## Melilotus alba; Melilotus officinalis

# sweetclover or white sweetclover; yellow sweetclover 

# Fabaceae

*Melilotus* spp. are monocarpic, annual or biennial herbs of the legume
family (Fabaceae), sweet- scented, with alternate,
pinnatelythrae-foliate leaves. The flowers are perfect and much like
those of *Trifolium* and *Medicago*, but in small, slender spike-like
racemes with a deciduous corolla, free from the filaments. The legume is
ovoid, leathery and wrinkled, longer than the calyx, and scarcely
dehiscent, with one or two seeds (Fernald 1950, Rydberg 1971).

*M. alba* (also known as *M. albus*) has an erect, branched, glabrous or
glabrate stem, 1-3 m high, with serrated, narrowly obovate to oblong
leaflets that are truncate or emarginate. It is the only white-flowered
sweetclover in the Midwest and Great Plains. The corolla is 4-5 mm long;
the fruit is 3-4 mm long and somewhat reticulate (Fernald 1950, Rydberg
1971).

*M. officinalis* is similar in height and appearance to *M. alba*, the
distinguishing feature being the two yellow corolla. The leaflets are
closely serrate, obovate-oblong and obtuse. The corolla is 5-6 mm long;
the fruit, 2.5-3.5 mm long, glabrous or glabrate and prominently
reticulate (Fernald 1950, Rydberg 1971).

Yellow-flowered *M. altissima* and *M. indica* can usually be told from
*M. officinalis* by their height and distribution. *M. altissima* is
found from British Columbia to Nova Scotia, is very tall (1.6 m) and can
be distinguished from *M. officinalis* by its pubescent ovaries and
pods.

*M. indica* is abundant in the South and Pacific states and grows to
just 0.5 m. Its flowers are smaller and more densely packed in the
raceme than those of *M. officinalis* (Turkington et al. 1978).
Sweetclover seedlings closely resemble those of alfalfa (*Medicago*
spp.). *Melilotus* may be distinguished from alfalfa by the absence of
pubescence on the underside of the leaves and by its bitter taste (Coe
1917). Anna Kummer\'s *Weed Seedlings* (1951, University of Chicago
Press) may be helpful in distinguishing *Melilotus* seedlings from those
of other similar legumes.

*Melilotus* spp. are native to the Mediterranean area through central
Europe to Tibet. They were reported in North America as early as 1664
and have been extensively used by agriculturalists as forage crops, soil
builders, and as a nectar source for honey bees (Turkington et al.
1978). The sweetclovers have spread from cultivation and thrive in waste
places and roadsides throughout the U.S. and Canada.

Both *M. alba* and *M. officinalis* are adapted to a wide range of
climatic conditions. They have long taproots and are drought tolerant
and winter hardy, but cannot withstand prolonged flooding. *M. alba* is
somewhat more tolerant to standing water than *M. officinalis* and is
occasionally found on gravelly, open river banks subject to periodic
flooding (Turkington et al. 1978). Sweetclovers are found most commonly
on calcareous soils and grow best on rich loams and clay loams with pH
levels of 6.5 or higher (Turkington et al. 1978, Martin and Leonard
1949). *M. officinalis* is apparently more salt tolerant han *M. alba*
(Turkington et al. 1978), although both species will grow on highly
alkaline soils (Martin and Leonard 1949).

Both species flower in June and July, *M. officinalis* usually preceding
*M. alba* by a few weeks. Unlike some other \"biennials\", which respond
to high densities by delaying reproduction, *M. alba* is an obligate
biennial and always flowers and dies in the second year of growth
(Klemow and Raynal 1981). *M. officinalis* is probably also obligately
biennial.

The following information on *M. alba* comes from Klemow and Raynal
(1981). In the first season of growth plants produce a vegetative shoot
which typically grows to 10-30 cm by October. Most root development
occurs in late summer, after crown growth has slowed. While the roots
develop, crown buds begin to form in the axils of the cotyledons and
around the crown just below or at the soil surface. The shoot dies back
in autumn, and the taproot and crown bud overwinter. The following
spring and early summer, one or more flowering shoots emerge from the
buds and rapidly elongate, often attaining a height of 100 cm by August.

Honey bees are the most important pollinators of both species, although
the flowers are visited by a number of other bees and insects as well.
*M. alba* is attractive to a wider array of insects, including many
wasps and flies. Barcikowska (1966) reported white sweetclover to
exhibit high self-fertility as well as cross fertility, and yellow
sweetclover to have very little incidence of self fertility.

