yellow sweet clover (Melilotus officinalis)
Melilotus alba Medikus, orth. var.
Melilotus albus Medik.
Melilotus albus Medik. var. annuus Coe
Melilotus arvensis Wallr.
Melilotus leucanthus W.D.J. Koch ex DC.
Melilotus lutea Gueldenst.
Melilotus officinalis (L.) Lam. var. micranthus O.E. Schulz
CONFIRMATION STATUS: Pending confirmation.
TAXONOMY: The currently accepted scientific name for yellow sweet
clover is Melilotus officinalis (L.) Lam. While some systematists treat
white sweetclover (Melilotus alba) Medik. and yellow sweetclover
(Melilotus officinalis) (L.) Lam as distinct species, others suggest they
are not distinct and recognize only one species, Melilotus officinalis.
Other systematists suggest recognizing both species, since they have
been identified as such for over 200 years; however, some research in-
dicates that the 2 species are genetically incompatible.
For the Nature Guide, primary discussion will be pertinent to Melilotus
officinalis; however, some reference will also be made to Melilotus alba.
NATIVE STATUS: Introduced, United States and Canada.
GENERAL BOTANICAL CHARACTERISTICS:
Habit: This introduced short-lived annual or biennial plant is 2-7' tall and
can sometimes be woody at the base. Larger plants branch frequently and
are somewhat bushy in appearance, while shorter plants are less branched.
The stems are generally erect, although sometimes they sprawl across the
ground. They are glabrous, furrowed, and angular; sometimes the lower
stems are ribbed light red.
Leaves: The alternate compound leaves are trifoliate and hairless. Each
leaflet is about ¾" long and ¼" across; it is oblong, oblanceolate, or obov-
ate 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.
Flowers: Spike-like racemes of yellow flowers are abundantly produced
from the axils of the middle to upper leaves, while the upper stems even-
tually 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. There is a mild floral fragrance; the sweet hay-like
aroma of the foliage is caused by coumarin (coumarin is a fragrant chem-
Fruit/Seeds: Each flower is replaced by a small seedpod with a beak that
is flattened and contains one (rarely 2) seeds. The seedpod fruit is small,
circular, wrinkled and light brown and usually has transverse ridges on
each side that are slightly curved. The tannish yellow seeds are somewhat flattened and ovoid-reniform in shape.
Roots: Sweetclover produces a taproot with secondary fibrous roots and
bacterial nodules. Taproots are semiwoody, and lateral roots can be exten-
sive. Lateral roots may extend 6 to 8 inches (15-20 cm) from the taproot.
Yellow sweetclover roots are generally shorter but spread farther than
white sweetclover roots. Roots can reach 1.5 m (5 feet) deep into soil.
REGENERATION PROCESS: Yellow sweet clover propogates itself
by reseeding; on favorable sites it can form large, even invasive, colonies.
HABITAT TYPES: Yellow sweet clover (as well as white sweet clover)
occupy a large variety of habitats. Throughout North America, sweetclover
is common in sand dune, prairie, bunchgrass, and meadow habitats. Sweet-
clover is also common in desert shrub, sagebrush (Artemisia spp.), pinyon-
juniper (Pinus-Juniperus spp.), and ponderosa pine (Pinus ponderosa)
communities. Sweetclover is often dominant beneath cottonwoods (Populus
spp.) or willows (Salix spp.). In arid regions, it may be most common in
riparian areas, including calcareous riverside seepage communities and fens
low- to mid-elevation streambanks, swales, meadows, and disturbed areas.
Throughout its nonnative range, sweetclover is described on open, disturb-
ed sites that include roadsides, railways, fields, and waterways.
SITE CHARACTERISTICS: Yellow sweet clover prefers full sun or
partial shade. Sweetclover is less "vigorous" and produces fewer seeds in
shade than in full sun; however, shade tolerance may be greater in hot,
dry climates. Sweetclover grows on a variety of alkaline or slightly acidic
soils. Very low nutrient levels and fine- and coarse-textured soils are
tolerated. Yellow sweetclover tolerates nutrient-poor and dry soils better
than white sweet-clover. Sweetclover is common in riparian areas and
typically tolerates short-duration flooding early in the growing season.
