white sweet clover (Melilotus officinalis)
white sweet clover
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 white sweet
clover is Melilotus alba Medikus. Some systematists treat white sweet-
clover (Melilotus alba) Medik. and yellow sweetclover (Melilotus
officinalis) (L.) Lam as distinct species, others suggest they are not dis-
tinct and recognize only one species, Melilotus officinalis (the Department
of Agriculture's PLANTS Database). Other systematists suggest recogniz-
ing both species, since they have been identified as such for over 200 years;
other systematists reports that the two species are genetically incompatible.
There are many sweetclover cultivars. For the Nature Guide, Melilotus alba
and Melilotus officinalis will be treated as separate species. Primary discus-
sion will be pertinent to Melilotus alba; however, some reference will also
be made to Melilotus officinalis.
Although yellow and white sweetclover are more alike than different, the
following morphological differences are common:
1) The most obvious distinction between the two species is flower color,
which is yellow for yellow sweetclover and white for white sweetclover
(when flowers are dry, however, both may appear cream colored).
2) White sweetclover is generally taller, has a more erect form, and pro-
duces coarser stems and branches than yellow sweetclover; white sweet-
clover may be up to 3 feet (1 m) taller than yellow sweetclover;
3) At peak flowering, white sweetclover racemes are much longer (8-15
times) than those of yellow sweetclover;
4) Leaflets produced by yellow sweetclover are often twice as wide as
those produced by white sweetclover;
5) Yellow sweetclover legumes are wrinkled; white sweetclover legumes
NATIVE STATUS: Introduced, United States and Canada.
GENERAL BOTANICAL CHARACTERISTICS:
Habit: White sweet clover is an introduced annual or biennial plant, grow-
ing from a long taproot, and may reach a height of 3-8' tall. It is occasional-
ly to highly branching. The light green stems are round or slightly terete
(i.e., furrowed on all sides), coarse, and glabrous to lightly pubescent.
Leaves: The alternate leaves are pinnately trifoliate; they are rather sparse-
ly distributed along the stems and have petioles up to 1" long. The greyish
green leaflets are up to 1" long and 1/3" across. They are hairless on the
upper surface and have an appressed pubescence on the lower surface.
Leaflets are dentate along the upper margins, and ovate, oblong or oblong-
ovate, with a truncate tip of each leaflet. There are a pair of small narrow
stipules at the base of each trifoliate leaf's petiole. The middle leaflet has
a conspicuous petiole of its own (i.e, a petiolule), while the lateral leaflets
are nearly sessile. Stipules are 7-10 mm long. Stipules of the lower leaves
have one to two teeth at the base.
Flowers: The upper stems terminate in numberous narrow racemes of
white flowers about 2-6" long. Each raceme has 40 to 120 flowers. The
small floppy flowers have a tendency to hang downward from the central
stalk of the raceme, and they sometimes appear on only 1 or 2 sides. Each
flower is about 1/3" long, consisting of 5 white petals and a light green
tubular calyx with 5 teeth. The flower is tubular at the base, become be-
comes broader toward the outer edges of the petals. These petals consist
of a standard, a keel, and a 2 lateral petals; the standard functions as a pro-
tective hood over the keel and is only slightly ascending. Both the foliage
and flowers are mildly fragrant.
Fruit/Seeds: Each flower is replaced an ovoid dark brown to black seed-
pod about 1/3" long that terminates in a beak and contains 1 or 2 seeds.
Seeds are oval (2-2.5 mm long and 1.5 mm wide) and yellow to greenish-
yellow. The surface of each seedpod is smooth or has slightly reticulated
Roots: The root system consists of a taproot that divides into abundant
REGENERATION PROCESS: White sweet clover propogates itself by
reseeding. Sweet clover is a highly successful invasive plant whose flower
structure encourages cross pollination by insects. When insects land on
lower flower petals, stigma and anthers bend and contact the insect body.
Bees are the most common sweetclover pollinators; honey bees, bumble
bees, and leaf-cutter bees were reported as important pollinators. Success-
ful pollination by insects can be affected by season and weather. Cloudy,
wet weather decreases bee activity. Honey production capacity of white
sweet clover is greater in early summer than late summer and greater for
areas west of the Mississippi River than areas east of the River.
Vegetative regeneration in sweetclover is rare and limited to damaged
HABITAT TYPES: White sweet clover occurs in a wide variety of
habitats over a very extensive distribution range. Habitats include lime-
stone glades, thinly wooded bluffs, prairies, weedy meadows, savannas,
dunes, old fields, vacant lots, areas along railroads and roadsides, water-
ways, open flood plains, and waste areas. This species can invade prairies
and other natural habitats, where it is regarded as a nuisance by ecologists.
However, it is even more common in areas with a history of disturbance.
Of the many invasive species to spread in North America, white sweet
clover can be said to have spread consistent with the spread of European
based agriculture and land usage.
