What is Green Manure?
To start with, Green Manure has nothing to do with the wrong end of a cow! The 'manure' in question is plants that are grown and then dug back into the ground to improve the soil. Its a simple process. So, they grow and then rather than harvesting, you dig them back into the soil. The plants chosen must be quick growing as this process only takes a few weeks.
Why Use Green Manure?
There are several reasons for growing green manure as part of your winter garden.
Weed suppression
The most popular reason for growing them is to stop weeds growing in an area where you aren't growing anything. This is mainly for veg beds in winter but can also be very useful when you take on an allotment and know you aren't going to be able to plant it all in the first year.
Improve the Soil
Many gardeners also grow them in order to improve the soil by adding extra nitrogen. It can also improve the actual structure of the soil. This will give better drainage or water retention - depending on what problem you have. If your site is prone to flooding or get a bit boggy then green manure might help with the problem. The other good thing to come from planting green manure is to provide a habitat to attract beneficial insects to your plot.
Stop Leaching
Bare soil, exposed to wind and rain can lose nutrients and be eroded. Green manure will not only stop the nutrients already in the soil from being lost but will also add extra nutrients. This makes green manure a particularly good choice for gardeners working on an exposed site.
Crop support
You can even plant it between tall plants such as sweetcorn to suppress weeds and between broccoli to deter pests. Research has shown that some flying pests can be confused if the outlines of their food plant are disguised. So planting underneath your brassica plants with vetch or trefoil disguises the outline of the crop and seems to confuse and deter cabbage root fly.
How To Choose The Right Green Manure
So which one to choose? The choice of green manure will depend on what task you want it to do and how long you want the ground to be covered.
As with any plants, you will also need to consider the time of year you are planting, your soil type, what was growing there before and what you intend to plant after. That sounds like quite a lot to consider but it is the same as growing any crop really.
So, take into account how long you want the green manure crop to grow for. And then decide if you want weed suppression, soil improvement or protection from erosion. It may well be possible to get more than one of these from the same plant but if you know what you are aiming for it will help you to narrow down the right plant.
Green manures with lots of dense foliage are best to suppress weeds. Those belonging to the pea and bean family are planted in summer and are best for fixing nitrogen. And those with deep roots are good for breaking up the soil pan and improving soil structure and therefore drainage.
Here is a list of popular Green Manures
How to Grow Green Manure
They can be sown from Spring through to Autumn. Sow them in rows or scattered over the area you wish to cover. Allow it to grow for the required time. The time needed will depend on the particular plants you have chosen to grow and when. Be sure not to let them flower as they are best dug in before they flower and set seed.
November is a little late for green manure but some like rye and mustard will still work. If the milder weather continues then you may still get away with others but it will be a bit of a risk and you may end up having to cover your beds with fleece or plastic instead to protect the soil.
Also, as the crop grows keep a watch out for, and remove, any slugs and snails as you are growing a perfect habitat for them! On the flip side it is also a nice cool habitat for frogs so you might be able to attract these to deal with them for you.
Once grown, chop back the leaves and leave them to wilt. After a couple of days, when they are wilted, dig them in to the top 25cm (10"0 of the soil.
If you don't want to do the digging then you can let the plants get caught by frost and leave them on the surface as a mulch.
Allow two to four weeks before planting your new seeds or plants. Err on the side of 4 weeks for sowing seeds as the decomposing plants release a substance that inhibits seed germination. Young plants could probably go in 2 weeks after digging in.