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I put some fava beans in my vegetable raised bed during the winter months to add nutrients to the soil. The seeds have sprouted and I have some healthy plants growing. Question I have is what next? As I understand it, the roots will provide nitrogen to the soil so I can remove the leaves and leave the roots in place but not sure what is the best time to do that. Also, what do I do with the leaves? Is there any use for the leaves?

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I like to sauté the tops of the plants just like spinach with some olive oil & garlic. – Brian Surowiec Feb 29 at 6:26
up vote 8 down vote accepted

An important thing that many people miss is that if a cover crop produces fruit and seed (beans in your case) it is no longer a cover crop or green manure, but a crop, which depletes the soil rather than rebuilding it. That is, all the nitrogen a legume has put into the soil during the growth stage, is consumed by the plant during the fruiting stage. So if you want a cover/green manure crop, cut it (at ground level) as soon as it flowers. The tops can either be taken to use in building compost, in which case you probably want to add some previously finished compost to the bed to replace what you have just taken away, or it can be mulched in place (spread the green material evenly over the bed and put some straw over it. The latter method requires more time (maybe six weeks) before planting the next crop, so use the take and compost if you need to replant quicker than that. We regard green manure as the first step in the rotation. Then comes heavy-feeding fruit/seed crops (referring to the part you eat), followed by flower or leaf crops, and finally, root crops.

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Welcome to the site Richard! Thanks for this great answer. We hope you'll stay around, share your gardening interests, and have some fun! – Sue Feb 24 at 23:13
    
(Now that I can comment) - BTW, if we are planting a cover crop we go wide bed rather than rows. It, well, covers better :-) – Richard Grevers Feb 25 at 7:54

Normally with cover crops you want them to grow as long as possible, where possible is influenced by:

  • When you plan to plant the actual crop (including some time for breakdown of the cover crop residues.)
  • Is the cover crop about to set seed and become a weed through self-seeding?

The tops/leaves are also valuable material - you can either incorporate them into the soil directly (sheet-composting in place), or cut and place in your compost pile.

Typically about 3 weeks from the time you intend to plant the next crop is a common recommendation for when to cut (or cut and till in) the cover crop, but that can vary. There are also no-till approaches where you either cut and leave or "crimp and roll" the cover crop, and plant through the residue, rather than trying to till it in.

If your schedule allows and you eat fava beans, you could also let them go all the way to harvest and harvest the beans, then cut the tops, but at that point we'd call it a "rotation" rather than a "cover crop" - semantics due to "growing a crop that you harvest" (even if it also helps the soil) .vs. growing a crop just for benefitting the soil.

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Excellent response and to the point! Just so I am clear, should i be cutting off only the top and leave the roots in the soil or do I uproot the plant entirely and compost in place? – JStorage Feb 24 at 19:47
    
Either works, actually - and if you are digging in the tops to compost in place, it's easier to do if you uproot the whole plant, IMPE. – Ecnerwal Feb 24 at 19:49

Usually, you would wait until the plants are in full flower, and have no ripe seeds, then cut/mow the tops down and turn them under. This will add the most nitrogen to the soil (the green tops are very high in Nitrogen), and the soil microorganism population will jump, increasing nutrient availability. In a raised bed, if you don't want to turn the tops under, you can cut them for compost, and plant crops after the roots have died.

Just be sure the tops are well decomposed, because they can have allelopathic effects on germinating seeds.

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Thanks. I think that answered a question I had. If the top is rich in nitrogen, what is the role of the roots? Instead of cutting the top (which can be laborious), can I just uproot the cover crop and bury them in the soil for decomposition? Also, if I am transplanting vegetable plants (since I am growing seeds indoors), does your concern about allelopathic effects go away? – JStorage Feb 24 at 19:51
    
@JStorage, the roots of a legume have a symbiotic relationship with bacteria in their root nodules. These bacteria fix nitrogen for the plant. If you kill the plant, the nitrogen moves to the soil. The tops will naturally add nitrogen as they decompose (as all green matter does). The reason for cutting the plants is that they die faster when cut, and that the smaller the pieces are, the faster they will decompose. About the allelopathy, if your seedlings have at least 3 true leaves (not counting the cotyledons), they should be fine. – J. Musser Feb 24 at 19:56
    
Just so I am clear, I can uproot the cover crop and cut it up into pieces rather than mowing the top only. Just want to make sure I get the most out of the cover crops. – JStorage Feb 24 at 20:00
    
@JStorage You can, if you want to. As long as it's all returned to the soil to decompose, you should be good. I usually do larger scale, so uprooting individual plants is harder work than pulling a flail mower through. – J. Musser Feb 24 at 20:03
    
@Jstorage The aim is to build a good microrhizal network - which helps transfer nutrients from soil to plant by a symbiotic relationship. Pulling roots out damages or destroys these fine filaments. So the theory is that you interplant between the roots of the last crop, and only dig the bed when you get to the root crop step of the rotation. This came from a very experienced organics and biodynamics assessor who was staying with us. – Richard Grevers Feb 25 at 7:59

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