SITE CHARACTERISTICS: White sweet clover prefers full sun to
partial shade (generally not in deep shade) and slightly moist to dry con-
ditions; moisture is important for sweetclover seedling establishment, but
once established, plants tolerate extremely dry conditions. It prefers rather
heavy clay-loam or gravelly soil, with a special affinity for calcareous soils.
Sweetclover grows on a variety of alkaline or slightly acidic soils. Very low
nutrient levels and fine- and coarse-textured soils are tolerated. Because
of its adaptability, the size of white sweet clover varies considerably with
the moisture and fertility of the soil. The seeds can persist in the ground
for several decades and remain viable. The roots can fix nitrogen into the
soil through rhizobial bacteria and they form a symbiotic association with
SEASONAL DEVELOPMENT: The blooming period occurs from mid-
spring to fall (April to October) and lasts about 1-2 months for a colony
of plants; this blooming period reaches a peak during mid-summer. It
should be noted that, generally, yellow sweetclover flowers 1 to 3 weeks
earlier than white sweetclover and the flowering rate in individual white
sweetclover racemes is usually about twice as fast as that for yellow sweet-
GENERAL DISTRIBUTION: White sweet clover is found throughout
all of North America north of the Rio Grande River with the possible ex-
ception of the Canadian province of Nunavut.
SKY MEADOWS DISTRIBUTION: To be determined.
IMPORTANCE AND USES: Sweetclover attracts a variety of insects.
Insect diversity on white and yellow sweetclover can be high. On eastern
Minnesota prairies, 15 insect species were collected from yellow sweet
clover and 19 from white sweetclover plants. White sweet clover may
attract a wider variety of insects than yellow sweetclover. The nectar of
the flowers attracts many kinds of insects, including long-tongued bees,
short-tongued bees, wasps, flies, butterflies, skippers, beetles, and plant
bugs (early beekeepers were responsible for some of the early spread of
sweetclover in North America). Short-tongued bees also collect pollen.
White sweet clover is considered an excellent nectar plant for Apis
mellifera (honey bee) by beekeepers. The caterpillars of Blue and Sulfur
butterflies feed on foliage, buds, or flowers, including Hemiargus isola
(Reakirt's blue), Glaucopsyche lygdamus couperi (silvery blue), Colias
philodice (clouded sulfur), and Colias eurytheme (orange sulfur). The
caterpillars of the moth Scopula inductata (soft-lined wave) feed on the
foliage, while the caterpillars of Washia miscecolorella (sweet clover root
borer moth) bore into the stems and roots. Sweetclover is a nectar source
for the rare and endangered Karner blue butterfly.
Sweetclover seeds and/or insect visitors are important forage for water-
fowl, game birds, and song birds. Bird use of sweetclover may vary by
season. Visitors to white sweet clover inlcude gray partridge, ring-necked
pheasants, California quail, sage grouse , white-crowned sparrows, house
finches, mourning doves, Gambel's quail, sharp-tailed grouse, and greater
prairie chicken. In the Great Plains, yellow sweetclover is used by game
birds for nesting, brood rearing, and winter cover. In parts of Canada,
ring-necked pheasants nest in sweetclover stands.
White-tailed deer, mule deer, pronghon 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. White sweet clover is also part of the diets of black-
tailed jackrabbit, eastern cottontail, and prairie dog.
Livestock feed on sweetclover, but bloating is possible from consuming
only sweetclover, and poisoning can occur if sweetclover hay is not pro-
perly cured (a problem shared with other memers of the Melilotus genus).
Sweetclover contains coumarin, which can break down into compounds
that prevent blood clotting. These chemical changes occur when hay molds
or is improperly cured. Internal hemorrhaging or excessive bleeding from
small wounds can lead to death. Death from internal hemorrhaging is more
common in cattle than in sheep or horses.
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
flowers in teas. Sweetclover leaves were also used to scent linens and
sleeping quarters by early settlers and members of Omaha and Dakota
tribes. Young leaves of Melilotus alba have been, and occassionally still
are, used for tea, cooked greens, salads, and flavouring.
The majority, but not all, field studies show that sweet clover negatively
impacts native grass and forb recruitment and growth. Species richness
and diversity increased over time without yellow sweetclover but decreas-
ed with it. Similar results are anticipated for white sweet clover.
Because sweetclover is a nitrogen fixer, soil nitrogen levels may be great-
er on sites invaded by sweetclover. Increased nitrogen levels on sites in-
vaded by sweetclover could alter species compositions, especially in nitro-
gen-limited ecosystems. Increased nitrogen from white sweet clover would
favor invasive species over stress-tolerant, long-lived perennials adapted to
low nitrogen accumulation rates.
As it may negatively impact natural settings, sweetclover may also nega-
tively impact agriculture. White sweetclover is associated with 28 viral
plant diseases including beet curly tip, cucumber mosaic, and tobacco
streak. If white sweet clover infects wheat crops and is still green at har-
vest, wheat can take on a sweet clover odor, referred to as "sweet clover
Crooked Run Valley