When it became clear at the beginning of the season that we would most likely be short of staff and volunteer help in the walled garden this year Bev, senior gardener in the walled garden, made the decision to not sow many crops. It takes a huge amount of work to grow, plant out, care for and harvest the crops we grow each year in the walled garden and we didn’t want to waste seeds on growing crops we would be unable to care for fully throughout the season.
In order to avoid having large areas of empty beds green manures were sown in large patches, this was to help with keeping the weeds down and to help with improving the soil. We use green manures often in the walled garden and have been experimenting with using them more and in different ways throughout the growing season for different reasons.
Green manures are plants grown with the intention of ameliorating the soil in some way. This could be by adding nutrients, improving the soil structure or increasing the organic content or the soil. Some green manures can be used to help with breaking up compacted soils and others can be used overwinter to help reduce soil erosion when beds may otherwise be empty. At the end of the growing season (or seasons for some green manures) the plants are incorporated into the soil, the aim is to ensure the green manure is fully aerobically decomposed. The tops of the plants can be shredded on the surface before hand and allowed to wilt, after then the roots can be turned in. This may be done by hand or with a machine, either way it is key that it is not incorporated deeply, around 15cm deep, as otherwise you will break up the channels that the roots will have opened up deeper down and potentially undo the soil structure improvements that will have happened. The soil needs to be left after green manure is turned in for a few weeks before it is sown or planted again with the next crop. This is due to the microbes that rapidly multiply in the soil to break down the organic matter require nitrogen to carry out this process and so the nitrogen in the soil is temporarily ‘locked up’ but will be available again for plants once the green manures have been fully composted in the soil.
The simplest way to use green manures is to incorporate them into the growing rotation plan in a kitchen garden or to use them as cover crops for the winter or when beds will sit empty for some time. They can also be used however in strips between edible crops or to undersow a crop with the intention of allowing the green manure to spread and take over once the crop has finished. Undersowing is a good way to incorporate winter green manures into the rotation as they will be more established at the end of the crop growing season and more ready to fully cover a bed for winter.
In the walled garden at Knightshayes we use green manures as a way to improve soil structure and nutrient levels but also to cover the beds for winter. We are experimenting with using them more throughout the growing season and using different plants for different reasons, chicory for example is a great green manure to break up compaction. This season we have used green manures to help cover large areas where we have not grown crops and so would have had large empty beds rips for weeds to take over. We have used Phacelia, which is a beautiful plant related to Borage and great for bees as well as good as suppressing weeds, white clover as a short season green manure to improve nitrogen levels in the soil (although this can be used for paths for many years if the right variety is chosen), red clover as a green manure for a few seasons (initially as strips between other crops but now a full bed after this year) which is great for nitrogen fixation and also improving soil structure thanks to its deeper tap root and branching root system and chicory which is a biennial plant (produces a rosette of leaves in the first year and throws up a flower stem the following year whilst putting down a tap root) that can help with alleviating compaction when it puts down tap roots in the second season. Overwinter we will use a combination of Italian ryegrass and field beans to protect the soil and help with weed suppression also.
A good example of making use of green manure as stated in the editorial .