Rainwater runoff and stream flow are probably the most important means
of seed dispersal, although wind can blow seeds up to several meters
(Turkington et al. 1978). Newly mature seeds will be soft, but as they
dehydrate they become temporarily \"hard\" or impermeable, and can
remain viable in this state for many years. Stoa (1941, in Turkington et
al. 1978) stated that hard sweetclover seeds could remain viable in the
soil for over 20 years. Factors that effect the scarification and
germination of hard seeds in nature have not been reported in the
literature. Given sufficient moisture, most germination and seedling
development occurs from late March through April, although some
seedlings continue to emerge until fall (Klemow and Raynal 1981). Martin
(1945 in Turkington et al. 1978) reported that temperatures of less than
15 degrees Centigrade are optimal for germination of *M. alba*, and
germination inhibitation occurs above 15 degrees Centigrade so seeds
become less likely to germinate during the summer. A high percentage of
seeds remain hard and contribute to the development of a large seed
bank. These seeds will remain dormant until conditions are optimal for
scarification, so abundance can fluctuate wildly from year to year.

Plant development and survivorship depend on temperature, rainfall
during the first year of growth, site conditions, and date of emergence.
Klemow and Raynal (1981) found survival of *M. alba* to be linear over
the first year when conditions were cool and moist, with 3% of the
population surviving to reproduce. When first year conditions were hot
and dry, more plants died soon after germination and less than 1%
survived to reproduce. Survival and development were better in a dense
than in a spare population in hot, dry years.

Sweetclover has been cultivated as a forage crop and soil builder
throughout the United States since the early 1900\'s. It has been used
most extensively in the north central and Great Plains states of North
Dakota, Minnesota, Montana, Iowa and Wisconsin (Martin and Leonard
1949).

It is a threat to recovering prairies because it easily invades open
areas and may compete for resources with native species or indirectly
affect the prairie community by altering edaphic conditions (see below).
It is aesthetically unpleasant on prairie preserves because it is a
highly visible exotic when in flower. It is considered desirable in some
areas of North and South Dakota and other states because it provides
good wildlife cover.

Monitoring is necessary to track the success of management procedures.
Patches should be monitored several weeks after prescribed burns to
locate and hand-pull, if possible, plants that escaped the fire.

Sweetclover is most easily identified when in flower. Seedlings are
easily confused with *Medicago* or *Trifolium* (see Description).
Unburned patches of second year plants in flower may be possible to pick
out in an aerial photo taken several weeks after a burn (Johnson 1987).

There are many questions for research that would be beneficial to
management of sweetclover. These include: Does sweetclover have a direct
ecological impact on native prairie species? Does it have an indirect
effect, for example, does it attract pocket gophers to the area, or does
it alter edaphic conditions to favor cool season exotics, perhaps by
enriching soil nitrogen? In general, what are the effects of a big
sweetclover year on the habitat and other plants?

What are the optimal conditions for scarification of \"hard\" seeds that
trigger germination and high survivorship of first-year plants some
years? How effective is grazing and what animals avoid or prefer
sweetclover over other species? In what ways do *M. alba* and *M.
officinalis* respond differently to similar treatment?

Sweetclover is part of an invasive community of exotics which require
special management to restrict growth and encourage the reestablishment
and recovery of the native prairie community.

Sweetclover response to fire is variable depending on the time and
frequency of burning, so prescribed burns in a sweetclover area must be
carefully timed to achieve the desired response. In Minnesota and
Wisconsin, spring burning from April through mid-May results in
increased germination and development of first-year plants (Heitlinger
1975, Kline 1984). The common prairie management practice of conducting
spring burns every few years can result in heavy

sweetclover infestations (Kline 1984). Late fall dormant season burns
also stimulate germination the following spring.

The best burn schedule for control of sweetclover in an even-age stand
(or nearly so) appears to be a dormant season burn (late fall or early
spring) to stimulate germination in the subsequent growing season,
followed by a later spring burn the next season to eliminate the second
year plants before they set seed (Kline 1984). The optimal time for the
second year burn will vary with year and latitude, so managers should
watch for the presence of elongating shoots. If the plants are burned
before the shoots elongate, the meristematic buds will be close to the
ground and may escape fire injury. If fire the first year is hot enough
over the entire area, close to 100% germination of the seed bank can be
achieved. New seed production can then be prevented if a follow-up burn
is conducted before the plants set seed in their second year (Kline
1987). Kline (1984) reported the virtual elimination of sweetclover from
experimental plots after conducting this two year burn schedule twice
over six years in an even age stand (years three and four having no
burns). No advantage was conferred by burning six years in succession
(Kline 1984).

