There is a whole group of plants in the Leguminosae (a.k.a Fabaceae) plant family and are referred to as legumes, a word that many people may have heard but may not know the special details about. Have you ever heard that legumes make their own nitrogen or that they are plants that never need nitrogen fertilizer? Well, both those statements are true!
Legume roots form a symbiotic (meaning both parties benefit – opposite of the word parasitic) relationship with soil bacteria —specifically Rhizobia— that end up providing nitrogen to the plant. It is said that legumes can “fix” or “capture” their own atmospheric nitrogen gas which is unavailable to plants and turn it into ammonia nitrogen, which is usable. Thus, legumes do not require any nitrogen fertilizers which can add up to a huge cost savings, particularly when grown in large scale agriculture systems. Earlier this spring in Western Maryland one ton of fertilizer with an analysis 20-10-10 was costing more than $700. For a refresher on what fertilizers are, visit this University of Maryland Extension Home and Garden Information webpage.
The Rhizobia bacteria will form nodules (little pockets/growths) on the plant roots, which can be found if you dig up the roots of the legume. Sometimes the nodules are round but other times they look kind of like a tiny little foot. If the nodule is active and healthy, the inside of the nodule will be pink or reddish; this is due to leghemoglobin. This webpage from University of Minnesota explains this process for checking for healthy nodules more in depth.
The red arrow above points to nodules on the roots of red clover. Photo: A. Bodkins
Legumes also provide protein food sources for both animals and humans. Alfalfa is a high-protein forage crop that many farmers love to have in their pastures and hay fields. Clovers are also highly valuable and can help reduce the need for nitrogen fertilizer in fields (and home lawns too!). Soybeans, chickpeas, and peanuts are examples of high protein legumes that are found in the human diet.
Inoculation of seeds means to introduce a beneficial microorganism. When planting legume seeds, they may benefit from the inoculation if no legumes have been grown in that soil/area before or not in a long time. However, there is never any harm in adding inoculation if you are unsure or are not having good success in growing a particular legume. Each plant needs a specific one of the more than 200 Rhizobium species. Gardeners can look for “pea and bean inoculants” where seeds are sold. Check your inoculation packet to be sure that your plant is listed as a suitable species for it to colonize. To learn more about soil inoculation check out this webpage from University of Georgia.
Cooler weather can inhibit nitrogen fixation so legumes that are used as cover crops late in the growing season may not be able to convert atmospheric nitrogen as quickly due to a decrease in the enzyme nitrogenase. They still capture nitrogen, just not as quickly because they slow down during cooler weather. Fall-planted legumes also provide good benefits to the soil because they capture excess nutrients and hold the soil in place.
There are many ornamental and native landscape plants that are legumes. Some examples include bird’s-foot trefoil, crown vetch, honeylocust, black locust, Kentucky coffeetree, sweet pea (perennial flowers), lupine, Eastern redbud, and American wisteria.
A few native plant legumes you might want to explore adding to your landscape include wild senna or American senna (Senna hebecarpa), false blue indigo (Baptisia australis), and partridge pea (Chamaecrista fasciculata).
American senna and partridge pea are two legumes that are native to Maryland.
As we get ready for the changing of the seasons, don’t forget to start thinking about some legumes that you can add to your landscape next year!
By Ashley Bodkins, Senior Agent Associate and Master Gardener Coordinator, Garrett County, Maryland, edited by Christa Carignan, Coordinator, Home & Garden Information Center, University of Maryland Extension. See more posts by Ashley and Christa.