Spinach and Swiss chard are two leafy plants that can be great additions to your polytunnel. Both produce an abundance of leafy green leaves that can be eaten in salads and in stir fries and a range of other recipes throughout the year. True spinach is a slightly more delicate crop, while Swiss Chard, and other related crops such as 'perpetual spinach' or Spinach Beet, as it is sometimes known will continue to produce for a longer time and are less prone to bolting. Since these excellent chards (like beetroot but without the bulb) taste very like true spinach, they are often an easier option for polytunnel growers and other home gardeners.
True summer spinach (Spinacia oleracea) is sown in spring between March and May and there are also varieties that can be overwintered successfully with the protection of a polytunnel. Winter cultivars will require a sunny spot while summer types benefit from some shade and will tend to bolt in hot weather if they are in full sun all day. It is best to thin seedlings to a spacing of around 7cm or so when they are large enough to handle and then to harvest every second plant for use a few weeks later to give the remaining ones space to grow. Make sure that soil is rich in nutrients and contains plenty of organic matter or leaves can taste bitter.
Chards are far more forgiving and can also be grown in a polytunnel all year round, often with only one sowing that just keeps on giving. There is swiss chard, perpetual spinach, and bright lights and rainbow chards that are as decorative as they are delicious. The leaves are cut and come again and should regrow as long as a small stump is left.
Both true spinach and chards will require plenty of water throughout the growing season, though chard is more tolerant of less than ideal conditions and is far less prone to going prematurely to seed. Both are leafy greens that will benefit from top dressing with a mulch that is high in nitrogen and/or nitrogen rich liquid feeds. True spinach in particular will benefit from nearby companion plants which can shade it from the sun in the height of summer, such as peas and beans. Brassicas and spinach may also benefit one another when grown as companions.
Pick true spinach from summer types between late May and the end of October and winter cultivars between October and April. Chards can be harvested any time throughout the year. Leaves may simply be harvested as soon as they are large enough to pick. Start from the outside leaves of chard. In summer you can take up to half of the leaves in one go, while you should be more sparing when harvesting from winter crops.