Potatoes

Introduction

Potatoes, Solanum tuberosum are a hugely rewarding crop for the first time gardener. Freshly dug new potatoes have a flavour that is never replicated in shop-bought potatoes. Furthermore, growing potatoes in newly cultivated ground "cleans" the soil; it is dug so much through their cultivation that most of the perennial weed roots missed when clearing ground can be found and removed.

Cultivation

For gardeners, potatoes are propagated from "seed potatoes", actually disease-free tubers rather than true seeds. These should be bought in late winter and placed in a cool, light place to "chit". Through chitting, small, strong and dark shoots are produced, rather than the large pale weak ones produced by leaving in a dark place. Potato tubers can be sown from early March onwards, in a 15cm × 15cm (6 inch × 6 inch) trench in well manured ground. They should then be covered with the excavated soil, which will protect the tender shoots from all but the hardest frosts. Watering is not necessary at this stage. Shoots will emerge from the soil 4-6 weeks after planting. When plants are 15-22cm (6-9inches tall) earth up. Draw soil around the stems to form a ridge, using a small spade or draw hoe. It may well be beneficial to apply concentrated manure or a liquid feed at this stage. From this point onwards, potatoes should be watered well after more than 5 days of dry weather.

Harvest

Do not despair if a late frost damages the leaves; while this slows the crop down it will not affect the tubers. Early potatoes may be ready as soon as 3 months after planting in a good year, although cropping can take as long as 4 months if the weather is cool. For most varieties, tubers are of harvestable size around a week after the plants flower. Tubers should be dug carefully with a fork. If tubers are to be left in the ground for more than a week before harvest, the foliage should be cut back to ground level, to "fix" the skins. Maincrop potatoes should be bagged up in paper sacks for storage. Alternatively, large crops can be stored in a clamp. This a shallow hole, lined with straw and covered with loose soil to protect the tubers from frost.

Problems

There are two main problems that can affect the amateur potato grower, blight and scab. Scab is a bacterial pathogen, Streptomyces scabies, that forms lesions on tubers in dry condtions. Scab will not in itself have a great effect on yield. However the lesions formed can allow other rots to attack the tubers if they are not harvested promptly. Scab cannot be treated, however heavy watering after earthing up will greatly prevent scab from occuring. Blight is a fungal rot caused by Phytopthora infestans, that can decimate a potato crop. Blight occurs in warm moist weather, needing 48 hours in which humidity is >90% for two 11 hour periods and temperature stays above 10°C- a Smith period. In such conditions, copper-based fungicides should be applied if there are any signs of blight in the local area. Further information on blight, including an early warning alert service can be found on the blightwatch website. Should a crop be affected by blight, all foliage should be cut at ground level immediately to prevent the infection spreading to the tubers.