Problems may arise when the first year burn is incomplete or patchy
leaving areas of ungerminated seeds, if conditions preclude burning
either after emergence or before seed set of second-year plants, or if
the stand is mixed first and second-year plants. In some situations
(high moisture, low fuel) it is difficult to achieve a good, hot fire in
the spring, and a late fall dormant season burn may be more effective at
promoting germination of a high percent of the sweetclover seed bank
(Johnson 1987). If weather conditions preclude burning the second
generation plants, one approach is application of a herbicide such as
2,4-D, which kills the plants and prevents seed production. Schwegman
and McClain (1985) stated that a fall burn followed by early spring
seedling treatment with 2,4-D was an effective control measure of
sweetclover. Since the sweetclover seedlings emerge before many prairie
plants, they may be sprayed before other dicots are present (Schwegman
and McClain 1985).

Another alternative to conducting a second year spring follow-up burn is
to mow the first year plants in mid-to-late August, leave the cut tops
to dry in the field for several weeks and then burn again in mid-
to-late September (Heitlinger 1987). Heitlinger (1975) stated that first
year sweetclover enters a \"critical growth period\" about the first of
September, when root weight begins to increase rapidly as food is
translocated from the tops for storage overwinter. Cutting or

burning alone before this critical period may leave room for a
substantial number of plants to resprout tops and store enough food to
survive the winter. Cutting followed by burning would provide further
assurance that plants would not survive, and allowing the cut tops to
dry for several weeks should also provide fuel for an effective burn.

Follow-up burns to kill second-year plants may present conflicts with
other objectives or financial constraints. In some areas, such as
Minnesota, burning after mid-May is strongly discouraged due to the
adverse effect on nesting birds and the consequent strong negative
reaction from the public. The logistics of burning a large number of
prairies in Minnesota calls for beginning prescribed burns in April, and
consequently, second year sweetclover is left uncontrolled on many
preserves (Johnson 1987). Johnson (1987) suggested that sweetclover may
be more of an aesthetic than an ecological problem on prairie preserves,
and as such it is a low priority for management, and herbicide treatment
is not considered justifiable. If sweetclover control is determined to
be a priority in an area, delaying burning to early or mid-May for one
or two years in succession after a fall or April dormant season burn
should effect a significant reduction in

abundance of flowering plants and the size of the seed bank, while still
being early enough to allow for birds renesting (Johnson 1987).

In an uneven-aged stand, a dormant season burn will stimulate seed
germination, but will not affect the second-year plants that will
subsequently resprout and set seed that summer. The later spring burn to
kill second-year plants the next season may stimulate germination of the
seeds set the previous year as well. Heitlinger (1987)

recommended later spring burns, after shoots emerge but before
second-year plants set seed, three to five years in succession, if
possible, for control of uneven-aged stands. Kline (1987) and Martin
(1987) noted that in slightly uneven-aged stands, burns prescribed to
control the most abundant age group can be followed up with hand-pulling
of the less abundant age group, before seed set for second-year plants.

Heitlinger (1975) and Martin (1987) noted that it is often good
management practice not to burn an entire prairie in one season. Thisis
especially true for areas where hand-pulling of sweetclover isnecessary,
such as in small patches or after spotty second year prescribed burns.
If a large tract of prairie is burned all at once, sweetclover, as well
as other xotics, can all come up again at once and be very difficult to
control, either by hand-pulling or by a follow-up burn, before flowering
takes place (Martin 1987). Splitting a large area into smaller tracts
for staggered burns also minimizes the potential for adverse effects on
non-target plants and animals. Optimal management of other exotic plant
species may also call for prescribing burns at different times during
the season. For example, a spring dormant season burn may not be
practical if control of cool season grasses, such as Poa pratensis, is a
priority in an area. Late spring/early summer burns, effecting a decline
in many cool season exotics while stimulating growth of warm season
natives, will also reduce abundance of first- and second-year
sweetclover. A good plan is to burn adjacent plots in different years
and at different times during the season (Heitlinger 1975).

