White and yellow sweet clover (Melitotus alba, M. officinalis)
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.
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 measures can also hurt native plants.