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Legumes: The Good, the Bad, and the Ugly

Updated: Apr 29, 2022

Legumes are in the family Fabaceae which includes beans and peas. Many are known for being able to "fix" nitrogen, that is capture it from the atmosphere and add it to the soil. Most of the time this is a good thing improving the quality of the soil. But on a prairie, it can sometimes lead to an overabundance of weeds that thrive on the enriched soil at the expense of native prairie plants that have evolved to grow in the more depleted soils on the bluffs.

There are many plants in this family. Worldwide this family has the third most numerous plant species behind Orchidaceae and Asteraceae. On the bluff prairies around La Crosse many legumes grow. Some are good, some are bad, and some are downright ugly!

The good includes leadplant, Amorpha canescens. This is one of the few native shrubs of the prairies and can be an indicator of a high quality remnant with high restoration potential. It grows well in dry conditions with full sun in limestone derived soils like we have on the bluffs. This plant was once wide spread in the prairie landscape but now exists in a tiny fraction of its previous range and is rarely found outside remnants such as on the bluffs. Amorpha is from the Greek word meaning "deformed" referring to the flower having only one petal, and canescens refers to the whitish gray haze from the tiny white hairs on the stem and leaves. Some say this plant was called lead plant in the mistaken belief that it indicated the presence of lead beneath the ground. Others say its was named for the gray haze on the stems and leaves. Other names for leadplant have included "Devil's Shoestring" because of its tough, deep roots that were difficult to plow. Indeed, the roots of this plant can grow up to 15 feet below the surface! It was also called "buffalo bellows" by some Native Americans because the flowering period of the plant coincided with the time the buffalo were in rut and bellowing. The flower is unique in having a single petal. It is purple in color and set apart by the bright orange stamens and anthers creating a gorgeous spike-like cluster especially when examined closely. It is a magnet for many native pollinators, especially bees. It is also the host plant for the rare Leadplant Flower Moth, a species of special concern in Wisconsin and Minnesota. This means this moth depends entirely on leadplant to live and reproduce. Leadplant tolerates fire well and even improves after fire. This year it was especially prolific on Zoerb Prairie because of the spring burn done in March. Other "good" legumes on the bluffs include Illinois tick trefoil, round-headed bush clover, and purple prairie clover. Here is a photo of leadplant on Zoerb Prairie:

The bad legumes include many nonnative species that can invade prairies and become dominant. One that has been a problem on the bluffs being restored in Hixon and other bluff properties is crown vetch, Coronilla varia. This legume was introduced in the 1950's for erosion control and is now found in all states except North Dakota. Unfortunately it spreads prolifically by root suckering and seed and sensitive invades areas like prairies. It is difficult to control because of this and the entire colony has to be treated to be effective. This requires vigilance to pull or chemically treat the resprouts many times in a season. Seeds can also remain viable in the soil for up to 15 years! Other "bad" legumes include bird's-foot trefoil, yellow and white sweet clover.

The ugly legume on the prairie is black locust. This is an especially tough plant to get rid of when it is on or surrounding a remnant prairie. Black locust is native in the United States, but not in Wisconsin. It was introduced in Wisconsin in the later 1800's for erosion control and because of its rot resistance, being used, for instance, for fence posts. Black locusts are easy to spot from a distance in the spring because of their conspicuous large white flowers that stand out in a forest and they also have a nice smell. They are fast growing and can reach heights of 90 feet and diameters of 4 feet. It is also a clonal species reproducing by suckering as well as by seeds. In this way it is a tenacious invader of prairies. It has a vast underground root system that connects the trees into a whole clone. Damage to the stems of any part of the clone by cutting, fire, or wind induces the root system to send up multiple new sprouts. Therefore the entire clone needs to be treated to be successful and because these clones can stretch for several acres, it can be a big job! Not just once, but many times. Friends of the Blufflands has spent many hours and days working on a clone surrounding Zoerb Prairie. It is a work in progress with stems still popping up on and around the prairie. From experience elsewhere, this will likely be a multiyear task. Here's a photo of a young black locust:

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