The Strategy: Perennial sowthistle (Sonchus arvensis L.) is a perennial noxious weed that spreads from seeds and rhizomes. The plant can grow to a height of about 4 feet and is usually found where there is an adequate amount of soil moisture. This is why it is abundantly found in the Snake River drainage, mostly on the islands, as well as the numerous springs on the outlying areas. The rosette resembles a Dandilion but the deeply lobed leaves have prickly outer edges and the upper leaves are quite rare and much smaller than the lower, basal leaves. The plant has a milky sap as well as it is quite hairy with small glands on the tips of them (this is what separates this plant from its very close relative, Marsh sowthistle (Sonchus arvensis L. ssp uliginosus which does not have the glandular hairs). The plants yellow flowers appear on the very top of a virtually leafless stem and are almost identical to a Dandilion.

The Attack: As this plant tends to grow in moist areas it is very competitive toward our native species along rivers and streams. It is also competitive due to its ability to cross boundaries via its vast root system, and the seed can be blown great distances in the wind. It can be foraged on by livestock, but once it gets too large it is no longer eaten and becomes a problem and develops stronger roots.

The Defense: As this is a perennial plant mechanical control, either shovel or plow, is not feasible. In-fact mechanical control will only perpetuate the problem as the plant has an aggressive root system. Herbicides such as Milestone, Redeem, Chaparral, or even Curtail are most effective. As with all perennial invasive weeds, the best application is at bud to early bloom or fall after the ‘killing’ frost. Keep an eye out for this one as it can invade fast and destroy your valuable riparian areas.

PLEASE NOTE – The proper use and application of herbicides can be an effective way to control and eradicate noxious and invasive plants. Before using herbicides, always carefully follow the label and safety instructions on the label. While we recommend the use of herbicides as one of the effective tools for integrated pest management, the Idaho Weed Awareness Campaign assumes no liability for herbicide applications.