Planning Winter Cover Crops

 

A cover crop mix of winter rye, hairy vetch, Austrian winter peas and crimson clover.
Photo Bridget Aleshire

If you haven’t already made a plan for winter cover crops, this is a good time to do it. Having plants growing through the winter, or at least into the winter until they get killed by cold temperatures, will improve your soil both physically (the roots hold the soil in place, preventing erosion, and they open up channels that improve the drainage) and biologically (the soil microorganisms thrive when they have active plants to cooperate with, exchanging nutrients). Cover crops will also reduce the number of weeds you have next year, because they crowd out weed seedlings. In some cases they even inhibit weed seeds from germinating.

I have some slideshows about cover crops, and am including one at the end of this post.

Choosing the “perfect” cover crops can be confusing, but any is better than none, so I encourage you to experiment and keep records, so you can improve your choices each year. It helps to know your first frost date, and your winter-hardiness zone (the lowest temperature your garden is likely to encounter). A two-week delay in sowing can seriously reduce the effectiveness of the cover crop, so follow these guidelines if you can.

A cover crop of overwintered oats the year they didn’t die.
Photo Pam Dawling

Short Simple Guide to Winter Cover Crops

  1. If the area has been fully harvested of food crops by 60-80 days before frost, sow a frost-killed cover crop or even a fast-growing food crop. Buckwheat, soy, cowpeas, spring peas, sunnhemp, Japanese millet, sorghum-sudangrass will frost-kill. Forage radish lab-lab bean or bell beans will die back and leave almost bare soil. Don’t sow a winter cover crop yet. If sown too early, oats head up in the fall and even drop seed.
  2. If the area is clear of vegetable crops by 40-60 days before frost, sow oats to winter-kill. If possible add a legume (soy and spring peas are easy, and will be killed by the frost, so they won’t complicate food crops next year). For us with a first frost date of October 14, the cut-off date for oats is September 7. This would be after growing early sweet corn, spring broccoli, cabbage, spring-planted potatoes or early season spinach, lettuce, beets, carrots. Oats will winterkill completely at 6°F (-17°C) or even milder than that, leaving the plot quick to prepare for early crops next year. So plan to put your early crops where you had oats in the winter. See the slideshow for more about oats.

    Crimson clover cover crop in flower.
    Photo McCune Porter
  3. If the area is ready for cover crops 20-40 days before frost, sow winter wheat. Add a legume such as crimson clover, if you won’t need to prepare the area before it flowers (in central Virginia 4/16-5/2, most usually around 4/20). You get the most nitrogen from the legumes if they reach the flowering stage before you kill them off in spring. If you have a legume that doesn’t reach flowering, it’s not the end of the world, you just get less nitrogen for your money. It is too late to usefully sow cover crops that are not frost-hardy, or even oats, which won’t make enough growth before getting killed.

    Cover crop of winter rye, hairy vetch and crimson clover.
    Photo Kathryn Simmons
  4. If the area is ready for cover crops up to 10 days past the frost date, sow winter wheat or winter rye and hairy vetch or Austrian winter peas. Winter rye is hardier than any other cover crop and can take later planting dates. But it is a bit harder than wheat to incorporate in the spring. Austrian winter peas can be sown later than other legumes.
  5. If you are up to 3-4 weeks past your average frost date, (we choose November 7 here, where our average first frost is October 14), sow winter rye alone. It’s too late for any legumes.

    Cover crop of winter rye still small in March, but holding the soil together.
    Photo Pam Dawling
  6. If you are later than 3-4 weeks past your average first frost date, leave the weeds or crop remains growing. It’s too late to sow a cover crop, and you’ll do more harm than good tilling up the soil. You can mow the weeds anytime you see lots of flowers and seed heads. The weed roots will hold the soil together and help feed the soil microorganisms until early spring. Be prepared to act soon in spring, so you don’t get weed seeds.
Quick Guide to Winter Cover Crops.
Pam Dawling

