Potatoes

Soil preparation

Potatoes require an open, frost-free site with deep, fertile, moisture-retentive and crumbly soil for high quality and heavy yields. Improve soils by adding organic matter, such as well-rotted manure, in the autumn. Before planting, supplement with a general fertilizer, such as Growmore or blood, fish and bone, applied to the soil surface or spread along the sides of the drill during sowing, at the rate of 1kg per 10m (2.2lb per 33ft) row. Half of this amount will be enough if the garden is known to be fertile.

Planting

Once chitted (see propagation section below), seed tubers can be planted in a drill or individual holes and earthed up as they grow.

Plant early potatoes in early April, with later cultivars being planted mid-April. In northerly districts and during adverse weather, you can delay planting up to mid-May. Potato ‘seed’ tubers are also offered in late summer for a winter or Christmas crop and these can be productive in greenhouses, but planted outdoors they are vulnerable to blight disease and frost.

Potatoes can be successfully grown in containers.

To grow outside, carry out the following steps:

• Draw a drill 7.5-15cm (3-6in) deep with a hoe or spade
• Place tubers with the chits (sprouts) uppermost in the drill. Early cultivars should be spaced 30cm (1ft) apart in the row and 60cm (2ft) between rows. Later cultivars should be spaced 35cm (14in) apart and have 70cm (28in) between rows
• Push the soil gently back over the tubers, ensuring that they are covered with at least 3cm (1¼in) of soil
• Lightly rake over the soil surface to level it and mark the drill
• Earth-up the plants by drawing soil around the stems to form a ridge. The final height of the ridges should be 20-30cm (8in -1ft). Earthing-up will exclude light from tubers preventing them from going green – green tubers are potentially harmful if consumed
• Alternatively, black plastic mulch can be laid before or immediately after planting and this excludes light and suppresses weeds, so earthing up is not required. Remember to cut a slit or cross in the plastic at each planting hole to allow the foliage to grow through 
• Protect the young growth from late frosts, either by drawing a little soil over them with a hoe or covering them with horticultural fleece
• Some hoeing might be needed to kill weeds, but earthing-up leads to the destruction of weeds and few will penetrate the foliage once it has covered the rows
• Removal of flowers is not thought to increase yield significantly, but watering boosts yields in dry spells, especially during the critical period when tubers are forming in early summer. Thoroughly soaking potatoes every 10 days at this period ensures numerous tubers are initiated and also helps to prevent common scab disease. Watering in late summer may help crops bulks up and avoid second growth (see Problems below)

Harvesting

Lift early potatoes carefully with a fork as soon as they are ready, when tubers are about the size of a hen’s egg or more. Flowering often occurs at this time, but the tubers may be ready before. Provided the crop is healthy, leave main crop plants until early to mid-autumn to bulk up (but the tubers are prone to slug damage).

Storing

For storing, lift main crop potatoes when the tops die back and the skin resists gentle pressure. All potatoes should be gathered by mid-October to avoid weather damage.

• Lift on a dry day and allow potatoes to dry on the surface of the soil for two or three hours
• Handle tubers gently, as they bruise easily and this impairs their keeping qualities
• Store in hessian or paper sacks or in boxes in a frost-proof shed. Avoid plastic materials, including plastic-lined paper sacks, as these promote condensation that can cause rotting
• Early and second early potatoes have a short dormant period and will sprout earlier so keep for a shorter time than main crop cultivars

Chitting

Potatoes are generally grown from tubers known as 'seed potatoes'. These are sprouted or ‘chitted’ prior to planting, particularly when growing early season cultivars. Chitting or sprouting tubers extends the growing period and leads to earlier tuber formation and higher yields.

• Place tubers on a tray in a single layer with the ‘rose’ end uppermost. This end has the most eyes or buds and sprouts will arise from them. Some suppliers offer 'pre-chitted' seed potatoes
• Keep trays of tubers in a cool, frost-free place with moderate light, such as an unheated room and avoid direct sunlight
• Sprouts form within a few weeks and, after about six weeks, shoots should be 5cm (2in) long and dark coloured.  High temperatures and dark conditions encourage unsatisfactory pale, leggy shoots
• Choose about four strong shoots and rub off the weaker shoots for early potatoes, but there is no need to thin shoots for later crops
• Tubers can chitted from January, but planting should be delayed until March in sheltered and southern areas; or April in less favourable areas. If weather is unsuitable for planting, tubers can be left to chit further – even until May without too much crop loss

Microplants

For cultivars that are rare or old and seed potatoes are not available, micro-propagation is used to produced microplants that are strong and virus-free. These microplants are grown in the same way as seedlings in containers or outdoors when all danger of frost is past. The subsequent tubers can be eaten and some seed potatoes, saved for future cropping.

