Managing Sweetclover in Natural Areas

Yellow (Melilotus officinalis) and white (M. alba) sweetclover are herbaceous, non-native legumes that are widely distributed in the United States. A native to Europe, sweetclover was introduced to North American by the mid 1600s. Spread of the plant was likely facilitated by beekeepers and agriculturalists.

Sweetclover is considered both a beneficial plant and a problematic weed. The plant has been cultivated for wildlife and livestock forage, for soil stabilization, and as a nitrogen-fixer and bee plant. However, both white and yellow sweetclover are classified as invasive in some Midwestern states and Alaska. Sweetclover is known to degrade grasslands in prairie and natural areas by overtopping and shading native plants, thereby reducing diversity. The success of prairie restorations is often hindered by sweetclover invasion (Figure 5.1). These clovers readily occupy open habitats and have successfully exploited many native prairies and open, mesic plant communities in the midwestern United States.

Management

Integrated management methods are used for controlling white and yellow sweetclover. An important consideration in managing these species is long-term viability of seeds in soil. Practices that stop flowering are important for seed bank depletion.
Plant community response to various herbicide treatments in a prairie restoration was evaluated in western Minnesota and Wisconsin field studies. Results one year following application indicated that sweetclover cover declined to about 0.1 percent post-herbicide application (99 percent control) with Milestone® at 5 fluid ounces per acre (fl oz/ac). On adjacent non-treated plots sweetclover cover increased to 36 percent during the same time period (Figure 5.2). 

Results from Wisconsin indicate that Milestone® at 5 to 7 fl oz/ac or Transline® at 1 pint/ac applied in the fall provides similar control of sweetclover the spring following treatment (Renz unpublished data).

Herbicide Application Timing

Sweetclover is capable of germinating throughout the growing season into fall. Herbicide applications applied in fall will control first year plants that haven’t flowered. However, soil residual properties of the herbicide are important to prevent seedling germination the following spring and summer. Milestone, Tordon® 22K or Transline applied in spring or early summer may control both bolting plants (second year growth) and seedlings that can germinate throughout the summer into fall. In prairie plantings Milestone and Transline would provide more selective control of sweetclover than Tordon 22K.  Design herbicide selection and application timing to meet management objectives for other desirable broadleaf species on the site and minimize non-target damage.

Sweetclover abundance can be reduced through integrated management strategies that are designed to encourage native vegetation and limit sweetclover growth and reproduction. Integrating late spring to early fall burns, seeding desirable species, mowing and herbicides will help reduce the competitive ability of sweetclover.

References

Gucker CL. 2009. Melilotus alba, M. officinalis. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [2014, May 5].
Panke B, M Renz. 2013. Management of invasive plants in Wisconsin: Sweetclovers. University of Wisconsin Cooperative Extension A3924-24. 4p.