Butterfly Garden - Pea Family Nitrogen-fixers

Land plants are believed to have co-evolved around 400 million years ago with a type of fungus that penetrates their roots and helps the plants to absorb phosphates and other nutrients in exchange for photosynthesis-derived sugar. However, more recently some plants have evolved a different symbiotic relationship which allows them easier access to nitrogen, an essential component of protein and other molecules necessary for plants to survive - this time with bacteria.

Although the air is 78% nitrogen, it is not in a form that plants can absorb. It must be fixed by special types of bacteria in the soil, many of which are free-living providers for plants. Others have evolved to provide an extra jolt to plants like legumes (pea/bean family plants) in exchange for carbon. In many of these you can dig up the roots and actually see the spherical "nodules" these bacteria call home. And then plant them again and hope they survive.

Three of the commonly planted prairie legumes are shown below. They are popular for use in prairie restoration because they increase nitrogen in the surrounding soil for other plants. And two of them are popular with grazing animals because of their high protein content.

From Butterfly Garden

Purple Prairie Clover (Dalea purpurea) is shown above and grows to about 1-3' tall. Native full sun prairie plants are hard to find this short and make nice border plants. The small purple flowers bloom from the bottom up along a 1-2" cylindrical spike. Be careful of rabbits - cages might help to get it established.

From Butterfly Garden

The White Wild Indigo (Baptisia leucantha) can grow very quickly in the Spring and can be 3-6 ft. tall by the time it blooms. The flower spikes can be up to 2 ft. long and although high in protein this one is actually poisonous to mammalian herbivores.

From Butterfly Garden

The Lead Plant (Amorpha canescens) is one of my favorites aesthetically. It blooms in early to mid-summer and grows from 1-3 ft. tall. It's very fine hairs on them stems and tiny compound leaflets make it look like it's dusted with white lead, hence the name. Rabbits love this one too but we haven't really had a problem in our garden.