More Options for Each of These Time-frames

  1. 60-80 days before frost: You could follow a frost-tender cover crop with an over-wintering cover crop, for best effect. If you leave the dead tender cover crops in place, in early spring the winter weeds will start growing in the open space, so be ready for fast action. For the very earliest spring crops, forage radish lab-lab bean or bell beans will die back and leave almost bare soil. While still growing, they suppress weeds. BUT fast-maturing spring vegetables will not do well with no-till cover crops unless you add N fertilizer, as they need nitrogen more quickly than can be got from no-till.
  2. 40-60 days before frost: Austrian winter peas, crimson clover, or red clover are other options for legumes, but they won’t die when the oats do. They are relatively easy to incorporate in spring. Frost-killed cover crops can also be combined with oats. Or for a cover crop to survive the winter, sow winter barley or winter wheat with Austrian winter peas, crimson clover, hairy vetch, red clover, fava beans. Hairy vetch takes a few weeks longer than crimson clover to reach flowering. Which you choose will depend what you want to grow there next spring and when you need to plant it. After oats or other winter-killed cover crop, we like to plant our early spring food crops, peas, cabbage, broccoli, carrots, March-planted potatoes, spinach and the first sweet corn.

    Cover crop height and thickness in late April.
    Photo Kathryn Simmons
  3. 20-40 days before frost: Winter rye, or winter barley are also options for the cereal grain part of the mix, if you have those seeds on hand. In central Virginia, it’s a mistake to sow rye as early as August, as it can set seed. Austrian winter peas, or red clover are other legume options if that’s what you have. Sometimes it pays to use what you already have, as it may not give good germination if saved over to next fall. Winter rye needs 3-4 weeks after tilling in, in spring, to break down and to disarm the allelopathic compounds that stop small seeds germinating. Plan for the next food crops to be ones planted after late April, such as late corn plantings, winter squash, transplanted watermelon, tomatoes, peppers, sweet potatoes, June-planted potatoes, fall brassicas, and second plantings of summer squash, cucumbers, beans.
  4. Up to 10 days past the frost date: it’s too late for clovers. Austrian winter peas winter-kill in zone 6, but are hardy in zone 7. Hardy to 0°F (-18°C). AWP bloom in late April at Twin Oaks, before hairy vetch. Suitable crops in the year before using Austrian winter peas are the late-finishing winter squash, melons, sweet potatoes, tomatoes, peppers, middle sweet corn, June-planted potatoes. The same group of crops are suitable for following AWP, as they are planted after May 1. You can sow AWP several weeks later than clovers, but at least 35 days before first hard freeze (25°F/-4°C) – in zone 7, 8/10–10/24 (11/8 is sometimes OK)

    Winter rye headed up. Mow or turn it under very soon! Don’t let it shed seed.
    Photo Pam Dawling
  5. Up to 3-4 weeks past your average frost date: no really, it’s too late for any other cover crop. If you don’t have winter rye, don’t till! Leave the weeds, see below.
  6. Later than 3-4 weeks past your average first frost date: no, really, do not till! You could mow and tarp, to kill the weeds before spring. I’m not sure what the soil life thinks about that, though! You could mow again in early spring, or till and sow oats, if you won’t be planting a food crop in the following 8 weeks, giving the oats time to make respectable growth before turning them under.

Create a crop rotation for vegetables that includes good cover crops

If you include winter cover crops when planning a crop rotation for your vegetables, you can tweak your plan to maximize your cover crop opportunities. Here’s the steps:

 Figure out how much area is needed for each major crop (the ones needing the largest amount of space).

  1. Measure and map the space available
  2. Divide into equal plots big enough for your major crops
  3. Group compatible crops together to fill out each plot
  4. Set a good sequence, maximizing cover crop opportunities
  5. Include best possible cover crops at every opportunity
  6. Try it for one year, then make improvements

For more details, see my slideshow Crop Rotations for Vegetables and Cover Crops on SlideShare.net

Advanced Options for Winter Cover Crops

Sweet corn with undersown soybean cover crop. Photo Kathryn Simmons

Undersowing

Sometimes you can undersow the cover crop between the rows of a growing food crop, to take over after the food crop dies of frost or mowing. We do this with our last planting of sweet corn, and with fall broccoli and cabbage.