Very small (one to several acre) preserves can also present a problem
for sweetclover management. Hanson (1987) stated that frequent burning
(i.e., annually for two or more years in succession) is not a good
practice for very small prairies because grasses tend to proliferate at
the expense of desirable forbs. On a small cemetery prairie near Urbana,
Illinois, burns are conducted in the spring about once every four years,
which is ideal for sweetclover germination and

survivorship. Since the prairie is small, hand-pulling is feasible.
Hand-pulling of first year stems is recommended in late summer/early
fall when the sweetclover is still green and easily detected among the
dried grasses (Hanson 1987). Clipping may also take the place of
follow-up burns, is easier and will cause less disruption of the soil
microenvironment than hand-pulling. If plants are clipped close to the
ground below the lowest branch axils, no resprouting will occur,
although this may be difficult in practice (Heitlinger 1975). Ladd
(1987) reported that on a loess hill prairie in northwest Missouri
approximately 30% of second year plants hand clipped as low to the
ground as possible had resprouted.

Sweetclover can be especially troublesome in areas not under management
for prescribed burns. On some state lands in North Dakota where this
situation exists, management procedure has been to mow in late
spring/early summer. This generally reduces but does not prevent seed
set, as flowering shoots can resprout from axils below the mow height
(Horner 1987). A power brush cutter can be used to cut the plants close
to the ground before flowering.

Little information is available on grazing effects on sweetclover, but
observations at the Ordway Prairie in South Dakota suggest that bison
tend to avoid it while cattle consume it quite readily (Heitlinger
1987). Kruz (1987) recommended a May burn followed by grazing in
September-October, but a high stocking rate is required.

Thick, almost monospecific stands can be difficult to burn due to the
low fuel content of sweetclover, and their size may preclude
hand-pulling or mowing. If left alone, the plants will die after
flowering and density will probably decline in subsequent years as
conditions for germination change (Heitlinger 1975, Plumb 1987).

Minnesota: There are several preserves where burns are prescribed to
effect a decline in sweetclover abundance. Since burning is often
conducted when sweetclover is still dormant in April, it is left
unmanaged at many preserves and is not considered a management priority.
An early spring burn in 1986 followed by a May burn in 1987 has
significantly reduced sweetclover abundance on Schaefer Prairie.
Contact: The Nature Conservancy, Minnesota FieldOffice, 1313 5th St.
S.E., Minneapolis, MN 55414. (612) 379-2134.

South Dakota: Aurora Prairie had abundant sweetclover on one 10-acre
tract in the early 1980\'s. This area was burned in late April of 1981,
\'82, \'83, and \'84, and in mid-May in 1987. Management plans are to
burn again in May of 1988 and \'89 before letting the site rest unburned
for three years (the other 17 acre tract on this prairie will be burned
during this time). Sweetclover abundance has declined dramatically,
although some plants can still be found.

Wisconsin: Sweetclover is a management concern on many Nature
Conservancy and state lands. Control is accomplished by several methods.
Martin (1987) and Kline (1987) recommend burning in early April first
year, burning in mid-May and/or hand-pulling early in the second year of
growth. Ahrenhoerster (1987) has conducted management by cutting right
at ground level when plants are in full bloom or burning in early spring
and then mowing close to the ground when seedlings are 6-8 inches tall.

Illinois: Sweetclover is considered a serious management problem by the
Dept. of Conservation in Illinois. On state lands fall burning is
prescribed for prairie management in general, and sweetclover is then
sprayed with 2,4-D in early spring when seedlings are very small, before
other prairie dicots are up. Contact: Illinois Dept. of Conservation,
Natural Heritage Division, 524 S. 2^nd^ St., Springfield, IL 62701.
(217) 785-8688.

Elisabeth Hanson manages two small (less than one acre) cemetery
preserves near Urbana where sweetclover is a problem. Management
procedure has been to hand-pull first-year plants at the end of their
first year.

Iowa: On Nature Conservancy land, Sioux City Prairie has significant
sweetclover patches, mainly in valleys that were most intensively grazed
in the past. Late spring burns (mid- to late-May) are conducted when the
sweetclover is in a good growth phase. Hand-pulling is also conducted in
spring and summer before second-year plants go to seed. Contact: Iowa
Field Office, The Nature Conservancy, 424 10th St. Suite 311, Des
Moines, IA 50309. (515) 244-5044.

