White and yellow sweet clover (Melitotus alba, M. officinalis)

Berries and leaves of Amur cork tree

 
 

White and yellow sweetclover are biennial plants in the legume (pea) family. Plants can grow up to 6 feet tall and overtop native plants in prairies and grasslands. As legumes, they can change nitrogen levels in the soil. Native species may be less adapted to the new nitrogen levels and not be able to survive and complete against other species under the new conditions.

Description

Appearance

White and yellow sweet clover are biennial herbaceous plants that look very similar. First year plants do not bloom. Second year plants grow 3 – 6 feet high and are bush-like. Yellow sweet clover has yellow flowers, is usually shorter and blooms earlier than white sweet clover. White sweet clover has white flowers. Sweet clovers are very fragrant.

Leaves and stem

Leaves are alternate (come off the stem one at a time). Leaves are divided into three leaflets and the middle leaflet grows on a short stalk. The edges of the leaflets are finely toothed.

Flowers

Flowers are crowded densely at the top four inches along a central stem. Individual flowers are small (about ¼ inch) and each flower is attached by a tiny stalk. Flowers bloom June through August on second year plants. Yellow sweet clover has yellow flowers and white sweet clover has white flowers. The sweet clovers are in the legume (pea, bean) family and have the irregular, distinctly shaped flowers common in that family.

Seeds

Seedpods contain one or two small seeds. Plants can produce more than 100,000 seeds. Seeds can stay viable in the soil for 40 years.

Roots

Sweet clovers have a deep taproot. Like other members of the legume family, the roots have root growths or nodules that house bacteria that can convert nitrogen in the atmosphere into a form of nitrogen that plants can use.

Biology

White and yellow sweet clovers are biennials. In their first year, they generally only grow leaves and stems. In their second year they flower, produce seed, and then die. Plants grow in open, sunny areas such as roadsides and grasslands. The sweet clovers reproduce by seed with plants producing thousands of seeds which can remain viable for 40 years.

Origin and spread

White and yellow sweet clover are native to Europe and Asia and were brought to the U.S. in the late 1600s. They have been planted as a forage crop and as soil enhancers in the Great Plants and Upper Midwest. White and yellow sweet clover are widespread in Minnesota.

Refer to EDDMapS distribution maps for white sweet clover and yellow sweet clover for current distribution.

Don't be fooled by these look-alikes

  • Alfalfa, Medicago sativa (non-native) – alfalfa leaves look similar to the sweet clovers, but alfalfa leaflets are only serrated on their top half while sweet clover leaflets have teeth along the entire leaflet. When in flower, alfalfa has purple flowers which distinguishes it from white and yellow sweet clover flowers.
  • Birdsfoot trefoil, Lotus corniculatus (invasive) – Birdsfoot trefoil has three clover-like leaflets on short stems with two additional leaflets at the base of the main leaflet stalk while the sweet clovers have only three leaflets. The leaflets have smooth edges. Individual birdsfoot trefoil flowers are larger than sweet clover flowers.
 
Regulatory classification

This species is not regulated.

Threat to Minnesota
  • Sweet clover invades and degrades native grasslands by overtopping and shading native sun-loving plants thereby reducing diversity. It grows abundantly on disturbed lands, roadsides and abandoned fields. The large, dead stalks can also alter habitat conditions.
  • Sweet clover hosts root bacteria that can increase soil nitrogen levels and potentially make the habitat less favorable for native species adapted to the lower nitrogen soil levels.
  • If sweet clover is cut for hay and the hay rots, it produces a chemical that can cause a bleeding disease in cattle if the spoiled hay is eaten.
What you should do

One way that invasive plant seeds and fragments can spread is in soil. Sometimes plants are planted purposefully. You can prevent the spread of invasive plants.

PlayCleanGo: Stop Invasive Species in Your Tracks

  • REMOVE plants, animals and mud from boots, gear, pets and vehicles.
  • CLEAN your gear before entering and leaving the recreation site.
  • STAY on designated roads and trails.
  • PLANT non-invasive species.
Native substitutes
 
Control methods

Mechanical control can be done by pulling the plant by hand or with equipment such shovels. A prescribed burn will stimulate germination of seeds in the soil. A hot, early season burn in the first year, followed by a hot late spring burn in the second year can reduce sweet clover. This burning process can be repeated after two years.  The key to controlling sweet clovers is to halt the flowering stage and then concentrate on depleting viable seeds in the soil. Be aware that too frequent fires can also hurt native plants.

Herbicide control can be done using systemic herbicides that are taken up by plants and move within the plant, which can kill leaves, stems, and roots. Spray emergent seedlings with 2,4-D amine or mecamine after a fall burn, or after a spring burn before native vegetation emerges.

Reporting

This species is unregulated, but you can add to the public information about this species by reporting new occurrences through EDDMapS.

Resources

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