SPECIES CHARACTER
Description
White and yellow sweet clover are biennial herbs that are somewhat easy to identify,
especially when in flower. Flowers are crowded densely on the top 4 inches (10 cm) of an
elongated stem, with younger flowers emerging nearest the tip, or apex. Each tiny flower
is attached to the stem by a minute stalk. The small pea-like flowers are white or yellow
and each produc-es one or two seeds during the second growing season.
The leaves of sweet clover are alternate and trifoliate. Leaflets are finely-toothed and
oblong. Mature plants (second-year) may appear bushy. These aromatic plants are members
of the pea (legume) family, but they are not true clovers.
Similar Species
White and yellow sweet clover are distinguished from other members of the pea family by the
following combination of characteristics. The leaves of yellow and white sweet clover are
divided into three leaflets that are finely toothed, the middle leaflet occurs on a
distinct stalk, and the flowers occur in a long narrow inflorescence. No other member of
the pea family has this combination of characteristics. Sweet clover should be accurately
identified before attempting any control measures. If identification of the species is in
doubt, the plant's identity should be confirmed by a knowledgeable individual and/or by
consulting appropriate books.
Distribution
Sweet clover is native to Europe and Asia. It can be found in all 50 United States. The
earliest records of its occurrence in North America date to 1664. More recently, around
the turn of the century, sweet clover was cultivated as a forage crop and soil builder.
Today it also is used as a wildlife cover crop and in production of honey. Each species of
sweet clover has been recorded from every county in Illinois, and adventive populations
occur in disturbed habitats throughout most of the state. Since this exotic is considered
economically important, and thus will continue to be planted, it will remain a problem for
land managers well into the future.
Habitat
Sweet clover has adapted to a variety of conditions, withstanding both hot and cold
climates. It grows well in direct sunlight and in partial shade. Sweet clover, however,
cannot tolerate dense shade. Even the thick growth of a second-year sweet clover produces
enough shade to cause its own lower leaves to fall. Sweet clover seems to prefer
calcareous or loamy soils with a pH level of 6.5 or greater. Areas most likely to contain
sweet clover include roadsides, abandoned fields, railroad ballasts, pastures and any
unflooded, open natural community such as a prairie.
Life History
Sweet clover is an obligate biennial, which means that the plant always puts its energy
into developing a healthy root system during the first season, and during the second
season, it flowers, sets seed and dies. In the first growing season, sweet clover is
entirely vegetative. A small, branched stem with clover-like leaves is visible. In late
summer, the root continues to develop. By the second year sweet clover has a strong
taproot and root crown from which new shoots emerge. The plants flower May-September,
producing hardy little seeds that may remain viable in the soil for years (up to 30 years
or more). After setting seed the plant dies. Because sweet clover dies after the second
year, seed production is critical for its continued existence, and is the key to
controlling it. If the flowering stage of sweet clover is halted, so is the spreading of
the plant - as long as management procedures continue long enough to deplete viable seeds
remaining in the soil.
Effects Upon Natural Areas
These clovers readily invade open habitats and already have successfully exploited many
Midwestern native prairies and open, unflooded communities.
CONTROL RECOMMENDATIONS
RECOMMENDED PRACTICES IN NATURAL COMMUNITIES OF HIGH QUALITY
Hand-pulling is effective if done when the ground is moist and most of the root can be
removed. The best times to hand-pull sweet clover are in the late fall, after the first-
year plant root-crown buds have developed, or anytime early in spring, before second year
plants develop flower buds. Fall weeding is recommended because: 1) the bright green sweet
clover is easily spotted within the yellowing prairie, 2) moist fall conditions and an
immature first year root may make pulling easier, and 3) fall weeding is less stressful to
native vegetation. However, sweet clover is easily located in the spring also, because it
becomes green before native prairie vegetation. Hand-pulling in summer can be effective if
done when the ground is moist. Hand-pulling is labor-intensive and must be done
consistently. This treatment is feasible for light and moderate infestations, but may be
too time consuming in heavy infestations.
