Why potatoes are worth growing in containers

Potatoes are one of the most forgiving crops you can grow, and they do remarkably well in a big pot, a sack or a recycled dustbin. If your allotment soil is heavy clay, stony or riddled with wireworm, containers sidestep the problem entirely. You control the compost, you control the water, and you can move the pot into shelter when a late frost threatens. Slugs are easier to spot, blight arrives later because the foliage is above the worst of the damp, and there is something deeply satisfying about tipping out a bag and finding a dozen clean, bright new potatoes underneath. Children love it too, which is reason enough on its own.

Choosing your container and compost

Size matters more than anything else. A skimpy pot will give you a handful of marble-sized tubers, so aim for at least 30–40 litres per plant. A 45cm-wide pot, a proper potato growing sack, a builder's bag or an old plastic dustbin all work well.

• Drill or check for drainage holes — half a dozen at the base is plenty. Without them, waterlogged compost will rot the seed tuber within a fortnight.
• Put 5cm of broken crocks or gravel in the bottom to stop the holes silting up.
• Avoid clear glass or thin terracotta, which dries out and bakes in the sun. Black plastic sacks can overheat in a hot spell, so shade them on really warm days.
• Site the container somewhere sunny — six hours of direct light minimum — and out of strong wind, which snaps the tall stems.

For compost, a good multipurpose mixed with a little John Innes No. 2 gives both moisture retention and structure. Own-made garden compost is fine if it is well rotted, but sieve out any perennial weed roots first. Add a handful of potato fertiliser or fish, blood and bone per 40 litres and mix it through.

Which potatoes to grow, and how to start them

First earlies and salad varieties are the best choice for containers, because they crop quickly and you can often beat blight altogether. Reliable choices include Rocket, Swift, Arran Pilot, Pentland Javelin, Charlotte, Nicola and the blight-resistant Sarpo varieties if you want to push into late summer.

Chit your seed potatoes from late January or February. Stand them upright in egg boxes or a seed tray, rose end up, in a cool, light, frost-free spot — a porch or an unheated bedroom windowsill is ideal. You want short, stubby, dark green shoots, not long pale ones. If a tuber throws out too many shoots, rub off all but two or three.

Plant out from late March in mild southern gardens, but wait until mid-April or even early May in northern England and Scotland. Fill the container with about 15cm of compost, sit the tubers on top with the shoots facing upwards, then cover with another 10–15cm. Two or three tubers fit comfortably in a 40-litre sack if you want a decent haul.

Watering and feeding through the season

Go easy until the shoots break the surface — damp, cold compost with no roots in it is a recipe for rot. Once the foliage is up and growing, water regularly and generously. In high summer a 40-litre container can drink four or five litres a day, sometimes more. Water in the morning, aim at the compost rather than the leaves, and never let the pot dry out completely and then flood it; that is how you get hollow heart and split tubers.

Start feeding once the flower buds appear. A high-potash tomato feed diluted as directed, given weekly, keeps the plants cropping without pushing too much leafy growth. Skip anything high in nitrogen at this stage.

Earthing up: the trick that doubles your crop

This is the single most important job, and it is where container growing beats the open ground for ease. Tubers form on the buried portion of the stem, so the more stem you bury, the more potatoes you get — and the compost also stops any tubers near the surface turning green and bitter.

• When the stems are about 15–20cm tall, add compost to bury roughly two-thirds of them, leaving 5cm of leaf showing.
• Repeat every two or three weeks, three or four times in all, until the container is full to within 5cm of the rim.
• Water before you earth up, and never bury bone-dry stems in dry compost.
• Stop once the pot is full or the flowers open. Leaking tubers at the surface are just a sign you were a little late.

Harvesting, pests and what comes next

First earlies are ready around ten to twelve weeks after planting, usually when the flowers open — that is the moment to start furtling with your fingers for a few precious new potatoes. Salad varieties can be ready in eight to ten weeks. Maincrops want longer, and the signal is yellowing foliage that is beginning to flop.

Either tip the whole container out onto a tarpaulin or rummage carefully from the edge and leave the plant to carry on. Freshly lifted potatoes have skins that rub off with a thumb, which is exactly what you want for boiling with mint and butter.

Watch for slugs, vine weevil grubs in the compost and the first sign of blight in a wet August. If blight strikes, cut the foliage off at once and leave the tubers in the compost for a fortnight before lifting, so the skins set. And when the crop is done, do not reuse that compost for potatoes next year — but it is perfectly good for beans, courgettes or flowers, or for topping up the beds.