Green manures

Colin Brain

It is generally accepted, among organic gardeners at least, that the ground should be covered at all times, to  discourage the spread of weeds and to protect the soil surface from the elements, be it from baking to dust during hot summers or from erosion.

A popular way of achieving this soil cover, particularly on vegetable plots, is the use of green manures. These are plants grown to benefit the soil, sown in the ‘vacant’ beds between planting of conventional crops thus. They can either be harvested and composted for return to the soil, or simply turned into the soil. In either event they cover the soil, and their nutrients are released back into the soil.

The range of plants suitable as green manures is quite wide some mature in 10 weeks, others will happily stand over winter. With a little care, green manures to fill the varying ‘time gaps’ can be chosen and incorporated into the regular planting programme to provide all year round soil cover. The basic requirement of a green manure is to achieve a dense blanket of vegetation.

However, as many green manures belong to the same families as normal vegetable crops, i.e. Trefoil is related to legumes and mustard to the brassica family, it is sensible to coordinate their planning with normal crop rotation.

Try to follow regular crops with a green manure of the same family so that although a similar second crop is grown in the same year, the standard four/five bed rotation is maintained.

An excellent ‘Step by Step’ guide to the cultivation, choice and use of green manures is published by Garden Organic (HDRA), however as this is currently out of print (?) the following basic information should be useful.

Generally green manures should be dug in before they go to seed. Those grown for longer term may be cut down when flowering to encourage new growth.

Rel to N fix Hardy Sow from-to Duration Dig Diff Height Note
Alfalfa L ? Y Apl-Jun 3m-1yr 2 100
Beans, Field L Y Y Sep-Nov 5-7m 2 60 1
Buckwheat P X X Apl-Aug 2-3m 3 80
Clover, Alsike L Y Y Apl-Aug 3m-1yr 2 30
Clover, crimson L Y ? Mar-Aug 2-3m 2 30-60
Clover, Essex red L Y Y Apl-Aug 3m-1yr 3 40
Fenugreek L ? ? Mar-Aug 2-3m 3 60
Lupin, bitter L Y ? Mar Jun 2-3m 3 50
Mustard B X ? Mar-Sep 3-8w 3 60-90
Phacelia X ? Apl-Sep 2-6m 3 60-90 2
Rye, grazing X Y Aug-Oct 5-6m 1 30-60 3
Trefoil L Y Y Apl-Aug to 1yr 2 30-60
Tares, winter L Y Y Mar-Sep 2m-1Yr 3 50-75
Related to
L – Legumes
P – Polygonum
B – Brassica
Nitrogen Fix
Y – yes
? – Possibly
X – No
Dig in Difficulty
3 – Easy to dig in
2 – More difficult
1 – Hard work
1 – Poor weed control
2 – Good weed control
3 – Excellent weed control

(Reproduced with the permission of B O G Birmingham Organic Gardeners)