Sweet potatoes: the watering mistake that costs up to 60 per cent of the harvest

Many amateur gardeners dream of plump, sweet sweet potatoes, only to end up disappointed by thin little roots in the bed.

At first glance, growing them appears straightforward: put the plants in the soil, water regularly, and that is it. Yet this is precisely where a large share of crops goes wrong. One particular watering habit can ruin up to 60 per cent of the tubers. Anyone who understands the plant’s needs for water and warmth can reverse the result entirely and fill their winter storage boxes.

Why sweet potatoes often struggle in the garden

Sweet potatoes originate in the warm parts of South America. They thrive in heat, dislike cold and do not cope well with heavy, wet soil. Below 10°C, growth virtually stops. In exposed, draughty positions, the plant can become chilled internally before the weather even feels cool outdoors.

They also need loose soil that allows the tubers to spread. Compacted ground produces narrow, misshapen roots. If you have clay soil, it must therefore be loosened with sand and compost worked in; ideally, you should also create ridges.

“Three factors are crucial for a plentiful sweet potato harvest: warmth, loose soil – and a wisely chosen watering time.”

July is an especially critical period: plants are growing vigorously, temperatures are rising, and many gardeners instinctively reach for the watering can more often. That is exactly when the trouble begins.

The watering mistake that costs up to 60 per cent of the harvest

For sweet potatoes, the greatest vulnerability is not too little water but watering at the wrong time. Regular watering at midday or in the early afternoon risks three problems at once:

• Plants can stall from cold shock when cold water meets overheated roots.
• Much of the water evaporates before it can reach the roots.
• Wet foliage in intense sunshine can suffer localised scorching and become more prone to disease.

Trials and observations from gardens in temperate climates have shown that poorly timed watering during high summer can lead to yield losses of up to 60 per cent. The plant then directs its energy into coping with stress rather than forming substantial tubers.

How to water sweet potatoes properly

The most important rule is to water in the evening rather than in broad daylight. At that point the ground is warm, the sun is losing strength, and the plant can take up the water undisturbed overnight.

“Watering in the evening supplies the roots for many hours and prevents stress from sun and evaporation.”

A practical watering schedule looks like this:

• Immediately after planting: Water in thoroughly so that the soil makes good contact with the roots.
• During the first few weeks: Keep the soil lightly moist, but avoid waterlogging. A finger test is enough: if the soil feels dry at a depth of 3–4 cm, water it.
• In high summer: Water deeply but infrequently. It is better to give larger amounts less often than small daily drinks.
• Towards the end of the season: Gradually reduce watering to prevent cracked tubers.

Anyone who mulches with, for example, dried grass clippings or leaves will need noticeably less water, as the soil stays cool and moist for longer.

The ideal location and soil preparation for sweet potatoes

Sweet potatoes favour a spot in full sun that is sheltered from cold winds. A south-facing slope, a bed beside a house wall or a raised bed is particularly suitable because the soil warms up more quickly there.

Experienced gardeners recommend a ridge system for preparing the soil:

• Ridge height: around 15–20 cm
• Distance between ridges: approximately 90 cm
• Spacing between plants on the ridge: 30–40 cm

The ridges heat up faster, channel excess water away to the sides and give the tubers room to develop outwards. In heavy soil, it is worth mixing in plenty of coarse compost and sand.

From sprout to vigorous plant: what “slips” are

Unlike ordinary potatoes, sweet potatoes are usually not planted from pieces of tuber but from what are known as “slips”. These are young, rooted shoots cut from a pre-sprouted sweet potato.

This simple home method works as follows:

• Insert three toothpicks around a healthy sweet potato.
• Suspend it over a glass of lukewarm water, with its lower section sitting in the water.
• Keep it somewhere warm and bright, but out of direct sunlight.

After a few weeks, several green shoots will emerge. Cut these shoots away with a small piece of the tuber attached and place them in a glass of water. Once a fine network of roots has formed, the slips are ready to plant out.

“A few warm weeks on the windowsill are enough to produce dozens of young plants from one sweet potato.”

Slips should not be moved outdoors until the soil has consistently reached at least 18°C and the risk of late frost has passed. Cool nights in May can otherwise halt the young plants straight away.

Summer care: earthing up, mulching and staying alert

Once the shoots are roughly 20 centimetres long, it is worth earthing them up. This means drawing soil around the base of the plant to form a small mound. Additional tuber sites develop on these mini ridges.

Then add a layer of mulch over the ridges. Particularly effective materials include:

• dried grass clippings
• leaves from the previous year
• straw-like plant debris

Mulch retains moisture, prevents surface crusting and suppresses weeds. Occasional shallow hoeing between the rows is then sufficient to keep the surface open.

Gardeners should watch for slugs on young plants and aphids on the shoots. A severe infestation weakens plants and leaves them more vulnerable to drought and heat stress, making the watering mistake described above even more damaging.

Harvesting, sweetness and storage: getting the most from sweet potatoes

Depending on the variety and weather, the period between planting and harvest is about 100 days. The foliage provides an initial clue: once it begins to yellow, it is worth carefully testing one plant with a small dig.

Care is needed when lifting the crop. The skin of freshly harvested tubers is extremely delicate and tears easily. It is better to insert a garden fork some distance from the plant and loosen the ridges section by section, rather than levering directly underneath.

“Freshly dug sweet potatoes are not yet at their sweetest – they need a period of ripening in warm, dry air.”

For this so-called “curing” phase, lay the tubers out in a shaded, airy place at around 20–25°C for two weeks. Their skins thicken slightly, cut surfaces heal, and starch is converted into sugar. Only after this should the tubers be moved to a cool store.

Why water and temperature management must work together

Anyone aiming to genuinely increase their harvest should always consider watering and warmth together. Heavy watering when the soil is cool leaves the roots “sitting in the wet”. The tubers split more easily, while rot finds ideal conditions. Large amounts of water applied to hot soil in blazing sunshine, on the other hand, cause the heat shock already mentioned.

One useful trick is to place a simple soil thermometer in the bed to show when watering is worthwhile. If the temperature remains clearly above 18°C in the evening, water can reach the roots effectively without stressing the plants.

Practical additions for more reliable harvests

In cooler regions, black mulch film or fleece covers in spring can help bring the soil up to temperature more quickly. With limited space, sweet potatoes can also be grown in large containers or builders’ tubs. Good drainage and high-quality, loose compost are essential in that case.

Combining them with other crops around the edge of the bed can also be worthwhile: low-growing herbs such as thyme or oregano attract beneficial insects that keep pests under control. Tall shade-casting plants such as maize planted directly beside them are unsuitable, however, because sweet potatoes truly need plenty of sunshine.

By keeping these points in mind – especially the right time to water – you can turn sweet potatoes from an exotic experiment into a dependable, high-yielding crop in your own garden. The previously “lost” 60 per cent of the harvest can be recovered surprisingly quickly.