Caring for the soil in your polytunnel should always be a top priority for organic growers. Without the valuable soil ecosystem we would not be able to grow our plants. Topsoil is a fragile thing, and so if we do not take care of it, we can lose it. Green manures are one of the things that organic growers use to care for, protect and enhance the soil that they rely on.
But what is a green manure, why should be use one, and what are the options that a polytunnel gardener in the UK might consider? In this article, we will aim to answer these questions.
What is a Green Manure?
A green manure is a crop that is grown not for your own direct use (for example as an edible crop). Instead, it is a crop that is grown to benefit and potentially improve the soil in which it is grown in some way. As you will discover below, there are a range of different types of green manure.
Usually, these green manures are chopped and dropped. Sometimes, the organic matter is dug into the soil. But in a no dig garden, the organic plant matter is left on the surface of the soil to rot down and be incorporated into the growing medium below through the agency of soil biota.
Why Use a Green Manure in Your Polytunnel?
Understanding which green manures to use involves understanding why we use them in the first place. There are a number of reasons why you should consider using a green manure in your polytunnel, or indeed elsewhere in your garden. These reasons include to:
- Fix nitrogen from the air to enrich the fertility of the topsoil for plants planted after them to take up.
- Suppress weeds through creating a dense soil covering.
- Gather nutrients, and to prevent these leaching away out of the topsoil during the winter months and protect the soil surface from compaction or erosion.
- Improve soil structure when chopped and dropped.
- Provide a valuable habitat for beneficial insects like ground beetles.
Types of Green Manure
In order to fulfil these different functions, there are a range of different types of green manure that can be utilised throughout the year. Green manures can form a crucial part of a crop rotation system, and can be integrated as stand alone crops to fill gaps in time, or to fill gaps in space in polyculture planting schemes. They can be a key part of making the most of both the time and the space that are available in your polytunnel garden.
Let’s take a look at some of the types of green manure that you could consider:
Green Manures for Nitrogen Fixation
An important category of green manures are those that fix nitrogen. Green manures that are legumes, belonging to the pea and bean family, are crucial to nitrogen fixation in the UK. During the summer months, these plants fix nitrogen from the air to their root nodules. Some useful nitrogen fixing green manures are listed below:
Alfalfa is a perennial legume that can be dug in after two -three months (or left for one or two years). It is generally sown between April and July, and so could be a good choice to fill a gap in rotation between spring harvested crops and overwintering greens sown in summer, for example. This is a good choice in particular for alkaline soils.
Bitter Blue Lupin
This legume is not only a nitrogen fixer but also boasts beautiful flowers. It is generally sown between March and June and left for two or three months before being chopped and dropped. This green manure is suited to polytunnels or gardens with light, sandy, acid soils.
A number of different clovers are also excellent green manures to consider. For wet, acid soils, Alsike clover could be a good choice to sow between April and August. Crimson clover is a good choice for those with light soils, and Essex Red clover is a good choice for loamy soils. Leave these in the ground growing for 2-3 months and sow these latter two options between March and August. Essex Red clover is also hardy and can overwinter well.
This is another legume to consider as a green in the summer months. It can also be sown any time between March and August and can be left for two to three months, or for a year or two should the need arise. It can overwinter well too, but does require a light, dry, alkaline soil.
Winter field bean
Winter field bean (Vicia faba) is a great green manure for heavy soils. Sow this legume between September and November and leave it for two to three months (up to flowering) before chopping and dropping.
This is another great overwintering option. You can consider sowing this green manure between July and September for overwintering. You can also sow any time between March and August to leave for two to three months before chopping and dropping.
Green Manures for Weed Suppression
Certain summer-grown green manures form dense foliage that makes them ideal for the effective suppression of weeds. They could be grown in gaps in a rotation, or between other crops. Green manures that are good for weed suppression include:
Buckwheat is a half hardy annual that is best sown in April-May and will only grow in spring and summer. It is generally left to grow for two to three months. One of the benefits of this green manure is that it not only creates an effective weed suppression coverage, but also that it can do so on nutrient-poor soils.
While fenugreek is a legume, it is believed to be unlikely to fix nitrogen in the UK. However, it could be useful as a weed suppressant and ground cover during the spring/ summer. Again, this is best planting in around April/May and chopped and dropped after two or three months of growth. It will only grow during the spring and summer.
Both of these green manures will also help to build soil fertility and improve soil structure over time.
Green Manures for Soil Cover
The final important category of green manures to consider is those that are grown primarily in order to cover bare areas of soil. The first of these may be used to cover bare patches between crops (in terms of time or space), but the primary use for both of the green manures described below is as protection for the soil over the winter months.
This fast-growing brassica is great for filling in intervals between crops when sown before mid September. It can be sown over the spring/ summer (between March and September) to be chopped and dropped after two-three months. However, it can also be sown towards the end of the summer/ early autumn and left in the ground over winter. Though the plants will succumb to frost, the remains can be left in the ground as a mulch and soil protection over the winter.
Like winter tares and other winter green manure crops, this is a hardy green manure that will keep growing all through the winter months. Sown in late summer or autumn, it will mop up nutrients and prevent these from leaching away over the coldest part of the year. Rye is a cereal crop that overwinters well, before being chopped and dropped in the spring. It is particularly beneficial for its ability to improve soil structure.
Sowing & Using Green Manures
Once you have chosen your green manures, and planned out your planting year, you will know when to sow any green manures that you have chosen to include in your year round growing regime.
When the time comes, simply broadcast the seed across the area where you wish to grow your green manure crop, and rake the seed into the surface.
Once the area is required for your next crop, simply chop down the foliage and leave it to wilt. At this point you may also wish to add additional sheet mulch in the form of a layer of compost or other organic material.
It is best to then leave this material to be incorporated into the soil ecosystem for a couple of weeks before you plant up the area with your next crop.
Using green manures is one of the ways to ensure that your polytunnel has healthy and productive soil, which will help you to grow food and other plants there for years to come. Do you use green manures in your polytunnel planting plan? Let us know in the comments below.
Elizabeth Waddington is a writer and green living consultant living in Scotland. Permaculture and sustainability are at the heart of everything she does, from designing gardens and farms around the world, to inspiring and facilitating positive change for small companies and individuals.
She also works on her own property, where she grows fruit and vegetables, keeps chickens and is working on the eco-renovation of an old stone barn.