Even-aged stands of sweetclover can be effectively controlled in areas
subjected to prescribed burns by conducting a dormant season burn in
late fall or early spring one year to promote germination, followed by a
later spring burn in May the next growing season before second-year
plants set seed. Uneven-aged stands may require three-to-five years of
May or June burns in succession to adequately control seed production.
Monitoring should be conducted to track the success of the burns because
incomplete or cool fires will leave patches of ungerminated seed or
incompletely burned plants. Small patches may be hand-pulled or machine
cut below the lowest axil before they set seed. Research should
concentrate on defining germination requirements, determining the
ecological impact of sweetclover on the prairie community, and
developing effective control methods where prescribed burns and
herbicides are impractical.

**Literature cited**:

Ahrenhoester, R. 1987. Prairie Seed Source, Northlake, WI. Telephone
conversation with N. Eckardt, The Nature Conservancy, Midwest Regional
Office. July 17, 1987.

Barcikowski, B. 1966. Self-fertility and inbreeding depressions in white
and yellow sweetclover (*Melilotus* alba Desr. and *M. officinalis* (L.)
Desr. Genet. Pol. 7:1-11.

Coe, H.S. 1917. Sweetclover: growing the crop. Farmer\'s Bulletin 797,
USDA.

Fernald, M.L. 1950. Gray\'s Manual of Botany. 8th Ed. American Book Co.,
N.Y. 1632 pp.

Heitlinger, M. 1975. Burning a protected tallgrass prairie to suppress
sweetclover, *Melilotus* alba Desr. Prairie: A Multiple View, M.K. Wali,
ed. Univ. of N.D. Press. pp. 123-132.

Klemow, K.M. and D.J. Raynal. 1981. Population ecology of *Melilotus*
alba in a limestone quarry. J. Ecol. 69:33- 44.

Kline, V.M. 1984. Response of sweetclover (*Melilotus* alba Desr.) and
associated prairie vegetation to seven experimental burning and mowing
treatments. Proc. 9th N.A. Prairie Conf. pp. 149-152.

Martin, J.H. and W.H. Leonard. 1949. Sweetclover. Principles of Field
Crop Production. Macmillan Press, New York. pp. 685-699.

Rydberg, P.A. 1971. Flora of the prairies and plains of centralNorth
America. Dover Publications, New York. 503 pp., 2 vol.

Schwegman, J.E. and W.E. McClain. 1985. Vegetative effects and
management implications of a fall prescribed burn on an Illnois prairie.
Nat. Areas Journal 5(3):4-8.

Stevenson, G.A. 1969. An agronomic and taxonomic review of the genus
*Melilotus* Mill. Can. J. plant Sci. 49:1-20.

Turkington, R.A., P.B. Cavers, and E. Empel. 1978. the biology of
Canadian weeds. 29. *Melilotus* alba Desr. and M. officinalis (L.) Lam.
Can. J. Pl. Sci. 58:523-537.

**Other Citations:**

Kline, V.M. 1987. Ecologist. Univ. of Wisconsin Arboretum. Telephone
conversation with N. Eckardt, The Nature Conservancy, Midwest Regional
Office. June 26, 1987.

Johnson, R. 1987. Director of Stewardship, The Nature Conservancy,
Minnesota Field Office. Personal communication with N. Eckardt, The
Nature Conservancy, Midwest Regional Office. July 9, 1987.

Kruz, A. 1987. U.S. Fish and Wildlife Service, Northern Prairie Wildlife
Research Center, ND. Telephone conversation with N. Eckardt, The Nature
Conservancy, Midwest Regional Office. June 29, 1987.

Hanson, E. 1987. Volunteer Manager, Tomlinson Pioneer Cemetary Prairie
Nature Preserve, Urbana, IL. Telephone conversation with N. Eckardt, The
Nature Conservancy, Midwest Regional Office. July 17, 1987.

Heitlinger, M. 1987. Midwest Region Director of Stewardship. Personal
communication with N. Eckardt, The Nature Conservancy, Midwest Regional
Office. July 14, 1987.

Horner, D. 1987. U.S. Park Service, Bismarck, ND 58501. Personal
communication with N. Eckardt, The Nature Conservancy, Midwest Regional
Office. July 1987.

Martin, M. 1987. Wisc. Dept. of Natural Resources. Telephone
conversation with N. Eckardt, The Nature Conservancy, Midwest Regional
Office. June 29, 1987.

Plumb, G. 1987. Research and Management Associate, Ordway Prairie
Preserve, SD. Telephone conversation with N. Eckardt, The Nature
Conservancy, Midwest Reigonal Office. June 25, 1987.

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