Timing is critical: Sow the cover crop late enough to minimize competition with the food crop, but early enough so it gets enough light to grow enough to endure foot traffic when the food crop is harvested. The leaf canopy of the food crop should not yet be closed. Often the best time is at the last cultivation, often about a month after planting the food crop. With vining food crops, it’s important to sow the cover crop before the vines run.

Choose vigorous food crops, but cover crops that are only moderately vigorous. Ensure the seedbed is clean and the soil crumbs small enough. Use a high seeding rate, whether broadcasting or drilling, and irrigate sufficiently.

A no-till cover crop mix of winter rye, hairy vetch, Austrian winter peas and clover.
Photo Bridget Aleshire

No-till Winter Cover Crops

In the spring, kill the cover crop without tilling it in, and plant food crops into the dying residue. There are three ways to kill cover crops without herbicides:

  1. Winter-killed cover crops for early spring food crops
  2. Mow-killed cover crops.
  3. Roll-killing (but it usually requires special equipment).

We have had one year in 10 as a no-till year. We use no-till cover crops before Roma paste tomatoes, which are transplanted in early May. We don’t need early-ripening for these, making them a good no-till food crop. The soil under no-till cover crops stays colder than tilled soil, slowing the plant growth down.

Late-spring transplanted crops such as late tomatoes, peppers, eggplant, Halloween pumpkins, or successions of cucumbers and squash can do very well after a winter-hardy legume-grass mix no-till cover crop.

To be an effective mulch, you need to get a thick sturdy stand of cover crops, which means sowing in plenty of time, and being generous with the seed. To make the timing work, you need a previous food crop that finishes before the sowing date 4-5 weeks before the average frost (that’s September 7-14 for us).

Timing is also critical in the spring. For maximum N, the legumes in the mix will be flowering right when you need to plant the food crop. Mow the cover crop mix close to the ground, and plant right into the stubble. Transplants or big seeded crops work well. The ground will be relatively hard – you probably can’t make a furrow for small seeds).

It’s not all over with the weed-prevention after that. In our humid climate the no-till mulch biodegrades after 6-10 weeks. In July we roll hay between the rows, to top up the mulch.

Cover Crops for Vegetable Growers 2019 60 mins

Planting kale, catching up on weeding and reading

Vates kale Photo credit Kathryn Simmons
Vates kale
Photo credit Kathryn Simmons

Our weather has been dry and sunny (no hurricanes in September this year!) and we’ve had a chance to catch up somewhat on weeding. It’s also meant lots of irrigation, more than usual at this time of year. And consequently, running repairs. This morning I switched out three sprinkler heads – one was stuck and wouldn’t rotate; one leaked too much at the stem and one old one had ground away its brass nut over years of use and finally fell through the hole in its stand, meaning it couldn’t rotate any more either. I fixed two, still not sure how to deal with the old one. Its homemade stand is also breaking up. I’ve also been replacing hose ends and connectors – we were the lucky recipients of a donated pile of about nine hoses, some in better condition than others.

Transplanting kale has kept us busy this week. We direct sow our kale, two beds every six days in August, to make it easier to keep them well watered – we only have to hand water two on any one day to get the seeds germinated. This year we’ve had disappearing seedlings, and we’ve been moving plants around in the beds to get full rows at the right final spacing. This means even more watering, but we all love kale so much, so it’s very worthwhile. Some of the disappearing seedlings were due to cutworms, some may have been grasshoppers, and some maybe rabbits.

I’ve also been pulling up drip tape from our watermelon patch and second cucumbers, rolling it on our home-made shuttles which I described last year. I found myself salvaging 23 late watermelons, I just couldn’t resist! Watermelons in October usually get as much demand as last week’s newspaper, but while the weather is so warm (85F yesterday), people are still grateful for juicy fruit. I’m looking forward to getting more of the gardens into their winter cover crops, so that this year’s weeds can become just a memory. I also like how the garden gets smaller and smaller in the process of putting the plots into cover crops. Less to deal with. (Although I am needing to water the cover crops areas overnight with the sprinklers).

