When to Turn Under Spring Cover Crops?

spring maple buds

I heard the peepers last night for the first time this year. There have already been a couple of sunny, almost warm, spring-like weeks in my neighborhood. Recently the overwintered rye has switched its dull reddish-green color scheme to bright green.

I remember reading a couple of years ago that stands of overwintered rye, if killed early, will provide the most nitrogen value.  Incorporating the young plants when they are just beginning to grow, will give as much as a 30-50 lb N credit. Providing soils are dry enough to drive onto, the best time to kill winter rye is when plants are no more than 6-8 inches tall and shortly after they have greened up.

Usually a light harrowing can kill winter rye when root crowns are small and the young stalks are not yet fibrous.  Allowing rye to continue to grow will put on biomass, however, the early spring nitrogen credit will be lost.  Nitrogen scavenged the previous fall and held in its roots throughout the winter will be utilized to put on rapid spring growth. Additional nitrogen will be required to mineralize it, when incorporated into the soil later in the spring.

If significant biomass is one’s goal, as well as field grown nitrogen, it’s better to seed a legume into one’s fall planted winter rye. Let the green manure cover crops grow to full maturity late into May or early June. Then turn them under and allow them to slowly break down to feed later summer cash crops.

Meanwhile, utilize the extensive root growth of an overwintered cover crop and benefit from the value of its winter carry over of “free” nitrogen. Many overwintering cover crops give the most value if you turn them under quite early.

— Eero Ruuttila,

Sustainable Agriculture Specialist – Scaling Up Program

UConn Extension – Tolland County

Information on early killing of spring cover crops came from the March 2011 Cornell VegEdge newsletter, authored by Thomas Bjorkman


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