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Green manures

Posted by Rosemoor Garden on 14 Apr 2009 at 11:48 AM

In the Fruit and Vegetable garden at Rosemoor we use a number of green manures in our many plots. Green manures have the ability to perform a range of useful gardening functions all at the same time. These include soil improvement in the form of adding organic matter and nutrients to the soil and also soil protection from the vagaries of the weather in the form of a cover crop.

Green manures encompass a multitude of crops that can, if correctly selected, span the whole gardening season and are not, as some may think, restricted to the winter months alone. Most are grown for a specific period of time before being incorporated in the soil. Green manures have a number of advantages which we value at Rosemoor; many of these advantages are also of great benefit to the amateur gardener.

These include:
The ability to fix atmospheric nitrogen, which is an essential nutrient, in a usable form for plants, by utilising the nitrogen–fixing bacteria contained in the root nodules of many Leguminous green manures such as the clovers and vetches. At Rosemoor we use the vetch winter tares to enrich and protect our 3 metre x 3 metre veg patch. Seed can be sown up to the end of September and the winter tares over-winters well. Come spring we turn it into the soil; importantly giving it at least a couple of weeks to rot down into the soil prior to seed sowing.

Some green manures are quite deep rooted and have the ability to bring up nutrients from deeper in the soil which benefits the shallower rooted crops planted subsequently. This also has the advantage of improving soil structure. Our use of Italian rye grass and rye corn gave us pleasing results this year, commented on by many of our visitors.

The increase in the level of organic matter added from the root and top growth of the manure aids its water retentive capacities and also aids aeration and it opens up the soil structure.

Certain flowering green manures such as Phacelia are very good at attracting pollinating insects by providing a valuable foraging food source. These insects are of significant benefit to the gardener. In addition, the flowers are often very attractive.


and at Rosemoor we combine the legume Crimson clover with its nitrogen fixing abilities along with Phacelia to get the best of both worlds. Phacelia unlike the clover can only take a degree or two of frost and so dies back over winter; however the benefits of its root structure to the soil remain. We sow these two generally by the end of August. 

When a green manure is applied as a cover crop it is very effective in acting as a weed suppressant and preventing soil erosion, compaction and panning from rain and sometimes stagnation at times when the soil would otherwise not be in cultivation. They do however take a bit of work to dig in to the soil and in the case of Italian rye grass which can be sown into November, needs to be completely incorporated otherwise it will start to re-grow and not rot down! Allow 6 weeks or more for the rotting process prior to sowing or planting.


Green manures are a great way for the hobby gardener with a small plot to gain many of the advantages of manuring when they are confronted with limited space and access difficulties. No backing in of trailers or wheeling around of barrows of manure, just the sowing of a few handfuls of the green manure seed and its digging in with the most basic of tools, a spade and old fashioned elbow grease!

Happy manuring folks!


bogweevil said:

Ah if only it were so.  You should be aware that green manures do not add organic matter to the soil as they are mostly water and indeed by enhancing soil microbe action green manures can actually deplete soil organic matter.  Neither do they add much nitrogen in winter as the nitrogen fixing bacteria need warmth and plenty of photosynthesised materials to function. Therefore vetches sown in autumn will make little difference.  Grown all summer they might well add a useful amount.  However if you are going to grow green manures in summer some might say that you might as well grow something useful and compost the crop residues and thus get a useful crop and much of the benefit of green manuring.  What autumn sown green manures will do is scavenge soluble soil nutrients in autumn and hold them to be released in spring, so that they are not washed out by winter rains. Ones dug in during autumn and winter can help make soil more workeable in spring, but they can also provide food and shelter for slugs. Boggy

on 14 Apr 2009 at 01:17 PM