Yellow Sweet Clover
Bean family (Fabaceae)
Description: This introduced annual or biennial plant is 2-7' tall. Larger plants branch frequently and are somewhat bushy in appearance, while shorter plants are less branched and rather lanky. The stems are usually more or less erect, although sometimes they sprawl across the ground. They are glabrous, furrowed, and angular; sometimes the lower stems are ribbed light red. The alternate compound leaves are trifoliate and hairless. Each leaflet is about ¾" long and ¼" across; it is oblong, oblanceolate, or obovate in shape, and dentate along the middle or upper margin. The terminal leaflet has a short petiolule (stalk at its base), while the lateral leaflets are nearly sessile. The petiole of each compound leaf is about ½" long; there are a pair of small linear stipules at its base. Spike-like racemes of yellow flowers are abundantly produced from the axils of the middle to upper leaves, while the upper stems eventually terminate in such racemes. Each raceme is up to 6" long and has dozens of flowers. These flowers are loosely arranged along the raceme and somewhat drooping. They may occur along one or two sides of the raceme, or in whorls.
Each flower is about 1/3" long and has a tendency to droop downward from the raceme, although curving upward toward its tip. The corolla has 5 yellow petals and is rather slender, consisting of a standard, keel, and two side petals. The tubular calyx is light green and has 5 pointed teeth. The blooming period can occur from late spring to early fall, peaking during early to mid-summer; a colony of plants will bloom for about 2 months. There is a mild floral fragrance. Each flower is replaced by a small seedpod with a beak that is flattened and contains 1-2 seeds. There are usually transverse ridges on each side that are somewhat curved. The tannish yellow seeds are somewhat flattened and ovoid-reniform in shape. Yellow Sweet Clover spreads by reseeding itself, and it often forms colonies at favorable sites. The sweet hay-like aroma of the foliage is caused by coumarin.
Cultivation: The preference is full or partial sun, moist to slightly dry conditions, and a somewhat heavy clay-loam soil. The mature size of this plant is highly variable, depending on moisture availability and the fertility of the soil. Its adds nitrogen to the soil by forming a symbiotic association with rhizobium bacteria.
Range & Habitat: Yellow Sweet Clover is a common plant that occurs in all counties of Illinois (see Distribution Map). It was introduced into the United States from Eurasia as a forage crop and a green manure to replenish old fields. Habitats include fallow fields, vacant lots, areas along railroads and roadsides, black soil and clay prairies, weedy meadows, and waste areas. Disturbed areas are preferred, although this species is sufficiently aggressive to invade prairie remnants, where it often becomes a nuisance.
Faunal Associations: The nectar of the flowers attracts many kinds of insects, including long-tongued bees, short-tongued bees, wasps, and flies. Less common visitors include butterflies, skippers, beetles, and plant bugs. Yellow Sweet Clover is a preferred nectar plant for honeybees. Sometimes bees collect pollen as well. The caterpillars of various Blue and Sulfur butterflies eat the foliage, flowers, or buds, including Hemiargus isola (Reakirt's Blue). The caterpillars of the moth Walshia miscecolorella (Sweet Clover Borer Moth) bore into the stems and roots. The seeds of Melilotus spp. (Sweet Clovers) are eaten by upland gamebirds to a limited extent, while the green foliage is readily eaten by various mammalian herbivores, including rabbits (young plants), groundhogs, deer, and livestock. However, improperly cured forage from Sweet Clovers can be mildly toxic to livestock.
Photographic Location: A vacant lot in Urbana, Illinois, where a gas station once existed, and along a railroad of the same city.
Comments: This is a tall lanky plant that sways back and forth in the wind and is difficult to photograph. Yellow Sweet Clover is quite similar to Melilotus alba (White Sweet Clover), which is another weedy species that has been introduced from Eurasia for agricultural purposes. This latter species has white flowers, greyish green foliage, and its slightly reticulated seedpods lack conspicuous transverse ridges. Otherwise, these two species are nearly identical. As a matter of fact, some authorities regard Yellow Sweet Clover and White Sweet Clover as different varieties of the same species. In my experience, Yellow Sweet Clover begins to bloom about 2-3 weeks earlier than White Sweet Clover, and it is more often found at sites that are moist and fertile.