Cover Crop Chats: Cool Season Legumes

Amanda Rollins, August 25, 2020

Looking to add cool season legumes to a cover crop this fall? Now is the time to get legumes planted in a cover crop, but don’t wait too long!

Nitrogen Fixing Legumes

       What is pictured above? Insects? Fungus? Nope, nodules! This is where the magic happens that allows legumes to fix atmospheric nitrogen and turn it into a plant usable form. That whole process is explained here! When planning what to put into a cover crop blend, legumes are often a first choice due to their ability to fix that nitrogen for the following crop. 

       Why does this matter this time of year? Late summer and early fall can be a great time to get cool season legumes established. Some cool season legumes such as lentils, crimson clover, and balansa clover will grow this fall and begin fixing nitrogen and taking up other nutrients and then they will winterkill. Winterkill legumes will break down throughout the winter releasing that fixed nitrogen back into the soil to eventually be taken up by the following crop (See the image below). Other legumes like medium red clover, winter peas, and hairy vetch can overwinter and continue growing in the spring. These legumes will need a spring termination plan, but can supply high amounts of nitrogen credits from that spring growth. 

Timing of Planting Cool Season Legumes

       Like we said before, do not wait too long this fall to plant your cool season cover crop mix that includes legumes. These cover crops need adequate time to establish to get you the best bang for your buck. Specifically the legumes that you want to overwinter need to be well established in order to build root reserves to survive the winter. In the Upper Midwest, Mid-September is a general timeframe to get diverse cover crop mixes seeded by. In any area, it should be at least 6 weeks before a killing frost.