Contributed by Dr. Rui Liu, Assistant Professor – Weed Science, Washington State University Irrigated Agriculture Research and Extension Center
Transition takes time and some familiarity helps. After working as an assistant scientist at Kansas State University Agriculture Research Center (KSU-ARCH) in Hays, Kansas, for about four years, I moved to Prosser, Washington and started my faculty position at the Washington State University Irrigated Agriculture Research and Extension Center (WSU-IAREC). The Pacific Northwest is different from the midwest. Each has their own unique beautiful scenery. The Columbia Basin offers a diverse range of cropping systems-grapes, tree fruits, potatoes, onions, mint, etc. All of them are new to me.
But as a weed scientist, the familiarity with weeds brings a little bit of comfort, especially when you encounter species you have some prior experience with. I ran into Russian thistle on a trail walk on a clear sunny day. They are huge compared to the ones I’ve seen in the midwest. Maybe the irrigated environment gave them more resources to develop into their extra-large size.
Russian thistle (Salsola tragus L.) belongs to the goosefoot (Chenopodiaceae) family. Another weed species in this family is kochia (Bassia scoparia L.). Both of them are often called “tumbleweeds” and are commonly seen in the western U.S. When a plant matures, it dries up and the stem breaks off at the base, becoming a tumbleweed. The seeds disperse as the plant tumbles with the wind.
Russian thistle is a native of Russia and was introduced to the U.S. through contaminated flax seed in the late 1800s. Now it is found in every state in the U.S., except Alaska and Florida. It is a summer annual weed species.
The seedlings look like grass seedlings when they first emerge, and later look similar to pine needles. The young leaves of Russian thistle are small, narrow, dark-green in color, and the older leaves have stiff spines with a sharp, pointed tip. The flowers are small and petal-less, located in the leaf axils between a pair of spiny bracts. The stems are erect with many branches, and often shown reddish or purple striping. The mature plants are woody and can grow up to 4 ft tall. The matured Russian thistle plants provide cover for pheasants. Its seeds and foliage can be food sources for small mammals and songbirds. However, they are toxic to cattle and sheep due to the nitrate level and oxalates, as well as the spike-like inflorescence. Only the young plants could be used as forage.
Russian thistle has a taproot system and is extremely drought tolerant. The taproot can grow several feet into the soil for moisture. It is well adapted for desert-like environments and is very competitive when moisture is a limiting factor. A Russian thistle plant can produce more than 200,000 seeds, yet the seeds can lose viability quickly in the soil. The seeds usually use the winter moisture to germinate in late winter or early spring. Normally more than 90% of seeds germinate or decay within the first year.
Photos: Russian thistle on Lincoln grade trail in Prosser, WA. Rui Liu, WSU-IAREC.
Management of Russian thistle can be achieved through using integrated weed management (IWM) practices. But the task is not easy. For young plants, mechanical methods like mowing, tillage, or other means to destroy or remove the plant can help prevent seed production.
There were multiple attempts throughout the years to use biological control methods, e.g., import control agents such as insects. However, the control level is not sufficient. Investigation on other potential biological control agents are still ongoing. Chemical control through herbicide use can provide good control of Russian thistle when the seedlings are small and actively growing. In the irrigated Coumbia Basin region, herbicide options are limited due to the variety of crops grown in the Basin and their sensitivity to many herbicides. Herbicide like 2,4-D, dicamba, and glyphosate are effective on Russian thistle but could also cause a lot of damage and potentially drift to fields of sensitive crops like grapes. Russian thistle plants resistant to glyphosate were identified in Washington in 2015, which will limit herbicide options availble to growers. Future research is needed to look at alternative chemicals and methods for managing Russian thistle in vegetable and specialty crops.