What are those little yellow weeds?

Have you noticed the carpets of a little mustard-like yellow flower this year? The patches seem to be everywhere from road ditches to hay fields to pastures.

You are likely seeing either desert or yellow alyssum, members of the Mustard family. Both species grow at low to mid-elevations in disturbed soils of grasslands, steppe, streambanks, or roadsides, as well as rocky areas. Yellow alyssum can invade undisturbed areas and grow with established perennial bunchgrasses. Desert alyssum is potentially a weaker invader that requires the site to be disturbed before establishment. As annual plants, both alyssum species rely on seed production for reproduction. Since they are annual forbs, their populations can fluctuate widely from year-to-year.

Seeds sometimes germinate in spring, but yellow alyssum and desert alyssum often grow as "winter annuals," with seeds germinating in autumn, plants overwintering as rosettes and resuming growth in spring, then producing seeds before the plants die in summer.

Cold winter temperatures cause yellow alyssum and desert alyssum plants to mature and disperse their seeds early during the following summer. These seeds may germinate after a rain shower in July or August, but the seedlings usually die when surface soil moisture is depleted with the return of dry, summer weather.

Neither yellow alyssum nor desert alyssum is listed currently as a noxious weed by the state of Montana.

Yellow alyssum and desert alyssum are native to northern Africa, Eastern Europe, and western Asia. Brought to North America for medicinal reasons, these plant species are purported to cure hiccups, mental illness, and rabies.

Why the current concern about these weeds, several decades after their arrival in Montana? Why have these weeds become more abundant and widespread in recent years? Reasons for the expansion of yellow alyssum and desert alyssum are not completely understood, but serious droughts that have weakened desirable plants; mild winters that favored seedling establishment of yellow alyssum and desert alyssum; and reduced tillage in cropland farming systems have most likely contributed.

On the positive side, the seeds and foliage of yellow alyssum and desert alyssum are eaten by western harvester ants, rabbits, pronghorn antelope, and sage-grouse.

Since these alyssum species do well in disturbed soils, it is important to take steps towards making your desired vegetation more competitive with weeds. Additionally, alyssum control should happen before seed production to limit persistence and spread. Plants may be mown just before flowering.

Some herbicides recommended for control of alyssum are chlorsulfuron, glyphosate, imazapic, and metsulfuron. Herbicide applications in the fall may be most effective because yellow and desert alyssum often grow as "winter annuals". In the fall, alyssum will be in a vulnerable seedling stage. Refer to the product label before using any herbicide.

For more information or for assistance in identifying weeds, contact your local MSU Extension Office.