Overwintering Green Manures for Crop Rotation

, written by Ann Marie Hendry gb flag

Using Phacelia as a green manure

At this time of year, patches of my garden start to look bare as crops are cleared. Left unclothed, winter rains and snow will wash out nutrients and strong winds will strip away topsoil, leaving the earth in poor condition by spring. Naked soil is not an option! Fortunately, there's a solution.

Sowing green manures, also known as cover crops, is a time-honoured way to protect soil over winter, while suppressing weeds and providing habitat for ground beetles, frogs and other pest predators. When these green manures are dug in at the start of the new growing season they will help to improve soil fertility and structure. Knowing what to sow late in the year, after edible crops have finished, will help you to give your soil a much-needed boost.

Forage Peas

Overwintering Green Manures and Crop Rotation

Late in the season in my cool, northern European climate, I have only a few options. There are just three types of green manures up to the job: legumes such as winter field beans, forage peas and vetch (aka winter tares); mustard, which is a plant of the cabbage family; and cereal rye (Latin name Secale cereale). Lucky gardeners in milder areas can add phacelia and clovers to their list of autumn/fall-planting green manures.

Sticking rigidly to a crop rotation plan while including one of these few overwintering green manures can become a real headache, so it's necessary to bend the rules a little. Just follow this one, simple guideline: grow a green manure that will leave a beneficial effect on the next crop.

Improving Soil Conditions With Green Manure

Fleshy legumes such as field beans and vetch fix the essential nutrient nitrogen from the air then rapidly release it back into the soil when they are dug in for the next crop to use (brassicas, for example, love a boost of nitrogen).

It's important to note that legumes only fix nitrogen when the soil temperature is above 8°C (46°F), so they may be less effective where winters are cold. Nonetheless, they will provide some protection for the soil, although mustard or cereal rye is more effective for this purpose. Legumes are fairly trouble-free, so using them as an overwintering crop does not normally cause problems within a crop rotation.

Mustard seedlings as a green manure

Mustard is exceptional at smothering weeds because it forms a dense, leafy growth very soon after sowing. Mustard, particularly 'Caliente' mustard, is also a a biofumigant, which means it may be effective in controlling some pests and diseases that affect other crops – for instance wireworm or potato eelworm. Grow it after your brassicas (unless they've suffered from clubroot, in which case avoid using mustard as a green manure at any point in your crop rotation as it can harbour the disease).

Mustard is not frost hardy, so it will be killed off after a hard frost. This is not necessarily a bad thing, as this will make it easy to incorporate into the soil in spring. You can often grow mustard within a greenhouse or tunnel right through the winter, though if it flowers the stems will become tough and harder to dig in.

Cereal rye and phacelia are useful green manures that don't belong to any of the common crop families we gardeners use, so they shouldn't compromise a crop rotation scheme. Phacelia has to be sown quite early in many areas to allow it to establish, and it won't survive a cold winter. Cereal rye on the other hand can be sown later than any other green manure, making it invaluable for following on from autumn harvests.

Cereal rye as a green manure

Cereal rye has several other benefits. Above ground, it produces a dense canopy of foliage that helps keep weeds in check. Below ground, it has an extensive, fibrous root system that loosens soil, laying the groundwork for root crops such as carrots and parsnips to follow. Cereal rye is unsurpassed among green manures in its ability to hold onto soil nitrogen, refusing to let it leach away, before releasing it gradually for the next crop once it's dug in.

It's important to cut cereal rye down at the right stage – too early and it may regrow (some gardeners cover it with black plastic after digging it in to help kill it off), while too late and it will produce a lot of dry material that's slow to decompose.

Be aware that green manures can produce chemicals that inhibit seed germination. Allow four to six weeks between digging in a green manure and sowing seeds.

Sample Overwintering Green Manuring Plan

Below is a sample crop rotation schedule showing which overwintering green manures to follow on from which edibles. Following this schedule should leave the soil in great condition for the next crop in the list.

Edible Crop Follow-on Green Manure
Legumes (peas and beans) Legumes (e.g. field beans, forage peas or vetch)
Brassicas (cabbage family) Mustard, cereal rye, phacelia, or legumes
Solanaceae (potato and tomato family) Cereal rye or phacelia
Umbellifers (carrot and root family) Cereal rye, phacelia or legumes
Cucurbits (squash family) Cereal rye, phacelia or legumes
Chenopodiaceae (beet family) Cereal rye, phacelia or legumes
   

This is just a guide, so feel free to experiment and find out what works best in your garden.

We'd love to hear your tried-and-tested ways to incorporate overwintering green manures into your crop rotation plan. Share them with us by dropping us a comment below.

By Ann Marie Hendry.

