There are three main types - first earlies, second earlies and maincrops depending on when they are planted and harvested; grow a selection of all three for a long cropping period, and store maincrops over winter. Extra early potatoes can be obtained by planting varieties such as first earlies from late February under fleece. Potatoes need a sunny site away from frost pockets - the newly emerging foliage is susceptible to frost damage. You can prevent this by earthing up the soil around the shoots. It's imperative to keep light away from the developing new potatoes as light turns them green and green potatoes are poisonous.
Tubers ought to be planted around late March for first earlies, early to mid-April for second earlies and mid- to late April for maincrops. This varies slightly depending on where you are in the country.
There are numerous ways of growing potatoes.
It's very important with earlies and a good idea with maincrops to chit the seed tubers first before planting; this means allowing them to produce sturdy shoots. Buy your seed potatoes in late January/February and stand them rose end up (the rose end has the most eyes) in egg boxes or similar in a light, frost-free place. The tubers are ready to plant when the shoots are about 2.5cm long.
The long-established way is to dig a narrow channel 12.5cm deep. This can be lined with compost for a better crop. The seed tubers are spaced 30cm apart for earlies and 37.5cm for maincrop varieties in rows 60cm apart for earlies and 75cm apart for maincrop. Slugs can be a problem. When the stems are about 23cm high start earthing up by carefully drawing earth up to the stems and covering to produce a flat-topped ridge about 15cm high. This can be done little and often or in one go.
The other method is to grow the potatoes under black polythene. The tubers are planted through the black polythene. The advantage of this method is that there is no need to earth up and the new potatoes form just below soil level which means there's no digging to harvest them.
Keep crops well watered in dry weather; the vital time is once the tuber starts to form. A liquid feed of a balanced general feed every fortnight can help increase yields.
First earlies should be ready to lift in June and July, second earlies in July and August, maincrops from late August through October. With earlies wait until the flowers open or the buds drop; the tubers are ready to harvest when they are the size of hens' eggs. With maincrops for storage wait until the foliage turns yellow, then cut it and remove it. Leave for 10 days before harvesting the tubers, leaving them to dry for a few hours before storing.
Sweet potatoes are well worth trying outdoors in milder areas - or in a glasshouse. Even in mild regions, indoor growing will produce more reliable crops.
Plants are best grown from cuttings or slips ordered from a mail order supplier, however you can grow them from shop-bought tubers, but you can't plant them like normal potatoes as they don't grow in the same way. Many shop-bought tubers are also treated with an anti-sprouting agent, so give them a good scrub to clean them first. Then place in moist sand in a hot propagator or in the airing cupboard. Once the shoots are 5-7.5cm long, they can be removed as cuttings and potted up into small pots of cuttings compost and placed in a warm propagator to root.
Plants are ordered as cuttings or slips, delivered from late April onwards. Pot the cuttings immediately on receipt into small individual pots of multipurpose compost. Should the slips be unrooted, simply cover the pots with a clear plastic bag or place in an unheated propagator until roots appear. Grow the plants on in a frost-free, well-lit spot until early June. In mild regions, sweet potatoes can be planted outdoors after a period of hardening off. They require a highly fertile but light, preferably sandy, soil. If your soil is not naturally sandy or free-draining, plant into ridges 15-30cm high, spacing plants 30cm apart, with 75cm between rows. Ideally, plant under a cloche or fleece tent. Alternatively, grow in a glasshouse in large tubs, growing-bags or the glasshouse border. Whitefly and red spider mite can cause problems on foliage under cover. Sweet potatoes crop best at temperatures between 21-26°C (70-80°F). Keep well watered, feeding every other week with a high-potassium liquid feed.
Tubers take from four to five months to mature. They can be lifted from the end of August, but it is usually better to leave them until the leaves begin to yellow and die