COVER CROPS and GREEN MANURE as Fertilizer and Soil Amendment



Green manures are plants that are grown to replenish the soil with organic matter and nutrients, especially nitrogen.

Most cover crops are planted in the fall and left alone all winter. The plant matter is then worked back into the soil in early spring – “overwintering”. Farms also plant cover crops in between production crops to suppress weeds, reduce erosion and enrich the soil for the next planting. Not all cover crops are grown to add nutrients to the soil.

To keep things simple, I’m using the term “cover crop” to also refer to green manures.

Why might cover crops be good for the edible garden?

Cover crops are not just for large fields. They can be used in smaller garden plots and even containers – reducing the dependency on store-bought fertilizers. Plant them in raised beds and pots well before the snow falls. Come spring, the soil will be much improved.

Cover crops can reduce or eliminate the need to bring in truckloads or bags of soil amendment. Earthworms and microorganisms will use the decomposing plants as food. The organic matter improves soil structure, drainage and water retention. You could plant a less than ideal area with cover crops with the goal of using the land later on for a garden.

Examples of Cover Crops

Some cover crops such as buckwheat and mustard protect the soil, block weeds and attract pollinating insects. Plants like hairy vetch and crimson clover draw in nitrogen from the air, store it in their root nodules and release it back into the soil – a process called “fixing” or “fixing nitrogen”.

Bell beans filter impurities out of the soil and send roots far beneath the surface to draw nutrients like calcium upward into the root zone. Fava beans are popular in the Pacific Northwest.

See the Bibliography for further reading on cover crops.

Using cover crops in a garden

Where can I find cover crops?

The recipes here focus on two edible cover crops that I use in my garden: Arugula and Mustard Greens. I already have the seed on hand so I don’t need to purchase anything special. Planted in the fall, they’ll keep weeds from taking over and I can incorporate them into the soil before planting tomatoes.

I’m not a large farming operation and need to make the most of every inch of growing area, so it makes sense for me to grow edible cover crops that have short growth cycles.

How to choose the right cover crop?

Determine when and why you want to grow a cover crop. Seed companies usually have charts that show which cover crops are ideal for warm seasons and which are best for cooler weather. There are annual cover crops that you grow once and perennial crops that are meant to stay put long-term in pastureland and fields, many times used as forage for livestock.

Cover Crops for Cool Weather

If you want a cover crop that fixes nitrogen, contributes lots of organic matter, grows quickly, and is easy to establish, then the top choices for cool weather cover might be peas (Biomaster) or clover (Dryland Mix, Nitro Persian, Subterranean). Seed companies may have different brand names for their pea and clover cover crops.

Cover Crops for Warm Weather

In warmer weather, consider black-eyed peas, red cowpeas or a cover crop mix, for example a buckwheat and cowpea combo. Take note that legumes often need inoculants mixed in with them for best results. Inoculants are specific strains of bacteria that thrive in the nodules of the plant roots and aid in the release of the nutrients back into the soil. It’s a powdery substance that adheres to the seed before planting.

Which plants may benefit from green manures?

Nitrogen-needy plants such as cabbage and lettuce benefit from cover crops such as peas, beans and clover.

Because some green manures have “wicked long” taproots, use them to loosen up the ground to ready the soil for deep rooted vegetables.

As mentioned above, I plant arugula as a cover crop and come spring, in that same space, I start my tomatoes.

Buckwheat is a popular cover crop planted before garlic.

Combine cover crops with…

You can employ different strategies depending on what you plan to grow before or after your cover crop. Here are several tips to get you thinking:

  1. Plant green manures to protect and enhance the soil before you grow vegetables. Plant them after you grow vegetables to refortify the soil.
  2. Do not plant vegetables directly after a cover crop from the same family. For example, do not plant cabbage or any brassicas in the spot you grew arugula or mustard.
  3. Start buckwheat in spring and let it reseed and grow again, then incorporate into the soil well 2-3 weeks before you plant garlic in the fall.
  4. Grow crimson clover in the fall and work back into the soil in spring or simply cut off the tops, then wait a few weeks to plant spring veggies.
  5. In warmer climates, plant borage 3 months before you’ll transplant tomatoes.
  6. Green Manure and Cover Crop “Recipe” using Arugula or Mustard Greens

19. Green Manure and Cover Crop “Recipe” using Arugula or Mustard Greens

At the school garden we grow lots of greens, especially mustards, although we renamed them “Asian greens” as a sneaky marketing ploy. (We wanted the students to take a liking right away without forming an opinion based on the name.)

