Lawnweed
(Soliva sessillis)

Description:
Description and Variation: Lawnweed is a low growing winter annual. The plant will grow to about two inches tall, with a spread of six inches in diameter. Each plant has one to ten light to dark colored stems. These stems grow from the base of the plant and they can be hairy to sparsely hairy, often with purple-spots. The leaves are pinnately divided which gives them a feathery appearance. The small composite flowers occur from February to July; they consist of greenish-translucent disc flowers and lack rays. The flat, hard seeds (achenes) are small, light-weight, and tipped with serrated spines, essentially forming a bur, that makes dispersal by human activities common. Lawnweed is probably self-fertile.

There have been taxonomic questions over species differentiation by achene characteristics. Research supports that only one species exists, Soliva sessilis. The following species are considered synonyms: S. pterosperma, and S. daucifolia. The following species are referable to S. sessilis: S. neglecta and S. valdiviana. (Ray 1987).

Lawnweed is associated with maintained recreational areas, particularly public swim areas or boat launches in Washington state. This low-growing and sprawling weed can successfully compete with planted and mowed grasses. As a winter annual, lawnweed dies back in the summer, leaving large dead spots during times of the heaviest use. It also produces the sharp seeds, or burs, during the summer which penetrate skin and tires. These seeds are then dispersed by attaching themselves to anyone using these high traffic areas. Beneficial: None known.

• Mechanical: stablishing thicker lawns and an appropriate timing of a fall applied herbicide should help prevent the establishment of lawnweed. (Joe Yenish, pers. conversation).
• Biological: Not Known
• Herbicide: elective herbicide applications are recommended, avoiding killing competitive vegetation. (Duble).

For More Information:
Detailed information about Lawnweed is available at the Washington State Noxious Weed Control Board Web Site.