Cover Crop Establishment Timing

Posted: August 21, 2012

When should I plant a fall cover crop? Results from some past PSU cover crop research is summarized.

Corn silage harvest is already getting started in parts of the state opening the opportunity for cover crop planting. Open small grain fields offer another opportunity, as do fields where vegetables have been harvested. In today‚Äôs agriculture, we talk about multi-functional cover crops, which are used for soil erosion protection, nutrient absorption and recycling, weed control, soil structure improvement and organic matter addition, as well as potential forage supply. Biomass accumulation is a measure of cover crop benefits in all these areas.  Over the years we have evaluated sensitivity of establishment date of several cover crops on spring biomass accumulation. The first species we tested were annual ryegrass, cereal rye, wheat, barley, crimson clover, hairy vetch, and rape. We tested these species in southeastern and central Pennsylvania. At the time we used 'VNS' or variety not stated for each cover crop. The research was done on our research farms without manure or fertilizer additions. Cover crops were established mid-September, early October, and mid-October. Biomass was monitored early May, mid-May, and early June.

  • Annual ryegrass accumulated about 1 ton/acre dry matter by early May in the southeast in two years out of three when established by early October. This increased to 2-3 tons/A by early June. In our research we did not have much success with annual ryegrass in central PA. Our conclusion was that annual ryegrass must be established before mid-September in central PA but can be established until early October in southeastern PA.
  • Cereal rye performed well in both locations at almost all establishment dates: it accumulated 1-3 tons/acre dry matter by early May in the southeastern location when established by early October, which increased to 2-5 tons/acre by early June (depending on the weather in winter and spring).  However, even a mid-October establishment date is very acceptable for cereal rye, which accumulated 1 ton/acre dry matter in two years out of three by early May in the southeastern location. In our central PA location, mid-October was already too late for cereal rye establishment with biomass accumulations less than 500 lbs/acre by early May. Our recommendation was therefore to establish cereal rye by early October in central PA.
  • Barley performed reasonably well in southeastern PA, but not in central PA where we recorded significant winterkill, even with a mid-September establishment date. In the southeast, in two years out of three, biomass accumulation was 0.7-1.1 tons/acre by early May when established by early October. This increased to 2-4 tons/acre by early June. However, establishment by mid-October was marginal for barley. We later discovered that barley variety selection is important in Pennsylvania - varieties from the south exhibit poor winter survival in Pennsylvania, and we may inadvertently have used southern varieties in our trials.
  • Wheat performed similarly to rye, although it accumulated less biomass due to the shorter varieties used. Wheat can therefore be an alternative to rye.
  • Crimson clover was the big surprise in our trials. Based on the literature, it was not expected to survive the winter anywhere in Pennsylvania, but to our pleasure it did well when established by mid-September in our southeastern location. In that scenario, it accumulated 0.7-1.5 tons/acre of dry matter by early May increasing to 2-3 ton/acre by early June. Because seeding rate is low (10-15 lbs/A), and seed is affordable, crimson clover has become the preferred winter annual legume of our farmers. In our central location, crimson clover was not successful with biomass accumulation of less than 0.5 tons/A by early May when established by mid-September. Later work showed that, if established by early September, crimson clover can do well in central PA.
  • Hairy vetch was expected to produce more biomass than crimson clover based on its winter hardiness rating, but in our trials it produced less. Due to high seeding rate, somewhat expensive seed, and some concern for weediness in winter small grains such as wheat, hairy vetch has become less favored as a leguminous cover crop, although it still has a role because it does provide a very dense mat by June that suppresses weeds and provides a fair amount of biological N. Unfortunately, hairy vetch winter survival can be erratic which is another reason it has lost favor as a cover crop. Some better more winter hardy varieties of hairy vetch have been released (e.g. Purple Bounty) since our research was completed, but our experience is that all issues have not been resolved yet.
  • Finally, rape (a precursor of canola) was successful in two years in three in southeastern Pennsylvania where it accumulated 1.5 ton/acre dry matter by early May if established by mid-September. Later establishment led to significant winterkill and poor stands. Rape did not perform well in our trials in the central PA location.

This research was done from 2002-2005, and in our cover crop work we now emphasize the value of mixtures of several species with complementary characteristics. We also recognize that on farms with manure history early growth of cover crops can be much faster, leading to improved winter hardiness and the potential to plant later than the optimal timings in our trials. Finally, we also discovered that it is possible to plant cover crops like crimson clover, hairy vetch, and annual ryegrass too early. When these cover crops put on a lot of growth in the fall they can die when the winter hits them hard. It is therefore recommended to mow a cover crop such as annual ryegrass when it grows taller than 8 inches prior to winter dormancy. Additionally, we recommend that the legumes crimson clover and hairy vetch be established in the first weeks of September and not in August in southeastern PA.

Contact Information

Sjoerd Willem Duiker
  • Associate Professor of Soil Management and Applied Soil Physics
Phone: 814-863-7637