Our favorites are tatsoi (mildest of the bunch and shaped like a spoon), mibuna (looks like a spear or a stretched version of tatsoi) and mizuna (a frilly, lacey leaf). You should see how fun it is to pronounce these names with the kids.Using green manure to grow greens

I also grow arugula as a green manure at home and in my small community garden beds. (To learn about using comfrey and borage as green manure, please see the Borage Recipes.)

I always seem to over-plant arugula and mustard greens. The seeds are so tiny. Luckily, I discovered years ago that in addition to its mild peppery flavor in salads, sandwiches and soups, arugula makes a wonderful green manure/cover crop. I eat some of it and the rest I work back into the soil or let sit in delayed planting areas.

How to Plant

If you live in a cold climate, you could plant arugula or mustard greens in the fall as a cover crop. The plants will be killed off by frost and naturally decompose over winter. If you’re in a warmer climate like me, or like to grow greens in spring, you can plant arugula for food and green manure and then chop it down to control weeds and amend the soil.

When planting, water the soil thoroughly first. Scatter the seed lightly. Cover with just enough soil to keep the birds at bay. Water thoroughly but gently so the seed stays in place.

When to Cut

Used as a green manure (no over-wintering), let the arugula or mustard greens grow as usual, enjoy your fill at meals and just when a few plants start to flower (4-8 weeks), chop them down to soil level and let the organic matter sit on top for a month or more todecompose. Worms and microorganisms will break everything down. You can work it all into the soil but realize you won’t have much left to protect the surface from sun and wind. Let a few plants remain to flower if you wish to save seed for future plantings.

When to cut cover crops and green manures

If you let green manures or cover crops grow too tall, cut the tops off first and then cut again at the ground, creating smaller pieces – the smaller the material, the faster it will decompose. Water weekly if no rain is in sight.

Many greens like mustards will grow back at least a couple of times if cut about an inch above the soil line. I teach the kids at school “cut and come again” techniques. However, when growing green manures, you want lots of plant material (biomass). In that case, cut the greens down very close to the soil level.

Green Manures for Potting Soil

If you grow in containers and pots, plant green manures in them every once in a while to rejuvenate the soil, especially before winter hits. Once they reach maturity and before seed develops, cut the plants down. Work some of the greens back into the pot and leave plenty on top for mulch.

Sometimes it’s best to remove tired soil from the pots. Place it all in a pile on the ground or in a large plastic tub with drainage holes. Make sure to remove old roots and debris. Add compost or worm castings to replace microorganisms and organic matter (at a ratio of 5 parts old soil to 1 part compost or castings). Mix it up, and then plant a green manure such as arugula or mustard greens. Before the plants start to go to seed, chop them up and mix into the soil. Leave some on top for mulch. Let it do its thing for a month or more, add perlite back in if needed to improve drainage and aeration and then refill your pots with the new potting soil.

Cover Crops and Green Manure Quick Tips

  • Try not to be too heavy handed when you sow seed. You want lots of biomass but plants also need space to thrive.
  • Cut green manure crops before they go to seed. Grow for the roots and organic matter. If you want to save seed, keep those plants in one area and collect the seed as soon as they are ready so that they don’t reseed in the wrong place.
  • If you keep chickens, and live in a cold climate, cereal rye will sprout in chilly weather. Plant in October and when spring comes, let the chickens crush and devour the stalks.
  • Buckwheat is a popular cover crop for summer and for good reason. It can choke out weeds and grows to maturity in less than a month! You can leave some plants to flower to attract bees and beneficial insects. The rest chop down and leave as a mulch or work back into the soil, or take to the compost. So many options!