If your garden is little more than a few paving slabs, a balcony, or a courtyard that catches the sun, you can still grow a genuinely useful crop of potatoes. Growing in bags is one of the most forgiving ways to do it, and it suits the way we actually garden in Britain — unpredictable weather, limited space, and a hankering for something home-grown with a bit of soil still clinging to it. Once you have tried a couple of bags of Charlotte or Rocket, tipped them out on a sunny morning and boiled a panful within the hour, you will wonder why you ever thought you needed a proper plot.

Why bags beat beds when space is tight

Potato bags work on the same principle as the old trick of growing spuds in a stack of tyres: you keep burying the stems, and they keep producing tubers along their length. The difference is that a purpose-made grow bag, usually 35–40 litres with handles and a flap at the front, is far tidier and can be moved when you want to rearrange the patio. You can also simply turn a spent compost bag inside out, punch a few drainage holes in the base, and use that instead. It is cheap, it works, and it stops the bag sitting about waiting for a trip to the tip.

There are real advantages beyond saving space. Compost in a bag warms up faster in spring than open ground, so you can start earlier. There are no soil-borne diseases lurking, no moles, and far fewer slugs than you would find in a wet border. Best of all, when the crop is ready you tip the whole thing out, pick up your potatoes, and use the spent compost on the flower beds.

Choosing and chitting your seed potatoes

Buy certified seed potatoes rather than using kitchen leftovers — supermarket spuds are often treated to stop them sprouting and may carry disease. For bags, first earlies and second earlies are the best value: they crop quickly, they taste superb as new potatoes, and they leave you time to grow something else afterwards.

• First earlies: Rocket, Swift, Arran Pilot — ready in roughly 10–12 weeks.
• Second earlies: Charlotte, Nicola, Kestrel — 13–15 weeks, excellent flavour and good for salads.
• Maincrop: Sarpo Mira, Cara — heavier crops, but they occupy the bag for 18–22 weeks.

Chitting means letting the tubers sprout before planting, which gives them a head start. Stand them upright in an old egg box, rose end (the end with the most eyes) uppermost, in a cool, light, frost-free spot such as a windowsill in an unheated room. Within about six weeks you will have sturdy green-purple shoots roughly 2 cm long. Rub off any extra shoots if you want fewer but larger potatoes; leave them all if you want a bigger haul of smaller ones. If shoots are long, pale and spindly, the spot is too dark and warm.

Planting and the earthing-up routine

Start in late March or April once the worst of the frost has passed, or in a sheltered spot under a cloche a couple of weeks earlier. Put 10–15 cm of peat-free multi-purpose compost in the base of the bag, then sit three to five chitted tubers on top, spaced evenly, shoots pointing up. Cover with another 10–15 cm of compost and water well.

As the shoots emerge and reach about 15–20 cm tall, add more compost so that only the top 5 cm of foliage remains visible. Repeat this earthing-up two or three times as the stems grow, until the bag is nearly full. Each time you bury the stems, new tubers form along them, which is exactly why bags crop so generously from such a small footprint. Keep the bag somewhere that gets at least six hours of sun a day — a south-facing wall is ideal, as the warmth reflected back off the brick will speed things along.

Watering and feeding for a proper crop

This is where bag-grown potatoes are won or lost. Compost in a bag dries out far faster than a bed, and irregular watering causes hollow heart and splitting. Check the compost by pushing a finger in to the second knuckle. In spring, water when it feels dry; from June onwards, in warm weather, that could be every day, sometimes twice on a scorching day. Always water until it runs out of the drainage holes, and never let the bag sit in a saucer of standing water.

Once the first flowers open, begin feeding weekly with a high-potassium tomato feed. This is the point at which the plant stops building leaves and starts filling tubers, and the extra potash makes a real difference to yield and flavour. Keep an eye out for slugs on wet evenings, and for blight — dark, greasy patches on the leaves — during warm, humid spells. If blight strikes, cut the foliage off immediately and leave the tubers in the compost for a fortnight before harvesting, so the skins firm up.

Harvesting and getting a second crop

For new potatoes, start rummaging from about ten weeks, feeling gently under the compost for a few tubers while the plants are still green and growing. Take only what you need for a meal; the rest will carry on swelling. For a full harvest of first and second earlies, wait until the flowers fade and the foliage begins to yellow, then tip the bag out onto a sheet, rub off the compost, and pick out your crop.

Maincrop varieties should be left until the tops have died right back, giving the skins time to set so the potatoes store well. Dry them somewhere airy for a few hours, then keep them in a paper sack in a cool, dark place — never in the fridge, as the starch turns to sugar.

One last thought: bags are perfect for a cheeky second crop. Plant a few chitted first early tubers in fresh compost in late July or early August, keep them well watered, and protect them from early frost with fleece. With a bit of luck you will be lifting your own new potatoes on Christmas morning, which is about as rewarding as patio gardening gets.