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An Assessment of Exotic Plant Species of Rocky Mountain National Park

Melilotus alba Medic. White sweetclover, Honey clover
Melilotus offcinalis (L.) Pall Yellow sweetclover ( Fabaceae)


Current level of impact
Known locations in RMNP: Found on road shoulders throughout RMNP. May have been introduced to help stabilize roadsides. Also found at RMNP utility area and Horseshoe Park alluvial fan.
Assessment: An intermediate number of patchy distributed populations in RMNP. Plants currently do not appear to be affecting native plant communities. White sweetclover and yellow sweetclover have very similar biology, ecology, and control methods.

Distribution
Origin: Introduced from Eurasia. Introduced as forage plant in some areas, now widespread.
Geographic distribution: Widespread throughout the U.S.
Ecological distribution: Neglected fields, pastures, roadsides, and waste places. Adapted to irrigation or mean annual precipitation of 10" or higher. Lowland sites of mountain brush and aspen woodlands. Have a broad habitat range, and are drought and cold tolerant.
Soils: Gravely or sandy soils. Commonly found growing on calcareous soils, and are most abundant in rich loams and clay loams having a pH of 6.5 or higher.

Reproduction
Annual or biennial, reproduces by seeds. Plants flower and die during the second year of growth. Flowers from June to October.
Seed dispersal: Seeds are distributed by the wind (short distances), and water.
Seed production: 14,000 to 350,000 seeds/plant.
Seed longevity: Long lived seeds, may remain viable in the soil for 11-50 years.
Germination: Only about 1/2 to 3/4 of seeds will germinate at once, the remaining seeds remain dormant until the seed coat becomes permeable. Germination occurs in March (although some germination can occur in summer), and is enhanced by temperatures > 15 C.

Competition
Level of impact: Sweet clovers readily invade open areas, and have the potential to inhibit natural succession processes.
Response to shade: Grows well in direct sunlight and partial shade, but does not tolerate dense shade.

Control
The key to controlling sweetclover is to prevent seed production. Spread of sweetclover can be halted by interrupting the flowering stage.
Mechanical: Hand pulling is effective for small to moderate infestations if done when the ground is moist and if most of the root can be removed. In general, the best times to pull sweetclover are in the late fall after the first-year plants develop root crown buds' or any time in early spring before second year plants develop flower buds. In large, dense colonies. cutting first and second year stems close to the ground with a hand held scythe is effective if done after the leaves on the lower stems have died.
Chemical: Spot application should be used to avoid contacting non-target plants with solution. 2,4-D can be used to hand spray individual seedlings following fall burns or when the plants are in the cotyledon stage. An amine formulation of 2,4-D is generally recommended to reduce spray drift. A 1% solution of Mecamine (2,4-D plus Dicamba) applied to the foliage is effective.
Biological control: Biological control options for sweetclover has not been investigated because of the plants value as an agricultural crop. However, heavy grazing has been observed to reduce sweetclover densities.
Other: Prescribed burning can control sweetclover. A burn in April combined with a burn in May the following year is most successful in eradicating an even aged stand of sweetclover. In uneven aged stands, a combination of other procedures such as later spring burns, or follow up with an early burn and hand pulling may be necessary.

Notes
White and sweet clover are grown extensively for forage, pasture, green manure crops in some regions. An excellent source of honey for domestic bees.

References:

Cole, M.A.R. 1991. Vegetation management guideline: White and yellow sweet clover 
    (Melilotus alba Desr. and Melilotus offcinalis (L.) Lam. Natural Areas Journal  
    11(4):214-215.

Eckard, N. 1987. Element stewardship abstract for Melilotus alba and Melilotus 
    officinalis. Unpublished report of the Nature Conservancy, Minneapolis. 9 pp.

Hanson, E. 1987. Melilotus alba control on a prairie remnant (Illinois). Restoration 
    and Management Notes 5(1):26.

Stubbendieck, J., C.H. Butterfield, and T.R. Flessner. 1992. Melilotus alba L. 
    pp. 139-145. In An Assessment of Exotic Plants of the Midwest Region. Final 
    Report. Department of Agronomy, University of Nebraska, Lincoln.

Turkington, R.A., P.B. Carvers, 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.

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