Fruits

Potatoes occasionally produce fruit that resemble small tomatoes, but the skin often does not colour and remains leathery. These fruit are poisonous and the seed is unreliable, less convenient and unsuitable for garden use.

Buy certified seed tubers as they are free of significant diseases, including viruses, and are widely available. Crops grown from home-saved tubers, including supermarket ones, often disappoint due to high disease levels.

The potatoes available are grouped according to their season of lifting;

• Early potatoes produce usable tubers in 100-110 days after planting
• Second earlies in 110-120 days
• Early maincrops after 120-125 days
• Maincrop produce usable tubers after 125-140 days
• Salad potatoes have firm flesh after cooking, as opposed to floury, so they remain whole when used cold in salads

Potatoes may be classed as 'determinate', with a compact and bushy habit, or 'indeterminate' with a wide-ranging, sprawling habit. The former are better for small gardens and include most modern cultivars. The latter, such as ‘Cara’ for example, give good yields and suppresses weeds, so are good for allotments.

Choosing wisely

• In dry areas and on sandy soils cultivars with drought tolerance should be chosen: ‘Ailsa’, ‘Cara’, ‘Cosmos’, ‘Desiree’, ‘Estima’, ‘Kestrel’, ‘Marfona’, ‘Picasso’ and ‘Rooster’
• Slug-resistant cultivars: ‘Charlotte’, ‘Estima’, ‘Golden Wonder’, ‘Kestrel’, ‘Pentland Dell’, ‘Pentland Ivory’, ‘Pentland Squire’, ‘Stemster’, ‘Sante’ and ‘Wilja’
• Scab-resistant cultivars: ‘Arran Comet’, ‘Arran Pilot’, ‘Charlotte’, ‘Golden Wonder’, ‘Harmony’, ‘King Edward’, ‘Maris Peer’, ‘Nadine’, ‘Nicola’, ‘Pentland Crown’, ‘Pentland Javelin’, ‘Sante’ and ‘Wilja’

Wisley Plant Centre has produced an extensive description of seed potato list each year, which is divided into categories depending on the harvest time of the crop

Links

RHS Potato Trials
RHS AGM lists

Potatoes are well-known for suffering from numerous problems. Thankfully, many of these can be controlled or managed with a little help.

Pest and disease

• Cutworms, slugs, wireworms and potato cyst eelworm may all cause damage to potato crops. Precautions against slug damage are particularly necessary when planting through polythene. Grow cultivars resistant to slugs (see Cultivar Selection above), as well as those resistant to potato cyst eelworm are available
• Potato blight can be a problem in warm, humid conditions but is not usually a problem with ‘earlies’
• Potatoes can also be effected by potato black leg, potato scabs and various potato tuber rots
• Spotting of tubers is caused by several fungal diseases, making them unsightly, but not affecting their taste, nutritional value or cooking qualities. There are no remedies available to gardeners except to try and buy disease-free seed, practise rotation and gather as soon as mature

As long as possible between crop rotations will help to limit the build-up of eelworm and soil-borne diseases. Tomatoes are closely related and suffer from the same pests and diseases and so occupy the same course of the rotation as potatoes. Due to the limited space, it is rare to be able to leave two or more years between crops, so growing cultivars resistant to eelworm is advisable.

Cultivation problems

• Second growth is caused during dry spells in late summer tubers cease to grow only to resume after rain. This can lead to misshapen, split and hollow tubers. Growing drought-resistant cultivars (see list above) and watering can help to reduce this problem
• Viruses may cause internal staining of tubers, but using virus-free seed, improving the soil and reducing stress during the growing period should avoid this
• The disintegration of tubers during cooking is associated with hot, dry growing conditions. There is no simple explanation or means of avoiding this problem. Although high dry-matter (high starch content) potatoes are said to be worst affected, there are also many instances of high dry matter ones resisting disintegration. However, low dry-matter cultivars such as ‘Kestrel’, ‘Nadine’ and ‘Picasso’ are worth trying. The damage is probably not caused by the swelling of starch, but by the separation of the cells of the potato.  Improving growing conditions with organic matter, fertiliser and watering in dry spells will help
• Frosts can damage new growth. Frosted shoots will often recover, but cropping will be delayed and reduced. Tubers frosted in the ground or in storage should be discarded

Chit early potatoes
Crop rotation
Crop sheet: potatoes
Potato cyst eelworms
Potatoes: growing for the festive season
RHS AGM Vegetables
RHS GYO Potatoes
RHS Trials: salad potatoes
Root vegetables: storing
Vegetables for exhibition
Vegetables in containers