Yellow sweetclover (Melilotus officinalis)
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Yellow sweetclover, a biennial and sometimes annual or short-lived perennial, occurs throughout California, except in the low deserts, to about 500 feet (1500 m). It inhabits agricultural land and open disturbed places. Two other common sweetclovers in California are Indian sweetclover, M. indica, and white sweetcover, M. alba. For management of yellow sweetclover, it is suggested to remove plants before seeds are formed. The seeds are resistant to both soil fumigation and solarization.
Yellow sweetclover can be considered both a beneficial plant and a problematic weed. Wildlife feed on sweetclover leaves and seeds. Their flowers attract honeybees. Sweetclovers are sometimes cultivated for use as cover crops and as feed for livestock. However, sweetclover leaves and flower buds contain a low level toxicant called coumarin. If sweetclover hay or silage is poorly cured, it can develop mold that converts the coumarin to a more toxic compound called dicoumarin (dicumerol, dicumarin). Feeding on moldy sweetclover hay can lead to an often, fatal hemorrhaging disorder called “sweetclover poisoning”.
Open fields, roadsides, agronomic crops, and other disturbed sites.
Cotyledons (seed leaves) are small, hairless, oblong, light green, and about 1/5 to 1/3 of an inch (5–8 mm) long. The first true leaf is somewhat egg shaped, tapers at the base, and is about 1/12 to 1/5 of an inch (2–5 mm) long, but is not divided. Later leaves, however, are fully divided into three leaflets. The middle leaflet has its own short stalk.
Plants are erect and can grow to about 6-1/2 feet (2 m) tall with spreading branches. Stems become somewhat woody at the base, especially in the second year. Leaves are fully divided into three leaflets, like alfalfa. Leaflets are egg shaped to oblong and mostly 2/5 to 1 inch (1–2.5 cm) long. The middle leaflet has a short stalk. Leaves are alternate to one another along the stem.
Flowering takes place from April through September. Small, yellow, sweetly fragrant, pealike, short-stalked flowers cluster along the end of each flowering stem and in leaf axils (where the leaf stalk meets the stem), creating slender flower heads. The flower heads are about 1-1/5 to almost 5 inches (3–12 cm) long.
Fruits consist of somewhat oval-shaped pods, about 1/8 to 1/5 of an inch (3–5 mm) long, with a sharply ridged or veined surface that is light brown and hairless. Pods hang from downward bent stalks.
Seeds are somewhat oval, smooth, dull yellowish green to orange-tan, and sometimes have purplish or brown spots.
Reproduces by seed.