Winter Green Manures

Winter Green Manures

Green manures or cover crops are really fertility maintaining or fertility building crops. They are grown for the purpose of covering the soil instead of being harvested. Green manures protect the soil against erosion and maintain and improve the soil structure and fertility. They can also help to bring biodiversity in and above the soil, which can reduce pest and diseases in subsequent crops.

Winter green manures are sown in late summer and early autumn after the harvest of crops. The winter weather can lead to a great loss of nutrients and structure in the soil if left unprotected. The earlier a cover crop can be sown in Autumn the better, as the crop has more time to get established and thus can be more effective. Also the choice of cover crop plants reduces as it gets later in the season. For example clovers can be sown up to mid September, but cereal Rye can still be sown all through until late Autumn.

Benefits of Green Manures

• Protecting soil – Leaving soil bare is a big no-no as valuable nutrients will leach away. A green manure will pick up these nutrients and stabilize the soil with their roots. This prevents erosion. Leaving soil bare is also asking for a weed infestation. Promptly seeding open soil with a fast growing green manure will keep weeds at bay.
• Improving soil – Incorporating the green manure back into the soil adds a lot of organic matter which is vital for feeding the biology of the soil. The byproducts of this biology are what gives good soil its characteristics; for example, qualities like the ability to hold nutrients and make them available to plants, moisture retention and good porosity. Soil can be improved structurally too such as with the opening up of deep or heavy soil with tap rooted species.
• Providing nutrients – The nutrients taking up or synthesised in the green manure will be released back into the soil for the benefit of the following crop. This is why, for example, leguminous plants such as clover, vetch, or lupins are often used as green manures. Their ability to fix atmospheric nitrogen into the soil is key for providing enough of this essential nutrient in organic systems where artificial nitrogen fertilisers are not used.

Different Types of Green Manures

Despite the name, winter green manures must be sown sufficiently early so there is enough development of the root system before colder and shorter days put a check on growth. A well established root system is especially important for holding on to nitrate which, unlike other nutrient ions, is not strongly attracted to soil particles. Sowing from early August is ideal and by the end of September the window has closed to reap the full benefits of a winter green manure.

Legumes (Clovers and Vetches) can produce good amounts of organic matter and some have impressive root structures. They can fix large amounts of Nitrogen from the air into soluble nitrogenous compounds in the soil which are available to plants. Now we know that this amazing example of natural chemistry is based on a symbiotic relationship between leguminous plants and the soil bacterium known as Rhizobium. Fast growing clovers, such as Crimson and Squarrose clover, are excellent choices as they grow rapidly producing a lot of bulk and nitrogen fixation in a short time. If later planting can't be helped then it is worth including Vetch which can be successfully established later than clovers. Vetch is also excellent at covering the soil and out competing weeds.

Grasses and Grains have great root systems, and will improve the soil structure preventing erosion and capping. Although they don't fix nitrogen like legumes, they are excellent at picking up existing nitrogen and holding on to it. Short lived rye grasses such as Westerwolds or Italian Ryegrass are excellent choices as they grow rapidly, grabbing any free nitrogen in the soil and prevent loses through leaching. Rye is one of the best winter green manures as it will grow at low temperatures, has a fabulous root system, is very good at scavenging for nutrients and it leaves lovely structure behind in the following Spring. Japanese Oats are good as they can help reduce root-knode nematodes.

Grass or Grain /Legume Mix Growing a mix of a grass species and a nitrogen fixing legume is an ideal and time tested winter green manure strategy. Check out Rye/Vetch, Landsberger mix, Wild Atlantic Way mix or Westerwolds and Vetch mix. There are a few advantages to this. First, mixes are generally always good because different species have different root structures, nutrient requirements and optimal temperature ranges. As such they fill different niches and make better use of soil and climate resources. Second, nitrogen fixing plants will work more efficiently at taking nitrogen from the atmosphere if nitrogen is more scarce in the soil - such as when in competition for it with another plant. Finally, when incorporated the release of nitrogen back to the soil will be buffered as the legume releases nitrogen fast in comparison to the slower release from decomposing grasses. This spreads out the availability of nitrogen for the following crop.

How To Sow

• Measure the area and work out the sowing rate according to the packet instructions.
• Broadcast or drill the seeds onto prepared soil. If you have a large area to cover it is a good idea to divide the seed and the land into 4 parts to avoid running out of seed. This Birchmeier Seed Spreader can be used to broadcast seeds.
• For larger seeds such as Rye, drilling is better as seeds need to be deeper in the soil. Netting may also be necessary to deter birds.
• Rake in, roll (or pat down). Water if soil is dry.

Managing Winter Green Manure

Once established, there is not much to do with your winter manure. However, if you are well organised and have the seeds in early you will be surprised how much mass will be produced before the winter. This is often considered a crop in itself and can be grazed or made into silage on a larger scale. In the garden, it can be cut and used as mulch, chicken feed or added to the compost heap. It can also be left in situ to add to the fertility of the site where the green manure is growing. Cutting delays the green manure from going to seed which keeps lush vegetative growth happening. Without cutting, or 'topping', your green manure can become woody and more difficult to incorporate when spring comes around. Cutting is also important in preventing a weed infestation in your green manure - Cutting about 15-20cm from the ground can prevent fast growing ephemeral or annual weeds that are present from setting seed. Cutting at this height leaves plenty of growing points on the green manure for it to take off again. Unfortunately, you might be left with the difficult decision of whether to prevent your green manure from flowering by cutting to minimise weed problems or enjoy it in bloom and deal with the consequences later.

Incorporating Green Manure

This should be done four weeks before you need the soil or before the Green Manure plants becomes woody, whichever comes sooner. You can -

• Cut down with strimmer or scythe and leave soften on the ground.
• Dig the plants back in using a sharp spade. Aim to bury them between 15-18 cms deep.
• If a frost tender manure is used in late summer you can let the cold weather kill it off. Leave on the soil, no need to dig in.
• Annual green manures such as mustard or buckwheat can be hoed in. Excess foliage can be composted.
• Mulching works well - annuals will take a few weeks to break down while perennials and rye will take a bit longer.

Check out our full range of winter green manures here.