In large, dense colonies of sweet clover, cutting first and second year stems close to the
ground with a hand-held scythe is effective if done after leaves on the lower stems have
died (before flowering occurs) and up to early stages of flowering (before seeds form).
Sweet clover usually does not resprout when the stems are cut close to the ground during
this time.
Prescribed burning can control sweet clover. A combination of an April burn in the first
year, followed by a May burn the following year is most successful in eradicating an
even-aged stand of sweet clover. A hot, complete, first-year April burn scarifies sweet
clover seeds, stimulating them to grow (a late fall burn will also have this effect). A
hot, complete, second-year May burn kills the emerging shoots before they can go to seed.
Heavily infested stands are best controlled with the above sequence twice, separated by 2
years without burning. Problems with this method may arise if the burn is patchy, leaving
viable seeds or second-year shoots unscathed.
In an uneven-aged stand of sweet clover, second-year clover may escape the harmful effects
of the early first-year burn because their shoots were not fully emerged. These plants
would live to set seed. In this case, a combination of other procedures can be used:
1) spring burns could be later (after shoots emerge, but before second-year plants set seed)
in a sequence of 3-5 years, or 2) follow up the early burn with hand-pulling, if
practical.
In an even-aged stand of sweet clover, fall mowing can speed up the 2 year burn program:
burn in April; mow first-year plants in August, leaving the stems behind to dry; and burn
again in mid-late September.
RECOMMENDED PRACTICES ON BUFFER AND SEVERELY DISTURBED SITES
Same as given above for high quality areas, with the following addition:
Herbicide can be useful in controlling large sweet clover populations in degraded areas.
Following a fall burn, hand spray individual seedlings with an amine formulation of 2,4-D
according to label instructions in spring, before native prairie vegetation emerges. This
treatment also is effective when plants are in the cotyledon stage (i.e. when the first
leaves appear in the development of the seedling). To reduce vapor drift, use an amine
formulation of 2,4-D rather than an ester formulation. A 1% solution of Mecamine (2,4-D
plus Dicamba) applied to the foliage as a spray is very effective. The herbicide 2,4-D
amine is selec-tive for broadleaf plants.
When applying either herbicide described above, spot application should be done such that
coverage is uniform with the entire leaf being wet. Precautions should be taken to avoid
contacting non-target plants with the solution. Do not spray so heavily that herbicide
drips off the target species. By law, herbicides may only be applied according to label
instructions and by licensed herbicide applicators or operators when working on public
properties.
FAILED OR INEFFECTIVE PRACTICES
No effective biological controls that are currently feasible in natural areas are known.
REFERENCES
Eckardt, N. 1987. The Nature Conservancy's element stewardship abstract for Melilotus alba and Melilotus officinalis.
Hanson, E. 1987. Melilotus alba control on a prairie remnant (Illinois). Restoration and Management Notes. 5(1):26.
Kline, V.M. 1984. Response of sweet clover (Melilotus alba Desr.) and associated prairie vegetation to seven experimental burning and mowing treatments. Proc. 9th N.A. Prairie Conf. pp. 149-152.
Turkington, R. A., P. B. Cavers, and E. Rempel. 1978. The biology of Canadian weeds: 29. Melilotus alba Desr. and M. officinalis (L.) Lam. Canadian Journal of Plant Science 58:523-537.
PERSONAL COMMUNICATION
Glass, Bill. 1989. Division of Natural Heritage, Illinois Department of Conservation, Springfield, Illinois.
Laurie, Dennis. 1989. Lake County Forest Preserve District, Libertyville, Illinois.
Schwegman, John. 1989. Division of Natural Heritage, Illinois Department of Conservation, Springfield, Illinois.
Packard, Steve. 1989. The Nature Conservancy, Chicago, Illinois.
Written for the Illinois Nature Preserves Commission by:
Maggie Cole
Illinois Department of Conservation
100 First National Bank Plaza
Suite 205
Chicago Heights, Illinois 60411