SEASONAL DEVELOPMENT: The blooming period can occur from
late spring to early fall, peaking during early to mid-summer (April to
September); a colony of plants will bloom for about 2 months.
GENERAL DISTRIBUTION: Yellow sweet clover is found in all states
and provinces north of the Rio Grande.
Although widely and similarly distributed in the United States, yellow and
white sweetclover are considered most common in the upper Midwest and
Great Plains regions. In the West, yellow sweetclover is rare west of the
Cascades, and in the East, white sweetclover occurs slightly farther north
and south of yellow sweetclover. In Hawaii, only white sweetclover is re-
ported. In Alaska and eastern Canada, white sweetclover occurs farther
north than yellow sweetclover. White sweetclover is also more common
than yellow sweetclover in the Canadian Shield region.
SKY MEADOWS DISTRIBUTION: To be determined.
IMPORTANCE AND USES: 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
and Bees are important to sweetclover pollination. Early beekeepers were
responsible for some of the early spread of sweetclover in North America.
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.
Insect diversity on sweetclover can be high. On eastern Minnesota prairies,
15 insect species were collected from yellow sweetclover and 19 from white
sweetclover plants. Two insect species were unique to yellow sweetclover.
Deer, antelope, elk, and livestock feed on sweetclover. When diets of co-
occurring elk, deer, and livestock were compared, sweetclover was often a
larger component of native ungulate diets than cattle diets. Sweetclover has
also been reported in black-tailed jackrabbit, eastern cottontail, and prairie
dog diets, eastern cottontails.
Sweetclover seeds and/or insect visitors are important forage for water-
fowl, game birds, and song birds. As with mammals, bird use of sweet-
clover may vary by season. Sweetclover seeds are eaten by gray partridge,
ring-necked pheasants, and California quail, sage grouse broods, white-
crowned sparrows, house finches, and mourning doves. Sweetclover seeds
are also utilized by Gambel's quail, California quail, sharp-tailed grouse
and gray partridge.
Although first planted for bees and soil improvement, soon sweetclover
was recommended for a variety of uses. Sweetclover was planted exten-
sively for livestock and wildlife forage and to stabilize roadside cuts.
During the droughts of the 1930s, sweetclover cultivation was again ac-
tively promoted, and cultivation reached peak acreages. In the 1960s and
70s, yellow sweet- clover was seeded on US Fish and Wildlife land to pro-
vide nesting cover for waterfowl on abandoned fields and other degraded
Sweetclover has several medicinal and household uses. Sweetclover pro-
duces a coumarin compound that can be converted to dicoumarin, which
is used medicinally as an anticoagulant. Yellow sweetclover has also been
used medicinally to treat external and internal inflammation and stomach
and intestinal ulcers. Sweetclover inflorescences have been used in eye
As its common name suggests, sweetclover has a pleasant smell. North-
eastern "woodland" natives used dried white sweetclover leaves and flow-
ers in teas. Sweetclover leaves were also used to scent linens and sleeping
quarters by early settlers and members of Omaha and Dakota tribes.
By the early 1900s, sweetclover was recognized and promoted for soil re-
clamation. Sweetclover was cultivated extensively after it was found sta-
bilizing abandoned tobacco fields on severely eroded slopes and had im-
proved soils enough to support tobacco agriculture again. Even as sweet-
clover was being hailed as a soil-building crop, some farmers hesitated to
plant sweetclover fearing it might interfere with future crop production.
In a 1917 USDA publication, successful use of sweetclover in crop rota-
tion was highlighted to provide farmers with "sufficient proof" that their
fears were unfounded. Several sweetclover growth characteristics make it attractive as a nurse crop in revegetation. Sweetclover produces strong
and deep penetrating taproots that can loosen and aerate compacted soils.
Roots are also the site for nitrogen fixation, and as roots decay, nitrogen availability is increased. These processes result in improved soil conditions
for succeeding plants. Yellow sweetclover has been used successfully as
a nurse crop revegetation of sagebrush ecosystems. Yellow sweetclover establishes and develops faster than seeded grasses and minimizes invasion
by other, less desirable invasive species.
Crooked Run Valley