I’m in the process of writing about no-till cover crops for Growing for Market magazine. We really like using no-till winter rye/hairy vetch/Austrian winter peas before our Roma paste tomatoes. We mow the cover crop in early May, when the vetch is starting to flower, then transplant into the dying cover crop, which becomes our mulch, and also supplies all the nitrogen the tomatoes need. Anyway, that’s for the winter double issue.

Hoophouse greens in November. Credit Ethan Hirsh
Hoophouse greens in November.
Credit Ethan Hirsh

Meanwhile, the October issue has just come out, including my article about how to minimize unhealthy nitrate levels in winter greens. During winter, when there is short daylight length and low light intensity, there is a potential health risk associated with nitrate accumulation in leafy greens. Nitrates can be converted in the body into toxic nitrites, which reduce the blood’s capacity to carry oxygen. Additionally, nitrites can form carcinogenic nitrosamines. Green plants absorb nitrates from the soil during the night and in the process of photosynthesis during the day, combine them with carbon-based compounds into protein (plant material). It takes about six hours of sunlight to use up a night’s worth of nitrates. In winter when the day-length is short, the nitrate accumulated can exceed the amount that can be used during the day, and the excess nitrate builds up in the plant, mostly in the leaves, stems and roots. Leafy vegetables can then exceed an acceptable adult daily intake level of nitrate in just one small serving of greens, unless special efforts have been made to reduce the levels. My article lists which vegetables are more likely to be higher in nitrates, and which circumstances are most likely to make the levels high. I give a list of 16 steps you can take to reduce the levels of nitrate in your crops. 

There are also articles about farms getting financing from crowdfunding websites (Lynn Byczynski), customizing CSA shares using LimeSurvey to let each sharer indicate what they want by email (Eric and Joanna Reuter whose blog I have mentioned before), building a seed germination chamber (Ben Hartman), and  making cash flow projections to avert disaster (Nate Roderick). A fine batch of useful articles, and I’m especially happy to see Eric and Joanna Reuter have “joined the crew” at GFM. They impress me with their attention to details and creativity.

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Twin Oaks Garden Task List for April

Asparagus in early April.Credit Wren Vi
Asparagus in early April.
Credit Wren Vile

All Month:

Lettuce Factory: In flats, (on greenhouse bench) sow lettuce #7, 8, 9 (romaines & small varieties to interplant with peanuts). Transplant 1/3 bed lettuce (120 plants)/week. Plant #4, 5, 6 this month.
Compost Needed for April: 6-9 tractor buckets for beds, 24-30 bkts to disk in.

Early April:

In greenhouse, sow lettuce #7;

Keep celery above 55°F, and celeriac above 45°F (don’t put in coldframe). 10 consecutive days <55°F for celery, <45°F for celeriac, causes bolting.

Spot lettuce, harden off in coldframe. Spot peppers, tomatoes, & eggplant. Protect new pepper seedlings from mice.  Keep tomatoes above 45°F at night, eggplant above 55°F.

Cut sweet potato slips at 6-12”, put in water.  Once a week, plant rooted slips in 4” flats.

Sow outdoors: carrots #5, beets (see March notes), parsnips with radishes #2, (in celery bed), sunflowers.

Weed and thin early crops. Side dress or foliar spray over-wintered spinach to boost production.

Take rowcover from turnips, senposai, cabbage #1, kohlrabi, little alliums, onions as needed for broccoli.

Transplant lettuce #4, main cabbage & broccoli under rowcover (12 pieces) within 6 weeks of sowing.

Till beds for mid-April. Compost beds for late April plantings.

Garlic bulbing is initiated on/after April 10 (13 hours daylight), and soil temperature above 60°F.

Mid April:

In greenhouse sow melons #1 in soil blocks or plug flats, replacement paste tomatoes, lettuce #8, and okra.

