Sowthistles

Annual sowthistle is a common landscape weed that plays host to several garden pests.

​While all weeds compete with your garden and landscape plants for sunlight, water, and nutrients, some weeds also carry diseases or provide habitat for pests. The annual sowthistle is one of those weeds.

Sowthistle (Pikist) Public Domain

​Annual sowthistle (Sonchus oleraceus) is also known as milk thistle, hare’s thistle, soft thistle, and hare’s colewort. As a member of the sunflower family, sowthistles have a taproot and they produce a milky white sap (latex).

​Annual sowthistle description

If you live in California, you’ve probably seen millions of these. Sowthistles can grow in only 1/2 an inch of soil, and they grow quickly. The cotyledons (seed leaves) are slightly football-shaped and less than 1/3 of an inch long. Early leaves are alternate, somewhat hairy, and feature margins (leaf edges) with backward facing teeth. Like dandelions, the stem is a smooth hollow tube. Mature leaves are a grayish blue-green. Upper leaves are smaller than lower leaves and there is no petiole (leaf stem). In many cases, the stem looks as though it is poking through the middle of the leaf, making them perfoliate. Lower leaves are deeply lobed and tapered. The flowers are yellow clusters that close at night (nyctinasty). These flowers matures into fluffy seed heads that blow in the wind, just like dandelions, but the seed heads are generally not a complete sphere.

Annual sowthistle (Forest and Kim Starr) CC BY 2.0

Sowthistle seedling (Salicyna) CC BY-SA 4.0

Pests carried on sowthistle

Pests carried on sowthistle include lettuce aphids (Nasanovia ribis-nigri), lettuce root aphids (Pemphigus busarius), green peach aphids (Myzus persicae), and nematodes. Sowthistle should also be considered a vector of several different plant viruses. Pulling these weeds from your garden can break the disease triangle for many of these viruses, reducing or eliminating the need for more extensive treatments.

Sowthistle control

Pulling up seedlings as soon as they are seen is the easiest control. Unlike many other weeds, sowthistle does not regrow from root fragments. Since sowthistle only reproduces by seeds blown on the wind, preventing those seed heads from forming can significantly reduce your workload in the one run.

Luckily, sowthistles are easy to pull up.​