin), Endurance (prodiamine), Lasso (alachlor), Predict (norflurazon), and Kerb (pronamide).

Postemergent herbicides are applied to plants, but timing is critical. For best results, these herbicides must be applied while the weed is in its early growth stages, preferably the early seedling stage, before it becomes hardened and starts producing its spiney branches. Do not use postemergent herbicides to try to control the mature seed (either on the plant or on the ground) as they are not effective for this purpose. Also, the later spiney stage of Russian thistle is not readily controlled by any postemergent herbicide. If rain or irrigation occurs after a postemergent application, additional seedlings may emerge and require future treatments. Postemergent herbicides that are effective when properly applied include Banvel or Vanquish (dicamba), Roundup (glyphosate), and 2,4-D.

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