Appearance: Biennial herbaceous plants, they are very similar. Yellow sweet clover is usually shorter and blooms earlier. First year plants do not bloom. Second year plants grow 3 – 5′ high and are bush-like. Sweet clovers are very fragrant.
Leaves: Alternate, divided into three finely toothed leaflets, middle leaflet grows on a short stalk.
Flowers: Crowded densely at the top four inches along a central stem, each flower is attached by a minute stalk; bloom June through August on second year plants.
Seeds: One or two hard small seeds per flower; they stay viable in the soil for 30 years.
Roots: Strong taproot.
Reasons Sweet Clover is a Problem:
- Sweet clover invades and degrades native grasslands by over-topping and shading native sun-loving plants.
- It grows abundantly on disturbed lands, roadsides and abandoned fields.
- Prescribed burns by scarify seeds and stimulate germination.
- First year plants are hard to detect.
- Prefers direct sunlight but can tolerate partial shade.
- Still used as a forage crop and soil enhancer predominantly in the Great Plains and Upper Midwest.
The sweet clovers are native to Europe but were brought to the United States as early as 1600 for forage and the production of honey. Their success as a forage crop, and the role as important nitrogen fixers means that both plants have a significant economic value and continue to be planted in large numbers despite their negative effects on natural ecosystems. Both white and yellow sweet clovers are biennials. After germination plants allot a large amount of energy into the creation of a healthy root system. First-year plants can be found in late summer. In the second year, plants may be seen in late April or early May. By that time, individuals have a strong taproot and a root crown from which new shoots appear. Sweet clovers flower from late May through September, set seed, and die. Both plants produce small, hardy seeds that remain viable in the soil for as many as thirty years. The sweet clovers are not a pressing problem in Cowling Arboretum. Each year there are usually a few isolated patches in the prairie restorations that must be dealt with, but plants are generally only found along the roadsides. McKnight Prairie is somewhat more at risk from sweet clover infestation because the former agricultural land surrounding the remnant was planted to sweet clover by the former owner and a large seed bank is still present in the soil.
Cowling Arboretum Procedures for Removal:
Sweet clover generally does not need to be sprayed to be controlled. Because the plant is a biennial populations can largely be kept in check by reducing or eliminating seed production. The most common procedure is to cut the second year plants back to the ground before they have begun seed production, which usually takes away the plants capacity to produce seed. Plants can also be uprooted if the soil is moist.