Sow beans #1 when lilac in full bloom, sunflowers. Sow edamame #1, corn#1, if warm, and soil >60F.

Till beds for late April (chard, cowpeas, peanuts). Compost beds for early May (okra, toms, melons, celeriac, lettuce 7,8,9, asparagus beans)

Hill up potatoes when 6” high. Cover half the vine. Repeat after 2 weeks. (Flameweed if too wet to hill.)

Take rowcover from kale, collards, early lettuce for raised bed tender crops.

Transplant broccoli #2, insectary flowers #1, bulb fennel, lettuce #5, cukes #1 w/nasturtiums, zukes #1; use spring hoops for cucurbits. Take rowcover from spinach to strawberries.

A fine bed of fava beans. Credit Kathryn Simmons
A fine bed of fava beans.
Credit Kathryn Simmons

Install stakes every 8-10’ for peas and fava beans, and stringweave them to final height of that variety.

Weed garlic [or flameweed it early in the morning after a good rain. Direct flame at base of garlic plants]

Harvest lettuce as heads rather than leaves, from 15 April

#3 Spring Tractor Work (mid April) – Disk areas for sweet potatoes, winter squash, watermelons, (Romas and peppers if no-till cover crop insufficient). Bush-hog late food crop plots when rye heads up, to help clover or peas develop. Also clover patches, eg Green Fallow (All Year Cover Crops).

Late April:

in greenhouse sow lettuce #9; watermelons #1 & 2 in soil blocks or plug flats; calendula and various insectary flowers, filler corn & Romas.

Sow corn #1 (1/2-3/4” deep) in two phases, and peanuts if soil temperature is 65°F. Also cowpeas #1, and sesame.

Sow more leeks if needed in Little Alliums bed outdoors. If not, sow more mini-onions and scallions #3.

Transplant lettuce #6, leaf beet, chard, insectaries; finish transplanting gaps in the main broccoli & cabbage plot, plant Alyssum. Take rowcovers from broccoli & cabbage for new crops.

If mild, plant tomatoes. Harden off nightshades by restricting water.

Till beds for early May (okra, toms, melons, celeriac, lettuce 7/8/9, asparagus beans). Compost beds for mid-May (edamame, eggplant, limas).

Store spring and fall seeds (spinach, peas, beets) in the basement for the summer.

Foliar feed the potatoes, ideally the morning before hilling up, and every 2 weeks.

Roll out Driptape and Biotelos corn plastic mulch for peppers and Romas where no-till cover crop not used.

Cover crops: sow rye to wimp out. Sow buckwheat in any beds not needed for at least 5 weeks eg. leeks limas; add soy if bed not needed for 7 weeks. 

Haybine or bush-hog vetch & rye for no-till planting of Roma paste tomatoes, late in the month (or very early in May). (Mow strips; or till strips through the cover crop for the rows, with narrow-set tiller). Water the area before digging holes, if dry.

Perennials: Weed blueberries, asparagus, raspberries, strawberries, grapes as needed. Mow aisles. If asparagus weeds are getting out of hand, mow down one or more rows to keep control. Monitor asparagus beetles, spray spinosad when bees not flying, if >10 adults/100 crowns. Spinosad: Shake well, 1-4 Tbsp/gall (1fl.oz=2Tbsp=30ml.) Repeat in 6 days.

The black center of this strawberry flower show that it was hit by frost and no berry will develop.Credit Kathryn Simmons
The black center of this strawberry flower show that it was hit by frost and no berry will develop.
Credit Kathryn Simmons

Cover strawberries if frost threatens – take rowcovers from spinach. (Pick flowers off any new spring  plantings.)

Visit grapes, log progress, remove flower buds from new vines. Note deaths and where replacement arms are needed.  Check and repair fruit drip irrigation, thin raspberries to 6/foot of row.

Harvest and weed: Asparagus, chard (hoophouse), collards, garlic scallions- pull at 8″, kale, leeks, lettuce, radishes, rhubarb, senposai, snap peas in hoophouse, spinach.