In July, many members of the pea or bean family (Fabaceae) bloom or are in fruit in our prairie and savanna gardens. Some species are welcome, even prized. Others we watch for because of their invasive and weedy properties. Many species in this family, weedy or not, fix nitrogen, making nutrients available for plant growth. N-fixation takes place through the plants’ association with Rhizobium bacteria in nodules on the root system.
This plant family is diverse in timing and form, with early blooming species like wild lupine (Lupinus perennis) in May to later blooming American hog-peanut (Amphicarpaea bracteata) in August. The family includes trees (non-native black locust, Robinia pseudoacacia), shrubs (leadplant, Amorpha canescens), vines (ground nut, Apios americana) and herbaceous species (tick trefoils, Desmodium spp.). While fruit type for this family is a pod, each with single or multiple seeds, flowers vary greatly in size from large and distinct as in wild white indigo (Baptisia alba) to tiny and clustered in clovers (Trifolium spp.). Despite the differences in size, pea family flowers are bilaterally symmetrical with fused petals. Shape and size of the pods vary, as well as mode of dispersal: wind dispersed in flattened pods, animal dispersed with stick-tight pods or simply released from splitting pods.
For sites with dry to moderate moisture and sandy soils, goat’s rue (Tephrosia virginiana) is an attractive native garden choice. With many finely divided gray-green leaflets and hairy stems, it does well in light shade. The lower (keel) petals are pink and the upper petals are cream or yellow. The pods curl when dry, dispersing the seeds.
In contrast, crown vetch (Coronilla varia) is a non-native and invasive plant. It also has finely divided leaves, but its stems are smooth and its smaller pink flowers are more tightly clustered at the ends of the stalks. Each flower gives rise to a pod that breaks into segments when mature. Originally introduced as a garden plant and for erosion control, this perennial species spreads by rhizome and is especially successful in disturbed sites. Its seeds can be transported in soil brought onto a site and a population may establish from those seeds. In the native plant garden, we prevent this species from setting seed and dig the plants as necessary. Although its stems are weak, it can grow as tall as the surrounding prairie plants by resting on them.
Bird-foot trefoil (Lotus corniculatus), with bright yellow flowers, is another invasive non-native species that, unfortunately, is sometimes planted for forage or on roadsides. It forms short, dense mats and has numerous flowers. Its stems are brittle and root systems persistent, making it difficult to remove.
Prairie clovers (Dalea spp.) are native perennials blooming in July. They thrive on dry or mesic sites, in full sun. Their tiny flowers are tightly clustered along the stem. Later, the tiny one-seeded pods are clustered along the stalk. The two species are easily distinguished by different leaf shapes and bud color. Purple prairie clover has narrow dissected leaves and dark flower buds, and white prairie clover has slightly wider leaves and green flower buds.
In contrast, non-native sweet-clovers (Melilotus spp.) have tiny flowers in loose racemes. We prevent this plant from seeding in the garden by hand-weeding. It pulls relatively easily because of its simple tap root. A small weedy legume called black medick (Medicago lupulina) is often mistaken for a clover. An annual or biennial, it does well in short gardens where there is not much shading, and at the edges of taller plantings. Growing in the open, it hugs the ground. In the tall grass prairie garden or prairie, it adopts a vertical, yet spindly, growth pattern. It is easily removed by digging, as there is no rhizome spread.
—Susan Carpenter, Arboretum native plant gardener