White and yellow sweetclover (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 sweetclover 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 sweetclover has yellow flowers, is usually shorter and blooms earlier than white sweetclover. White sweetclover has white flowers. Sweetclovers 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 arecrowded 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 sweetclover has yellow flowers and white sweetclover has white flowers. The sweetclovers 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

Sweetclovers 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 sweetclovers 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 sweetclovers reproduce by seed with plants producing thousands of seeds which can remain viable for 40 years.

Origin and Spread

White and yellow sweetclover 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 sweetclover are widespread in Minnesota.

Refer to EDDMapS distribution maps for white sweetclover and yellow sweetclover for current distribution.

Don't be fooled by these look-alikes

  • Alfalfa, Medicago sativa (non-native) – alfalfa leaves look similar to the sweetclovers, but alfalfa leaflets are only serrated on their top half while sweetclover leaflets have teeth along the entire leaflet. When in flower, alfalfa has purple flowers which distinguishes it from white and yellow sweetclover 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 sweetclovers have only three leaflets. The leaflets have smooth edges. Individual birdsfoot trefoil flowers are larger than sweetclover flowers.
 
Regulatory Classification

This species is not regulated.

Threat to Minnesota
  • Sweetclover 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.
  • Sweetclover 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 sweetclover 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 & mud from boots, gear, pets & vehicle.
  • CLEAN your gear before entering & leaving the recreation site.
  • STAY on designated roads & 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 sweetclover. 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 Midwest.

Resources