Common Sowthistle Sonchus

Sonchus oleraceus, or Sowthistle, is a genus of annual herbs, considered a winter and summer weed, and includes several species that are common in nurseries and landscapes. Most of the species are annual herbs, but a few are perennial and a few are even woody. The species can overlap the winter and summer annual categories. Seeds germinate in the fall or spring and are typically carried by the wind or water to propagate the plant. Seedlings form a rosette of waxy leaves with variable leaf margins. One of the most common species, spiny sowthistle, has prickly leaf margins. In contrast to wild lettuce (Lactuca spp.), sowthistle lacks prickles on the mid-vein of the leaves. In warm weather, plants bolt to about 3 to 4 feet tall but may be shorter. Flowers are bright yellow. Seedhead is similar to a dandelion but smaller.

This plant spreads by reseeding itself. Seeds have a pappus that aids wind and water dispersal and the plant can spread aggressively. Do not let plants go to seed in the nursery. You can manage the spread of the plant with applications of preemergence broadleaf herbicides labeled for use. This plant can often be controlled by mowing, because it does not regrow from root fragments.

Insects, Diseases, and Other Plant Problems

Sowthistles are common host plants for aphids. This can be a blessing or a curse; aphids may spread from Sowthistle to other plants, but alternatively the Sowthistle can encourage the growth of beneficial predators such as hoverflies. In this regard, Sowthistles make excellent sacrificial plants.