Sweet potato

Light and exposure

Sweet potato does not compromise on sun: it needs the warmest, sunniest spot in the garden, sheltered from the wind, ideally against a wall that catches the midday sun and gives back its warmth in the evening. Below six hours of full sun a day, the plant just ticks over without ever swelling its tubers properly.

Where the summer stays temperate, growing under a tunnel or in a small greenhouse really changes things: the heat stored during the day keeps working on the plant all night, which makes up for cool nights. In unprotected open ground, give it without hesitation the warmest, most sheltered corner you have, even if that means sacrificing a spot usually kept for another crop.

Watering

Sweet potato drinks a lot during full summer growth, especially in the dry, warm summers it likes best. Water regularly and generously right through the stem-growth phase, until about six weeks before the expected harvest, deeply rather than in frequent small doses, so the roots go looking for moisture deep down instead of staying near the surface.

Then cut back gradually on watering from that point on. Soil that is too wet while the tubers are swelling encourages disease and rot, and too much water just before harvest leaves them blander and less well suited to storage. Soil that stays moist without ever being waterlogged remains the rule, as with most root vegetables.

Soil and repotting

The ideal soil is light, sandy, deeply loosened and well drained. Heavy clay soil, which holds water and stays cold in spring, delays the start of growth and produces misshapen or forked tubers. Work the soil deeply before planting and form mounds or raised rows about twenty centimetres high: the soil warms up faster there in spring and drains better once the rain returns at the end of the season.

A sheet of black plastic mulch laid down before planting is the single most rewarding move wherever the summer lacks heat: it warms the soil by several degrees and keeps that warmth going all summer, a real boost for a tropical plant. Plant the slips through small slits cut in the plastic. On poor soil, working in some well-rotted compost at preparation time supports tuber formation without the excess fresh nitrogen that would make them split.

Temperature and humidity

This is the critical point in any temperate climate, like basil but worse: sweet potato tolerates absolutely no frost, neither in spring nor in autumn, and it stalls as soon as night temperatures drop below 15 degrees. A single cool night is enough to stop tuber growth dead for several days.

Never plant before the soil is genuinely warm, meaning above fifteen degrees ten centimetres down, which often takes three to four weeks after the last frosts, far later than the date that is enough for basil or tomatoes. At the other end of the season, watch the first frost forecasts as soon as the nights turn cool and harvest before they hit: a single frosty night on tubers still in the ground is enough to damage them and ruin their storage life.

Pruning and upkeep

Sweet potato is not pruned like a herb, but its long trailing stems do need regular attention: lift them every two or three weeks to stop them rooting at every node. A stem that roots along its length spreads the plant’s energy over a scatter of small secondary tubers instead of concentrating it on the main tubers closest to the crown.

You can also pinch out the tips of the most invasive stems to curb their spread and push the plant to branch rather than run indefinitely. The young shoots and leaves are actually very good to eat, sautéed like spinach: a bonus harvest that costs the plant nothing as long as you keep it modest.

Pests and diseases

Outside its tropical range, sweet potato still escapes most of the diseases and pests that trouble other vegetable garden crops, a crop too recent at these latitudes for its natural enemies to have really settled in. That is a clear advantage over the ordinary potato, which has to deal with blight from the first summer rains.

Slugs remain the main worry, especially on young plants freshly put in the ground, still tender and vulnerable before their leaves toughen up. Voles sometimes attack the tubers in the soil, particularly late in the season when they sense the harvest is near: fine mesh laid at the bottom of the mound at planting time limits the damage on soils prone to them.

Propagation

Propagation is done using slips, young rooted shoots taken from a tuber. The simplest method at home is to stand a tuber half in water and half in the air, somewhere warm and bright, eight to ten weeks before the intended planting date: shoots appear after a few weeks and then grow their own roots.

Once the slips are about ten centimetres long and carry real roots, gently detach them from the parent tuber and let them soak in water for a few days to strengthen their root system before planting. In a cool climate, it is often easier to simply buy ready-grown plants from a nursery or specialist garden centre in spring: growing your own slips needs a warmth and a length of time that a temperate early season rarely provides in enough quantity.

Month-by-month care calendar

Green Hub tips

• Never plant until the soil has risen above fifteen degrees: count on three to four weeks after the last frosts, far later than the date that is enough for basil or tomatoes.
• Black plastic mulch or growing under a tunnel changes everything wherever the summer lacks heat: it supplies the extra heat this tropical plant is missing.
• Lift the trailing stems regularly to stop them rooting along the way, otherwise the plant’s energy scatters over a mass of small tubers.
• Harvest without fail before the first autumn frosts, then let the tubers cure for a few days somewhere warm: they gain sweetness and store far better.