HWDAA | Green Manures

What are they?

The slightly misleading name for various fast-growing plants which we use between crops (a) to avoid leaving soil bare (b) to suppress plants we don’t want (weeds) and (c) to dig into the ground before planting ‘proper’ crops, in order to help soil structure and (in some cases) add nutrients.

Why?

The bare earth between removing one crop and before planting is not great for the gardener or the soil. Weeds will grow or, if you keep the ground clean by hoeing scrupulously, rain will wash some nutrients down to lower levels and (especially in summer) can smash the top particles apart leaving a dried crusty top that water runs off in heavy rains. Of course you can avoid weeds and protect the soil structure by covering with black plastic, but green manure is an alternative to consider - and feels, well, a little more ‘green’.

Are there any other alternatives? If you have sufficient quantity, other alternatives are to cover the surface of the soil with organic matter eg well-rotted manure, compost or leaf mould. It needs to be at least 5 cm thick to suppress weeds, and may contain active weed seeds if it was not decomposed at a high temperature. The nutrient content varies - cow manure (fairly high) to leaf mould (virtually none) – but of course, depending on the next crop, your soil will need different levels of nutrients.

Green ‘Manure’? Only some varieties of green manure add significant nutrients to the soil when they are dug in. These are deep-rooted plants which bring up nutrients from deep in the soil which are otherwise not reached by crops, and the clover / bean family plants which develop nitrogen nodules on their roots. (nitrogen is an essential plant nutrient and is reckoned to be washed away by winter rains)

Aren’t annual weeds green manures in a way?

All organic matter added to the soil improves it when it decomposes and maintains the soil ecosystem so, in this sense, weeds dug into the soil act a green manure. The big difference is that weeds seed very freely at irregular intervals, so you can’t leave them to grow between planting crops as some will be bound to flower and seed and their seeds will come up year after year (‘one year’s seed: 7 years weed’) - whereas you know the green manure plants will all grow at the same rate and you dig them in BEFORE they seed. ‘Cunning plan!’ (Baldrick)

What types?

The grid lists the different plants that can be used as green manures to cover the soil over winter. (There are some half hardy annuals for infill over summer months – bitter lupins, mustard, fenugreek, buckwheat - but of course you can grow crops for this!) Some add nutrients, some just provide ground cover; some cover the ground well, others are a bit sparse, some have the bonus of pretty flowers (but don’t let them go to seed).

How does it fit into crop rotation?

Garden Organic recommends using green manures of the same family as the crop it follows on from, so as not to compromise the crop rotation pattern. Essentially, all nitrogen fixing green manures have to be treated as legumes but the others fit in anywhere.

What are the disadvantages?

The main thing is to be organised, so you have allowed enough time for the green manure plants to grow, then to be dug in and decompose a bit before the next crop. And to work out the best type for the season, length of growing time, crop rotation etc. (With thick rooted plants, there is a bit of effort involved in chopping up the roots as you dig them in.) The information is in the grid so you can plan now for the coming growing season.

Where do I get the seeds?

The stores!

Over this coming year, please let me know what types(s) you have used and how you got on, so I can report back and update the grid to reflect our local conditions.

NAME + type	Nitrogen Fixer?	SOWING TIME	GROWING PERIOD	COMMENTS + Uses + crop rotation
Field beans Vicia faba (hardy annual)	Yes	Sept - Nov	Over winter	Doesn’t smother ground very well – may develop blackfly like broad beans. Deep roots. Treat as legumes in crop rotation
Hungarian grazing Rye Secale cereale (hardy annual)	No	Aug/Oct	Over winter	NB will inhibit germination of small seeds for a few weeks after digging in so dig in as soon as poss in early spring, just before flower heads form Fits anywhere in rotation
Annual Ryegrass Lolium multiflorum (hardy annual)	No	Mar/April or Aug/Sept	Few months or over winter	Fits anywhere in rotation (good variety is named ‘Westerwolds’)
Winter Tares Vicia sativa (hardy annual)	Yes	March - September	2 – 3 months + over winter	Good weed suppressant (spring sowing can provide supply of N if dug in before planting out winter brassicas). Treat as legume in rotation
Crimson clover Trifolium incarnatum (hardy annual)	Yes	April - September	2 – 3 months + may overwinter	Prefers lighter soil. Treat as legume in rotation
Phacelia Phacelia tanecetifolia (hardy annual)	No	March - September	1 – 3 months may overwinter	Mainly for ground cover during growing season but if planted late may over winter. Easy + mauve flowers attract bees. Fits anywhere in rotation

NAME + type	Nitrogen Fixer?	SOWING TIME	GROWING PERIOD	COMMENTS + Uses + crop rotation
Essex red clover Trifolium pratense (hardy perennial)	Yes	April - August	3 months + stand over winter	Short and long term ground cover Treat as legume in crop rotation
Alsike Clover Trifolium alsike (hardy perennial)	Yes	April - Aug	3 – 18 months	Shallow rooted so prone to drought. Treat as legume in rotation
Trefoil Medicago lupulina (hardy biennial)	Yes	March - Aug	3 months + stand over winter	Dense foliage, will tolerate some shade Can use to underplant eg sweetcorn Will stand light dry soils, Treat as legume in rotation
Alfafa Medicago sativa (hardy perennial)	Yes but poor	April - July	A few months + stand over winter	Rich in calcium + major nutrients, deep-rooting so brings up trace elements Treat as legume in rotation