Winter Green Manures

Green manures or cover crops are mainly grown for the purpose of maintaining or improving the soil fertility instead of being harvested, but some can be grazed or cut for fodder as well. Green manures protect the soil against erosion and will scavenge for nutrients and will improve the soil structure. They also bring biodiversity in and above the soil, which can reduce pests and diseases in subsequent crops.

Winter green manures are sown in late Summer and early Autumn after the harvest of crops. The harsh Winter weather can lead to a great loss of nutrients and structure in the soil if it is left unprotected. The earlier in Autumn a cover crop can be sown the better, as the crop has more time to get established and thus can be more effective. The choice of cover crop plants reduces as it gets later in the season; for example clovers can be sown up to the end of August, but a winter cereal like Rye can be sown all through the Autumn.

Benefits of Green Manures

• Protecting soil – Leaving soil bare will cause valuable nutrients to leach away. A green manure will pick up these nutrients and stabilise the soil with their roots. This combined with the green growth above the soil prevents erosion. A bare soil also leaves space for weeds to get established, so promptly seeding open soil with a fast growing green manure is important.
• Improving soil – Incorporating the green manure back into the soil adds organic matter which is vital for feeding the biology of the soil. This biology improves the soil  characteristics; it enhances the ability to hold nutrients and make them available to plants, moisture retention and good porosity. Soil can be improved structurally too, for example the opening up of deep or heavy soil with tap rooted species.
• Providing nutrients – The nutrients taken 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 some 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 (apart from Rye).

Legumes (Clovers and Vetches) can produce significant amounts of organic matter and some have impressive root structures. They can fix large amounts of Nitrogen from the air into nitrogenous compounds in the soil which will be available to other plants later. 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 for August sowing as they grow rapidly, producing a lot of bulk and can Nitrogen fix 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 which will improve the soil structure and the plants will prevent erosion and capping. Although they don't fix nitrogen like legumes, they are excellent at scavenging for free soil nitrogen and holding on to it. Short lived rye grasses such as Westerwolds or Italian Ryegrass are excellent choices as they grow rapidly, are hardy and have deep root systems. Cereal Rye is one of the best winter green manures as it will germinate at even 4C and will continue to grow at low temperatures. It has a fabulous root system which is very good at scavenging for nutrients and it leaves lovely soil structure behind in the following Spring. Japanese Oats are good as they can help reduce root-knode nematodes. It is not as hardy as Rye, but that suits some situations.

Grass/Grain + Legume Mix Growing a mix of a grass/cereal species with 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. The advantages are: 

1) mixes are generally better 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.

2) nitrogen fixing plants will work more efficiently at taking nitrogen from the atmosphere if nitrogen is more scarce in the soil - The grass or cereal will reduce the Nitrogen in the soil.

3) 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 no work with your winter manure. However if the seeds are sown early you will be surprised how much mass will be produced before the winter. This is often considered a crop in itself and depending on the variety of green manure it 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. Make sure not to let the green manure get woody as  that will make it more difficult to incorporate in Spring. Cutting can be important to prevent weeds getting hold 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, it also prevents the crop from getting smothered by the mown crop.

Incorporating Green Manure

This should be done four weeks before you need the soil or before the Green Manure plants become 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 green manures here.

Updated: 6th August 2024