A polytunnel provides just that extra little bit of heat that allows us to expand the range of things that it is possible to grow in the UK. Sweet potatoes are a warm climate crop and have traditionally been difficult to grow in most parts of the UK. Now, however, hardier cultivars have made sweet potatoes a more feasible proposition for the home grower. If you have space in your polytunnel, you may like to give sweet potatoes a go. Not only do the plants provide sweet tubers, their leaves are also edible and tasty and can be cooked as a spinach-like green.
Sweet potatoes are usually grown not from seed but from 'slips' that are taken from a mature sweet potato plant. Such 'slips' are usually available to order online or by mail order in the spring. When choosing your supplier, it is important to make sure that you choose a variety that is suitable for growing in the cooler conditions of the UK, one that can cope with our shorter and more variable growing season.
Sweet potatoes will do best in a warm and free-draining sandy soil that is also rich and fertile. If your soil where you live does not fit the bill then it will usually be easier to grow sweet potatoes in large containers or raised beds filled with a suitable growing medium and placed in a warm, sunny spot in your polytunnel, in a greenhouse or indoors. Black polythene can be used to warm the soil of your polytunnel preparatory to planting.
Since sweet potatoes have a long growing season it is important to plant the slips as soon as possible – cover the soil as soon as possible, in late March or April and plant the slips as soon as temperatures warm in late May or early June. Do not be tempted to plant out slips too soon. Keep them on a sunny windowsill inside until you can be fairly confident of a reasonable temperature in your tunnel. Do not forget to harden off your slips before you plant them out. You can further increase the soil temperature with cloches and horticultural fleece placed over your crop.
Sweet potatoes will crop best at a temperature of around 21-26 degrees Celsius, so extra warming measures may be needed in some parts of the UK. To save space, the foliage can be trained up canes or strings. Water consistently. It is also a good idea to feed your plants with a high potassium organic feed every fortnight. Overwinter plants in a frost-free location.
Tubers take four or five months to mature and are best harvested when the foliage begins to yellow and die back. Lift the tubers carefully and try to avoid bruising your crop. Tubers are difficult to store and so it is best to use up your sweet potatoes right away. ber.