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Turning Cover Crops into Green Manure

Cover crops planted in the fall are now growing like weeds. But now what do you do with them? They should be turned into Green Manure! Time to knock down your cover crop. In our video on Green Manure, Tricia talks about when and how to turn your cover crop into a fantastic soil additive.

Timing of the Knock Down

The timing is determined by when the next crop will be planted and the life stage of your cover crop.

  • Cut down your cover crops before they go to seed.
  • A good indicator for when to knock down the cover crop is when one half of the crop is flowering. This allows you to take advantage of maximum biomass.
  • After cutting, let it decompose so that you get the maximum benefit to the crop that you're going to plant afterward.
  • After cutting, allow at least 3 to 6 weeks before planting your next crop. It's important to wait because your cover crop will be decomposing and during this decomposition process you will temporarily lock up some of the nitrogen in the soil.

Speeding up Decomposition

You can speed up the breakdown process of your green manure by adding something like our Biodynamic Field Spray. The spray adds beneficial bacteria that speeds up the break down of the green manure to about two weeks. When using this product, make sure you use dechlorinated water when mixing. After application of the spray, turn under the plant matter.

What is the Best Way to Knock Down the Cover Crop?

There really is no best way. It depends on the tools you have on hand and the amount of cover crop you want to turn into green manure. You can cut it and compost it, you can weed-eat it or mow it and just let it lay on the ground, or you can till it into the ground.

The three main methods of cutting down cover crops are: undercutting, mowing and rolling. Undercutting is when you draw a blade under the soil and you slice the cover crop underneath the soil. For mowing you're going to mow down the crop with your lawn mower a weed whacker, or a scythe. This chops the crop up fairly finely and it will decompose quickly. Rolling is basically running your tiller over the plants with the tines turned off. However, this may not always kill the plant and thus not decompose.

Tilling or Rolling

  • Tilling it in is the fastest and easiest way to incorporate your cover crops.
  • Rototill the crops into the ground.
  • The advantages of this method are faster decomposition and less nitrogen loss into the atmosphere.
  • The disadvantage is that you don't get the weed suppression or water conservation while the crop decomposes.

Cut it and Compost it

  • You can use a hand sickle, a scythe, weed-eater or mower.
  • The advantages are that you're adding finished nutrient-rich compost back into the soil (eventually).
  • This is a great option for raised bed cover cropping.
  • The disadvantage is that it's more work to cut, compost and then add the compost back into the soil and it can take several months instead of weeks.

Mow or Weed-Eat

  • After cutting the cover crop down, let it lay on the soil surface as a mulch as it decomposes.
  • This will help with water conservation and weed suppression.
  • This method chops the cover crop into small pieces and will speed up decomposition.

No matter what method you choose, the most important thing is that you will be improving your soil with the addition of green manure!

22 comments

  • Elle, sent your question to our garden advisor, Janice, and this is what she says, “Unknown if any research on success of cover crop to thwart this type of oxalis exists. It thrives on disturbed soils so preparation for cover crop introduction would be counter-indicated. However, a dense stand of a grass that stabilizes the soil would probably hinder spread. Maybe introduce as plugs well rooted so they can establish – obviously a long term project.
    Sheet mulching is effective if repeated and wide enough to deal with perimeter issues. Work is being done with tilling in the fall/winter when oxalis emerges and before it flowers and then repeating 2 weeks later. Thought is that bringing root and growth material to surface with degrade.”

    Suzanne
  • I have oxalis growing on a steep slope in my SF east bay backyard—the hill is covered. I’ve tried digging them out to no avail. I tried solarizing with plastic, but it didn’t work either, the bulbs are too deep. I thought maybe a strong weed suppressing ground cover might help. The ground cover would need to work in the fall and winter as Oxalis pes-caprae (yellow buttercup strain) is from South Africa and grows during our rainy season. I remembered reading somewhere that higher nitrogen might retard it, so was thinking a nitrogen fixing ground cover that forms a strong weed suppression mat might help. Any suggestions? The other issue is planting since on a fairly steep hill with dirt over serpentine rock, so would have do seeding by hand. I’d appreciate any suggestions.

    Elle
  • Don’t till. Just chop and drop. Any plant material left will eventually decompose and become mulch if you want to plant something before it’s all dry and crumbly, just scooch the greenwaste to the side and plant…no till is the way to go. Tilling destroys soil structure and causes erosion and lack of tilth

    Pam
  • Early spring planting is my favorite for peas as cover. Sometimes I mix a 1:1 ratio of cut biomass to finished compost so I don’t burn anything. Laying it on top works for light fertilizing, weed control, and water retention for a raised bed. The big garden gets tilled in early spring when the pea plants are young. A lawnmower and tiller is fine for this. Just cut and turnover. Hope this helps

    Michelangelo Morse
  • Helen, the large clumps are a bother. You can know out as much soil out of them and put them in your compost or just turn them under. You could also put some weed fabric over it and let it just die out and it should break down over 2-3 weeks.

    Suzanne
  • I’m also in the Puget sound and I’m finding large clumps that is hard to till. First time doing this and not sure if it’s going to work that great for my raised beds.

    Helen Brunson
  • Elizabeth, the red clover is a perennial clover. It will not die back in the summer. If you want to remove it, you will have to pull it up, or just let it grow.

    Suzanne
  • I planted red clover in the fall and it has made a nice mat over my flower bed. I have a lot of bulbs and perennials that I don’t want use a rototiller, pluse I don’t have one. I am going in with a hand hoe, cutting into the clover, and yanking it up. I’m just leaving the clover on top. I’m not sure that I’m getting all the roots out. Am I going to have weed issues? Ps I live in south Puget sound Washington state if that matters c

    Elizabeth Rush
  • Laurie, you should not be having issues with the cover crop seeds unless the plants were allowed to go to seed. Cover crops should be cut down before they go to seed. With cover crops many people rotate planting in beds so not to hold up any planting.

    Suzanne at GrowOrganic.com
  • I last commented on this on April 4, 2018. As an update, I did plant covers recommended for cold climates, and I used the Pfeiffer Biodynamic Field Spray suggested in the article to speed up decomposition. I did this for two seasons, and will not be doing this again. I got some growth from the cover seed and was able to till that in, but I believe it was more trouble than it was worth. There just wasn’t enough time for really adequate growth prior to the time I needed to get seedlings in the ground, and many of the cover seeds hadn’t sprouted prior to tilling. These seeds haunted me all summer by becoming weeds in my beds which then had to be removed.

    Laurie
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