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Comments

 
"I am planning my first garden on this property. The growth of healthy grass is sparse. Even weeds don't do well. The soil is visible through the sparse vegetation. It is the end of October in region 8a. I'd like to plant a cover crop to till up in the Spring. I'm reading "don't till in the Fall", and then "do till in the Fall" I'd like suggestions of what and how to plant a recommended ground cover/green manure. I plan a 50'x40' garden. I'm pretty sure just broadcasting seed on top of the ground is not a good idea."
charley on Monday 26 October 2015
"Hi Charley. You can broadcast small-seeded green manures then rake the seed in. For larger seeds such as winter field beans you'll need to turn the soil with a fork or spade to incorporate them. It's just possible that you could get away with sowing grazing rye now, but it's getting quite late in the year for even that. Barbara Pleasant has written an excellent article which you might find useful for advice on tilling your soil - use the search box above on the right to search for The Pros and Cons of Cultivating Soil. "
Ann Marie Hendry on Tuesday 3 November 2015
"Well, too late, or not, I plan to broadcast rye grass seed, this week, then broadcast fertilizer, and water well, every other day."
charley on Tuesday 3 November 2015
"we have a raised bed garden about 30 ft x 30 ft. This past autumn we put in a winter cover-crop of rye in some beds and peas with oats in others. We also added some mushroom compost to the mix last fall. Is it still necessary to rotate the summer crops when using a winter cover crop? (Bed space is somewhat limited and some beds lend themselves better to some crops than other beds -- i.e., deeper bed for carrots, shallower bed for onions, larger bed for tomatoes.) "
B Mumper on Friday 3 March 2017
"Personally I would rotate your summer crops, because some pests and diseases can build up in the soil if they repeatedly find a host crop every year. Regular additions of organic matter (such as your mushroom compost) may help to suppress some diseases however. You might find it useful to grow alternative varieties of plants that will suit the beds they're currently growing in, for instance more compact varieties of tomatoes or shorter-rooted varieties of carrots. "
Ann Marie Hendry on Saturday 4 March 2017
"Many thanks for your response. Guess it's back to the drawing board to figure out what goes where this year. Or perhaps we'll add a new bed into the mix! B"
B Mumper on Saturday 4 March 2017
"Can I grow heat prepared onion sets in soil that has been previously over wintered with Caliente Mustard Green Manure? I dug the veg patch over, taking a soil sample to find that the ph is 7.5 Alkaline soil. Please, can you advise me if I need to add anything else ie, blood fish and bone meal? Look forward to your reply, Kind regards, Roy Doran."
Roy Doran on Sunday 26 March 2017
"Hi Roy, I'd recommend adding plenty of compost to your soil. You might also wish to incorporate a balanced organic fertilizer into the soil before planting."
Ann Marie Hendry on Tuesday 4 April 2017
"Hi Ann Marie, Thank you for your help. I only have a small garden so no room for a compost heap, so I can't make my own compost. Would a good quality multipurpose compost be alright I use Humax from my local garden centre? Which balanced fertiliser do you suggest? I do have blood, fish and bone would this be suitable or should I use another one perhaps Growmore? Hope to hear back from you soon. Kind regards, Roy. "
Roy Doran on Tuesday 4 April 2017
"Hi Roy, multipurpose compost from the garden centre will be OK too. Personally I'd steer clear of peat-based composts, both due to the environmental impact and because peat doesn't add much in the way of nutrients. Blood, fish and bone would be fine as an additional fertiliser, though it's not strictly essential if you're using good quality compost."
Ann Marie Hendry on Tuesday 4 April 2017
"Hi Ann Marie, Thank you for your prompt reply, I will take your advice and prepare my raised bed accordingly. Happy gardening. Kind regards, Roy."
Roy Doran on Tuesday 4 April 2017
"Hi, I'm in gardening zone 8b in south Louisiana. We are still having temperatures in the highs of 70s and lows of 50s as low as 48F in the next couple of weeks. I still have a good number of crops growing strong, but I know my first frost date is looming. I'm interested in cover crops and thought a blend might be good, but I worry about crop rotation. I'm doing 4'x8' raised beds and some rows as well. If I plant legumes as cover crops or in a blend does that mean I can't plant legumes in the season that follows? My garden keeps getting bigger as I worry about crop rotation. Thanks, Wanda"
Wanda H on Thursday 9 November 2017
"Hi Wanda, you may be pushing it a bit to get a cover crop in if your first frost date is coming up soon, as it's unlikely to grow enough to become established before cold weather slows it right down. It might be better to use an organic mulch to help protect and feed the soil over winter at this point. Unless you've had pest or disease problems with legumes before I wouldn't worry about growing them after a legume cover crop, as they tend to be fairly trouble-free. The sample overwintering cover crop plan in the article above is a good place to start when deciding what to grow after which edible crop. "
Ann Marie Hendry on Thursday 9 November 2017
"Hi dear! I f you are volunteer I need some help from you, Just I have an experiment which have a treatment supporting conservation agriculture. Just as we all already know Conservation agriculture have its own principles. with this guideline my experiment is aimed to sustain sorghum production at moisture stressed area with improving soil moisture through conservation agriculture. now there is enough rain fall around that area that can be enough to grow Mung bean so If I grow a mung bean it may be counted as cover crop but if crop may reached to harvest it could not be a cover crop. so can I think the Belg as crop rotation for main season with this phenomena?"
Ayele Desalegn on Monday 19 March 2018
"Hi Ayele. I don't have any experience with sorghum or growing mung beans as a cover crop I'm afraid, however all legumes are said to fix nitrogen in the soil and will certainly add organic matter, so it could be worth trying. "
Ann Marie Hendry on Friday 23 March 2018

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