Sweet potatoes are an important source of vitamins and minerals, especially vitamin A if you consume the orange variety. The purple varieties are one of the reasons why the Okinawan diet is so special. In Africa, they are seen as the vegetable which will help alleviate serious shortcomings in vitamin A shortages.
Most sweet potatoes are purchased as tubers known as slips in the USA. It takes between 90 and 120 days from planting a tuber to harvesting. We reckon that 5 slips will produce between 7.5 kg and 10 kg of produce but this depends on location. The further North we go the lower the yield. It can get as low as 1 or 2 kg of tubers per slip but at least 2 kg when grown in the south of the country.
Preparing The Bed
Sweet potatoes like to be grown in a weed free environment so make sure the bed is free of these critters before and during growth. Just till and hoe the bed about 2 weeks before planting. Plant the slips directly in the ground or start off in a raised ridge a bit like asparagus. If they are higher up in the ridge, they warm up faster in the Spring sunshine and it helps with the drainage.
Always plant after the danger of frost has passed. I plant mine in the greenhouse which is relatively frost-free but an unheated greenhouse. The soil should be at least 65°F. Place the slips about 3″ deep. Space them a foot apart as the plants will grow relatively big.
If the sweet potato is a vining type, the rows can be just 3 or 4 inches apart. Rows are closer for the bushier varieties.
The tubers/slips often look careworn on arrival but they soon perk up on planting. To revive them immerse in a glass of water so that just 3/4 of the stem is submerged. Leave for a week as they revive and then start potting up.
The tubers like to grow in full sun but can stand a small amount of shade.
Handling Plug Plants
Either plant directly into a heated poly tunnel, greenhouse or 30 cm container or nurture in 9 cm pot indoors until ready for final planting. Fill a 9 cm pot with compost and shake soil level with top of pot. Do not firm. With your finger make a hole large enough to take the root ball of the plug plant and push in the plant so that the root ball sits just below the surface. Shake the pot so compost levels, water and keep warm indoors (+15C).
Sweet potatoes take a little bit of time to put out leaf. Don’t worry as they will soon begin to sprout when the soil starts to warm up. keep weeding but take care not to cut off emerging shoots. Add some fertilizer to promote leaf growth but this will not necessarily increase root content. Look out for slugs and snails eating the foliage. In the USA, deer and squirrels are a problem but in the UK it’s likely to be a voracious hedgehog that is the culprit.
Make sure the slips/tubers are well watered but they do not like waterlogging at all ! The tubers will rot standing in water.
Harvesting Sweet Potatoes
Pick the leaves as these are edible. The leaves can also be clipped to neaten the bush. Stop watering about 3 weeks prior to harvesting the roots. It stops the mature tubers from splitting. Harvest before the first frosts appear. You should also notice the leaves turning yellow as the plants mature. This indicates they are entering senescence.
The tubers are only small if they were grown in cold conditions. Some UK summers are now so warm that the tubers resemble those grown in Spain or Italy or the southern USA states.
Storage Of Sweet Potatoes
Allow the sweet potato to convert its starch to natural sugar. It also toughens the skin so that they retain their properties for long-term storage. Store in a warm room for between 80°F to 85°F and with high humidity for at least 10 days. keep then in a dark, cool room with good ventilation. Prevent mice and rats gnawing the roots by keeping in a mouse-free box. Most harvesting will start from the end of June and end before October.
Caroline Ruby – a common variety for growth in the UK