They suppress weed growth and hold nutrients in the soil that might be washed away by Autumn and Winter rains. Some - particularly if from the bean family of plants - actually add fertility to the soil by fixing nitrogen - just like beans grown to eat. When they have done their work, start to flower or die back - they can either be dug into the soil to improve its structure, or provide a useful addition to the compost heap.
Finally - my personal view - I prefer to at least have something growing on my plot rather than empty beds. As long as it adds to the plot within broadly "green" principles - I'm keen on a Stakhanovite level of allotment plot productivity that would put a Stalinist collective farm commissar to shame ! ........
Caliente Mustard - because when its cut down and dug in it can act as a biofumigant to suppress s number of soil-borne diseases.
Preparation was simple - I cleared each bed of weeds, roots and stones.. Then forked over lightly to loosen the soil, broadcast the seed by hand and watered in.
One thing I've learned his year though is the importance of timing.
My first bed was sown with mustard in early August - following on from a crop of first early potatoes. This germinated quickly, got well established and started to flower by the end of the month. I then cut it down to prevent it from seeding and put it into the compost bin. That bed is now home to my Autumn Sown Peas.
I'll probably leave this to over-winter unless it gets damaged by frost - but in our mild Cornish climate it will probably last through. In early March I'll use the strimmer to chop it up, leave a few days to wilt down then dig it into the bed. French Beans will follow in April or May.
As you can see from the pic has grown ok but nowhere near as well.
But its providing a fair amount of ground cover and may grow on a bit through Autumn.
I'll let this grow on for at least a couple of weeks to see what happens - mustard is relatively fast growing even in the Autumn chill - so might just get on top.
I'll write another post soon about other green manures and and update you on progress of the mustard currently growing on my St Ives allotment.
More about green manure:
Allotment Growing - Green Manure