Identifying perennial legumes
Identifying perennial legumes
Leaf traits can be used to identify individual legume species. Legume leaves are compound (more than one leaflet per leaf) and often have large stipules. The leaves are borne on petioles which are attached to stems (figure B).
Though leaves of clovers and alfalfa typically have three leaflets per leaf, they sometimes have four or five. The frequency of four or more leaflets per leaf is influenced by both the genetic makeup of the plant and the growing environment. Because they occur relatively infrequently, “four leaf” clovers are said in folk lore to impart good luck.
Four arrangements, or organizations, of leaflets are found on the leaves of legume species that are commonly grown in the north central region of the United States. These are palmately trifoliolate, pinnately trifoliolate, odd pinnate and even pinnate with tendrils.
- Palmately trifoliolate – red, white, alsike, and kura clover;
- Pinnately trifoliolate – alfalfa and sweetclover;
- Odd pinnate – birdsfoot trefoil, crownvetch and cicer milkvetch;
- Even pinnate with tendrils – hairy vetch.
Figure B. Legume leaves are compound (more than one leaflet/leaf) and often have large stipules. The leaves are borne on petioles which are attached to stems. Typical leaf arrangements are shown (sizes are not to scale).
Legume flowers are usually showy and colorful. These features enhance the plants' ability to attract its insect pollinators. Legume flower parts are the standard (also called the banner), wings and keel (figure C). The keel surrounds the male and female sexual parts.
Legume flowers are borne in groups called inflorescences (figure D). The most common legume inflorescences are the head (in red, white, alsike and kura clover), raceme (alfalfa, sweetclover and cicer milkvetch), and umbel (birdsfoot trefoil and crownvetch). A head will typically contain many flowers while racemes and umbels contain few.
After pollination, legume seeds develop in pods. The pods can contain several seeds, as in birdsfoot trefoil, or only one seed, as in sweetclover. Northern Minnesota provides proper conditions for plant growth, pollination and seed harvesting. Thus, it is the site of an agricultural industry focused on commercial production of birdsfoot trefoil and clover seed.
Figure C. Legume flower parts: the standard, wings, stamen, and keel.
Figure D. Typical compound inflorescences of legumes (sizes are not to scale).
Forage legumes are usually tap-rooted plants that have fine secondary roots produced from the tap root. It is these secondary roots that are usually nodulated by nitrogen fixing bacteria. This is illustrated for birdsfoot trefoil and red clover in figure A.
A very large tap root gives legumes such as alfalfa, kura clover and sweetclover greater drought tolerance than other forage legumes. In contrast, the more fibrous and shallow root systems of other legumes, such as white and alsike clover, reduce their drought resistance.
Stolons and rhizomes
Stolons are horizontal above-ground stems (figure E). Rhizomes are horizontal below-ground stems. Stolons and rhizomes allow for vegetative reproduction without seeds. New stems and roots can arise from nodes on stolons and rhizomes. This enhances plant persistence while creating more root sites for nodule growth. Stolons are found in white clover; rhizomes are found in kura clover, cicer milkvetch, and crownvetch. Legumes with rhizomes are among the most persistent species.