In the last issue of GFM, I wrote about using clovers as cover crops. If you are looking for a legume that can be sown several weeks later in the fall than clovers, Austrian winter peas could fit the bill. Like clovers, they will discourage weeds, add nitrogen and biomass to the soil and prevent erosion. When flowering, Austrian winter peas also attract beneficial insects (especially honeybees) and reduce aphids. The SARE book Managing Cover Crops Profitably which I mentioned last time, has more good information. sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition/Text-Version/Legume-Cover-Crops/Field-Peas

Austrian winter peas are a hardy type of Field Pea. They are also known as black peas, and are related to Canadian field peas which are sometimes simply called spring peas.

Austrian winter peas are best used when the next crops will be planted from mid-May to July. They make rapid spring growth, and bloom in late April at Twin Oaks, earlier than hairy vetch. They can be mixed with oats, rye or barley, for vertical support, more biomass and better weed suppression. Peas alone do not add much organic matter to the soil, as the vines break down quickly.

They are high nitrogen-fixers (90-150 lbs N/acre) and a good cover crop stand can provide enough N for the following food crop, when incorporated into the soil. For comparison, crimson clover produces 70-150 lbs/ac. Grow your own fertilizer! Also, the tendrils and shoot tips of the peas in the spring make a nice addition to salads or stir-fries. And you’ll have a whole field of them.

Austrian winter peas can produce 5,000-8,000 lbs dry matter per acre, about 18% more than hairy vetch (which produces more than crimson clover) in the Southeast. They make rapid growth in cool, damp weather, but grow poorly in hot (or even warm) weather. My experience is using them as a winter annual in the mid-Atlantic. In cooler climates, they can be grown as summer-sown annuals.

Overwintering cover crop
We consider 10/14 to be our last date for sowing crimson clover. We plant winter rye and Austrian winter peas (fondly known to us as AWP) during late October or early November (up to 11/8) after our later-finishing food crops (winter squash, melons, sweet potatoes, tomatoes and peppers, middle sweet corn). We benefit from the nitrogen-fixing ability of the peas when the next year’s crops are ones that are planted from May onwards. When we don’t need to disk the plots before mid-April, the peas have time to flower and maximize their nitrogen-fixing.

Examples in our crop rotation include the watermelons to be followed by next year’s middle corn sowings; the winter squash plot going to sweet potatoes and late sweet corn; the tomatoes and peppers to watermelons; and the June potatoes to winter squash. We used to leave the squash as late as possible to get every last fruit, but more recently we came to the view that a good stand of cover crops is worth more than a few more squash. We go through the winter squash field for the last time just before Halloween and pull out the large semi-mature squash and give these away for lantern carving. Then we disk that patch and sow rye and winter peas. We aim to harvest our sweet potatoes in the week we expect the first frost (mid-October), then disk that plot and sow wheat and Austrian winter peas. (Wheat is easier to incorporate in the spring, and is less allelopathic than winter rye.) We sow winter wheat from mid-September  until 11/5. The crop following sweet potatoes the next year is spring-planted white potatoes, so the peas don’t get a chance to flower, so we only include peas in the mix if we have got spare seed leftover.

We sometimes use a mix of rye, crimson clover and AWP if the date is borderline and the weather uncertain, especially if we have the seed on hand and don’t want to use it the next year when the germination rate and seedling vigor will be less good.

We use a 3-way mix of winter rye, hairy vetch and AWP after our spring broccoli and cabbage (perhaps with an interval of buckwheat first). The next year’s crop will be our paste tomatoes, and we use a no-till method. We mow the cover crop in early May and immediately transplant tomatoes into the stubble. Later in the summer, as the weeds start to grow, we unroll big round bales of hay between the rows. By this time, the soil has warmed up. AWP is said to suppress Septoria leaf spot in following tomato crops and greens. Because it decomposes rapidly AWP is not a good no-till cover crop without sturdier companions, hence the mix with hairy vetch.

Unsuitable for AWP
   AWP will be winter-killed in zone 6, at 0°F. They are hardy in zone 7, where we are. The most winter-hardy field pea varieties are Granger, Melrose and Common Winter.  Magnus and Miranda are early maturing. For the best chance of winter survival in cold areas, choose your sowing date to get plants 6-8” tall before the soil freezes. Sowing in a mix with a winter grain will improve cold weather survival by reducing soil freezing and heaving. If you are in zone 6 or colder, you could use AWP intentionally as a winter-killed cover crop by sowing them in summer with oats, which will also die in winter. In zone 5, SARE recommends sowing from mid-August to mid-September. If the AWP do winter kill, they can be a slimy mess which is challenging to deal with (less bad if mixed with a grain). Hairy vetch is more cold-tolerant than AWP.

Austrian winter peas are best for fall sowing. They may not do well if sown in spring, as they require a cold dormant spell. If you want to sow in spring, get Canadian peas instead, as they make faster growth than AWP in spring.
AWP are not tolerant of flooding, drought, salinity, heavy shade or of long cold spring weather below 18°F. Nor do they regrow after mowing or grazing once blooming starts. Likewise, this is not a good cover crop choice for high traffic areas. It does OK in an understory mix in orchards in California.

