The issue: Yellow and desert alyssum
Yellow Alyssum (Alyssum alyssoides) and desert alyssum (Alyssum desertorum) are winter annual mustards that are very similar in appearance. Both are from Eurasia and they grow from 3 to 10 inches tall. They do well in gravel driveways and roadsides, and quickly establish in disturbance areas. They can displace desirable vegetation if not managed.
The seeds germinate fall through early-spring. The small, yellow to white flowers give a brief spring show then quickly go to seed. The disc-shaped fruit on yellow alyssum is hairy, while on the desert alyssum it is hairless—about the only way you can distinguish them visually. There is a seed compartment on each side of the disc, and the membrane between them is persistent after the seeds have dropped. They flower and fruit from April to July.
They reproduce only by seed. Seeds may remain viable several years in field conditions. Control efforts must focus on eliminating seed production and spread.
Integrated pest management options:
Prevention: Keep ditch banks and roadways free of seed-producing weeds; clean vehicles and equipment.
Mechanical: Hand dig or cultivate young plants in late fall or early spring; flame weed young seedlings in the fall or early spring.
Cultural: Provide growing conditions for healthy competition from desirable plants; soil solarization; organic mulch at least 3 inches deep.
Chemical: Pre-emergent herbicides applied in the fall; post-emergent broadleaf herbicides with 2,4-D or dicamba on young plants—don’t apply when temperatures will reach above 85 F for three days; glyphosate for bare ground areas. Be sure the target weed and crop, or landscape situations are listed on the product label. Always read and follow herbicide label directions.
These control options are all more effective when combined with the other efforts described.