What are green manures?

Green manures play an important part of soil care but are often neglected by gardeners. Growing a green manure over the winter rather than leaving the soil bare is where they can have most benefit. They protect the soil, improve soil structure and prevent hard-earned nutrients from being washed out by rainfall. Wintergreen manures are ideally established in early September. The problem is that, at this time, we are often still harvesting from our food plants, so the space isn’t ready for sowing green manure.

What is undersowing?

We wanted to look at the possibility of sowing a green manure under the food plants whilst they are still growing. That way, you can get the green manure established for the winter whilst still continuing to take harvests from your crop in the autumn. This technique is known as ‘undersowing’.

Here is a timeline for the process:

Late Spring - Plant out French beans, sow green manure underneath.
Summer - Green manure grows slowly underneath the beans.
Autumn - Take down the beans when they are finished, green manure is already established.
Winter - Green manure is there to protect your soil.
Spring - Cut down green manure and mix it with compost to break down and boost soil fertility.
Spring - Plant next crop.

Why is undersowing a challenge?

The challenge is to get the green manure established without it competing against the main food crop. Yellow trefoil (Medicago lupulina) was chosen as a low-growing non-competitive green manure that is often used for this purpose.

Our trial

Participants grew two plots of climbing French beans.

Control plot:

Beans grown normally, then tried sowing yellow trefoil in late September, after the beans had finished producing.

Undersown plot:

Sowed yellow trefoil under the bean plants in late May, at the time of planting, then removed the beans in September and allowed the trefoil to keep growing over the winter.

Did undersowing work?

In the undersown plot, the trefoil grew well, and by autumn was providing a substantial covering for the ground to protect the soil for the winter. Conversely, in the control plot, which was sown in September, the plants remained very small and never took off.

Even by the following, spring, the autumn-sown plants remained much smaller than the undersown plants:

_	Controlled plot (Sown in September)	Undersown plot	Effects of undersowing
Plot yield of beans (g)	2762	2650	A reduction of 4%
Ground cover of green manure in October (%)	26	79	Big boost to getting green manure established