They may increase 39 species of plant-parasitic pest nematodes (according to ASI at UC, Davis), so if you are already having trouble with those, this may not be a good cover crop for you. Field peas are susceptible to Sclerotinia crown rot, which can completely destroy crops during winter in the mid-Atlantic. This risk of infection is one reason not to grow pea crops on the same land two years running. AWP can also be host to Sclerotinia minor, Fusarium root rot and Ascochyta blight. We have not had trouble with any of these, happily. If you might, grow AWP in a mix with a cereal grain, so that if the peas die, you’ll still have a cover crop. And then practice longer rotations after that, if you see diseased winter peas.

After early November, we use winter rye alone, as a cover crop, as it is too cold for anything else to make enough growth. Rye needs three to four weeks after tilling in, in spring, to break down and to disarm the allelopathic compounds that prevent small seeds from germinating. Rye is our Last Chance cover crop, and we reckon that we can sow until mid-November, although it’s only worth sowing in November if it will have time to make growth in spring.

Nitrogen availability
How much nitrogen becomes available to the next crop depends on the C:N ratio in the biomass of the cover crop. C:N ratio is as important here as in compost-making. The soil microbes that digest the cover crop have a C:N ratio of 10:1. When a cover crop is incorporated into the soil, the microbes use the carbon and some of the nitrogen in the cover crop to build more microbes, tying it up until they die, meaning it is not immediately available to the next crop. If the cover crop has a C:N ratio of 50:1 (like sorghum-sudan), the microbes will need to find extra N to make use of all the C. They will use N from the soil, tying it up until they die. Hence any crop following immediately after a high C:N ratio cover crop will need a different source of nitrogen.

Legumes have a lower C:N ratio (from 30:1 down to 12:1) and when they are incorporated, soil N is unlikely to be tied up, so is available immediately for the next crop. Mixtures of small grains with AWP (or vetches) have C:N ratios of 34:1 to 13:1, so are easily incorporated without tying up soil N. This process is explained in the North Carolina Extension Service Horticulture Information Leaflet 37 http://www.ces.ncsu.edu/hil/pdf/hil-37.pdf . This explains the advice to incorporate high-C cover crops a few weeks ahead of planting the following crop. It also allows time for any cover crop allelopathy to subside. Leaflet 37 does not mention AWP specifically.

If cover crop residues are left on the surface rather than incorporated, the rate of decomposition is slowed. Some nitrogen is lost to the air (denitrification), but the increased organic matter can boost the diversity of microorganisms at the surface. Some of the carbon from cover crops is below the top 8”, where almost all soil data are collected. Remember to value the roots below 8” too!

Planting AWP
   AWP are easily broadcast, or may be manually seeded for small areas with the EarthWay seeder #14 plate. The ideal sowing date if your last frost is 4/30 and first frost 10/14, such as our zone 7 farm, is 8/10–10/24 (11/8 is possible). Adjust the dates as needed, sowing up to 35 days before the first hard freeze. The optimum temperature for germination is 75°F, and the minimum germination temperature is 41°F.

Seeding rates: Drilled, 60–120 lbs/acre depending on the quality of the seedbed. (The highest rate is only needed if the soil structure is poor, the sowing is late in the season, or the seeds are not well planted under the surface of the soil). AWP is relatively strong at emerging through crusted soil, and will tolerate a wide range of soil types. Broadcast: 90-120 lbs/acre, 5–6 oz/100 sq ft in a mix. The high-sounding rates (compared to clovers) are because the seeds are large and heavy. The cost/area is fairly high, similar to medics and subterranean clover, a little higher than vetches, but considerably lower than cowpeas (a summer cover crop).  
       
Using mixes can give the plot the advantages of each of the components, and also ensure that regardless of the weather or rainfall, some cover will grow. Additionally, most mixes include some crops that attract beneficial insects and some legumes to add nitrogen. Mixes can generally be sown at a depth of one inch (2.5 cm), regardless of seed size. Up to 3” deep will be OK.

When legumes and grasses are mixed, sow in the date range for the grass. When two grasses are mixed, the seeding rate of each is reduced by a third (not a half), but do not reduce the seeding rate of legumes in mixtures. Use at the same rate as a pure stand, or if you want to take the chance, reduce the legume seeding rate by a maximum of 25%.
Pea seed cannot be stored for long. The germination rate could go down to only 50% after two years. Run a germination test if you have seed you are unsure about. If you haven’t grown peas or beans on that plot for some years, inoculate the seed before sowing with Rhizobium leguminosarum, the same type as used for various peas and vetches.
Some livestock farmers graze their animals on pea plants in early spring, then incorporate the residue in late spring, and find that the N from the roots is still sufficient for the following crop.
See last month’s article for a list of useful resources.

Pam Dawling is the garden manager at Twin Oaks Community in central Virginia. Some of this material is from her book, Sustainable Market Farming: Intensive Vegetable Production on a Few Acres, © Pamela Dawling and New Society Publishers, 2013. The book is available at www.sustainablemarketfarming.com, or by mail order from Sustainable Market Farming, 138 Twin Oaks Road, Louisa, Virginia 23093. Enclose a check (payable to Twin Oaks) for $40.45 including shipping. Pam’s blog is also on facebook.com/SustainableMarketFarming