Yellow and white sweet clover

Yellow and white sweet clover

The issue: Yellow and white sweet clover

Yellow sweet clover (Melilotus officinalis) and white sweet clover (Melilotus alba) are biennials from Europe that were brought to the United States in the 1600s for forage and honey production. As legumes, they fix nitrogen from the air and improve soil fertility. Sweet clovers can throw the native plant ecosystem out of balance. Moldy hay with sweet clover is toxic to cattle and horses.

The seeds germinate in the summer and form a rosette. The second season they bolt and bloom. Yellow is shorter and begins blooming a couple of weeks earlier than white. They flower and fruit from June through August. The small flowers are densely crowded along narrow, terminal stems. The middle leaflet of the trifoliate leaves has a petiolule (leaflet stem).

They reproduce only by seed. Seeds may remain viable up to 30 years in field conditions. Control efforts must focus on eliminating seed production and spread.

Integrated pest management options:

· Prevention: Feed weed-free hay; 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, before seed formation—remove as much root as possible; mow bolted plants.

· Cultural: Provide growing conditions for healthy competition from desirable plants; organic mulch at least 3 inches deep.

· Biological: None.

· Chemical: 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.

Combine them:

These control options are all more effective when combined with the other efforts described.

For more information, contact Ron Patterson, University of Idaho Extension horticulture/agriculture educator in Bonneville County, at 208-529-1390 or

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