War on Weeds: Prickly Lettuce

Prickly lettuce (large plant in center)

Prickly lettuce is an annual or biennial weed with prickly leaves. When it is cut or broken, it emits a milky sap. It is found throughout the US, though it originally came from the Mediterranean area. Other names for it are horse thistle, milk thistle, and wild lettuce. The leaves have prickles along the leaf margins and along the midvein on the lower leaf surface. It can reach up to six feet in height.

Prickly lettuce looks a lot like another weed called sowthistle, but sowthistle does not have the prickles on the vein on the bottom of the leaves. Both plants have the milky sap, which differentiates them from "true" thistles.

Prickly lettuce sprouts in fall or early winter. It grows as a rosette of leaves the first year that looks a lot like a dandelion. The flowering stem develops at maturity. It blooms from mid-summer until frost, with individual flowers that look like small dandelion flowers.



Prickles on back of leaves
 

Since the plant reproduces only by seeds, preventing seed production is key to controlling this weed. Each plant can make several thousand seeds. The seeds are blown by the wind, so a single plant can result in a large infestation the following year, for yourself and also for your neighbors.

Prickly lettuce can be dug or pulled up when the ground is moist. Getting the root up is essential. Mowing is not effective, as the base leaves lie close to the ground, and will pass under the mower.  If the stem is cut or broken off, it will simply regrow and flower.



Prickly lettuce

Too many weeds to pull? There are several herbicides whose active ingredients are natural substances. Those that contain clove oil (eugenol) do best on controlling broadleaf weeds as long as they are young. Products containing acetic acid, often in combination with citric acid, do a good job on young grasses. Some products contain both clove oil and acetic acid, so they are useful for a broad variety of weeds. Soap-based herbicides dehydrate leaves by cutting through their protective layer of cutin. All of these types of organic herbicides work best on young weeds, and will only cause a temporary setback to well-rooted perennial weeds. Whatever kind of herbicide you choose to use, be careful to spray only in dry, still weather so that spray does not drift onto nearby desireable plants. Regular use of pre-emergent herbicides like Preen or corn gluten could also be helpful in preventing this weed from taking root.