Make good use of any summer gaps that appear in the vegetable plot to sow wildlife-friendly green manures, says RHS Chief Horticulturist Guy Barter
Summer's in full swing and the veg plot is bursting at the seams with productive crops. However, this abundance isn't infinite and soon the odd gap will start to appear as spent crops are cleared and sent off to the compost heap.
Bare soil is bad news on many fronts. Not only is it an open invitation to whatever weed seeds are lurking, a lack of cover also leaves soil prone to erosion and leaching (having its nutrients washed away by the rain).
Luckily there are plenty of plants you can sow as cover crops (also known as green manures) that will not only help keep your soil in good shape, but will provide a boon for birds and bees too.
A good kind of cover-up
It's not just gardeners that make use of this technique. Farmers are paid subsidies to sow areas of plants for birds and biodiversity in late summer. This ensures the ground is covered over the winter and supports wildlife the following year with spring flowers and, later, seeds.
Gardeners will get no money alas, but the opportunity to use hardy annuals as green manures to help wildlife before planting late crops (such as pumpkins) next summer is potentially pleasing. These plants also smother winter weeds and take up nutrients otherwise lost to winter rainfall. As early crops come to an end in August and September, ground becomes available for sowing.
Wildlife-friendly plants to sow in late summer
Phacelia tanacetifolia (see photo, top) is a hardyish annual to sow until mid-September. Its purple-blue flowers are relished by insects. Crimson clover (Trifolium incarnatum) is another plant with masses of insect-friendly flowers and is hardy in all but the coldest regions. Both can be pulled up in June and composted, leaving the ground fertile and weed-free for planting.
A relatively new variety of English marigold, Calendula ‘Winter Sun’ (see photo) has looked amazing at RHS Garden Wisley. Sown in July, it flowers sporadically all winter and into spring, supporting hardy insects on the wing in the cold season. Both phacelias and calendulas smother weeds and set seed freely if you let them.
After mid-September or in northern regions, only large-seeded plants will make worthwhile growth and flower in spring. Field beans and vetches flower in spring from a late autumn sowing, but as they share diseases and pests with broad beans it is unwise to grow them where broad beans are an important crop.
Over-wintered plants on clay soil can be a nuisance the following year as you try to get the ground ready for planting. If your ground is heavy, try white mustard (see photo) and buckwheat. They are particularly quick-growing and will flower to some extent in late autumn, after which they can be left for frost to kill and then raked up in spring.
Linseed or flax (Linum usitatissimum, see bottom photo) is vigorous and free-flowering, and along with gold of pleasure or false flax (Camelina sativa) it is extensively used in farm mixes, although seed is not widely sold to gardeners. However, health food outlets often sell unprocessed seeds for their beneficial oils; these should germinate well.
Kales and other brassicas are important in farm mixes, but there will be plenty of winter brassicas in most vegetable gardens that can be left to flower for the bees in spring. Ideally remove them before planting out your new brassica crops, to limit re-infection of any diseases the older plants may be carrying.
As seeds typically ripen in mid- to late summer, gardeners wishing to make full use of their available space usually only let a small proportion of their plants set seed; some for the birds and some to re-sow.
Pick of the crop
Look for the RHS Award of Garden Merit (AGM) when buying vegetable seed or small plants. You can also download the RHS lists of recommended cultivars.