Seedlings of the Russian thistle germinate rapidly when exposed to temperatures between 52 and 90 degees F. It is enabled by the ability of its taproot to extend into the soil in a span of 12 hours. The seedlings themselves are slender, flexible and usually have reddish or purple streaked stems. Cotyledons are similar in structure to pine needles and up to 1 inch long. True leaves are softer and more flexible, forming tiny spines at their tips. They alternate along the stem, often appearing to be opposite due to the short stem. Later on, leaves become more fleshy.
Contrary to the younger, skinnier plants, mature Russian thistles become bushy and wide. Growing up to 3 feet tall, the plant becomes ball-shaped as a whole. Leaves tend to become more fleshy to leathery and turn a bluish green. Leaves have sharp tips or spines and remain hairless or obtain short, stiff hairs. The upper leaves are shorter, stiffer, and more prickly overall. Towards the end of its life, the base of the stem turns grayish brown and breaks off. The plant begins to tumble along, dispersing its seed as it's blown about by the wind.
The tumbleweed's flowers bloom form July through October. They are generally hard to see, but have 5 sepals that are green or pink. The fruit produces a round, compressed to conical seed that has a gray to brown translucent seed coat. It usually produces about 250,000 seeds but is capable of producing upwards of 1 million.
The tumbleweed is particularly dangerous because of its ability to rapidly deplete soil moisture. This will reduce both yield and quality of surrounding plants. It can also host a variety of crop pests, as its shape allows harbor and leaves provide food. One such pest is curly top. Russian thistles can also become a serious fire hazard during extended dry periods.
Cutting, mowing, and tilling are always effective measures of controlling this weed. Since Russian thistles thrive in loose, dry soil, maintaining a compact and well-irrigated field will prevent it from competing with desireable crops. Burning will not be effective, as this weed thrives on disturbed areas.
Dicambia, glyphosate, and paraquat have been effectively used to control Russian thistle. There has been some resistance following treatments of chlorsulfuron and sulfometuron. Postemergence herbicides can be useful but timing is extremely important. It must be applied during its early growth stages before it hardens. Please contact your local weed specialist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.