Why a Bag Beats a Bed

Potatoes take up a lot of room in a bed, and if you only have a patio they can seem out of reach. Growing them in bags solves that neatly. A strong 40-litre bag of compost against a sunny wall will give you a couple of kilos of new potatoes, and you can shuffle it about to chase the sun. There is no digging, no weeding between rows, and no stray tubers left behind to sprout in next year's carrots.

The other advantage is control. Bag-grown potatoes sit in fresh, clean compost, so they dodge a good deal of the eelworm and scab that build up in open ground. You can start them early under cover and shrug off a late frost with nothing more than a sheet of fleece. For a small patio, few crops give back as much for as little space.

Choosing Bags and Compost

Look for woven plastic sacks of 40 to 50 litres with handles, or simply turn a used compost bag inside out and give it a second life. Whatever you use, drainage comes first: punch six to eight holes in the base and a few along the lower sides, then stand the bag on pot feet or bricks so water can run away freely. A bag that sits flat on paving will waterlog in a wet spell and your tubers will rot.

For compost, plain multipurpose works but dries out alarmingly fast in a bag. A 50/50 mix of multipurpose and a soil-based compost holds moisture far better. Stir in a handful of slow-release potato fertiliser or blood, fish and bone before planting. Never use garden soil — it is heavy, compacts badly in a bag and arrives complete with weed seed.

• 40-litre bag: three seed potatoes
• 50-litre bag: four to five seed potatoes
• Position: full sun for at least six hours a day

Chitting Your Seed Potatoes

Buy certified seed potatoes rather than supermarket ones, which are often treated to stop them sprouting and may carry disease. Stand them rose end up — that is the blunt end with the most eyes — in egg boxes or trays in a cool, bright, frost-free spot. A north-facing windowsill or an unheated porch is ideal. Start first earlies in late January, second earlies in February and maincrops in March.

Light matters more than warmth. In the dark you get pale, brittle shoots that snap off; in good light you get short, sturdy green-purple ones. After four to six weeks they should be about 2cm long. Rub off all but the three or four strongest shoots on each tuber.

Reliable varieties for bags include 'Rocket' and 'Arran Pilot' as first earlies, 'Charlotte' as a second early, and 'Sarpo Mira' as a blight-resistant maincrop.

Planting and Earthing Up

Put 10–15cm of compost in the bottom of the bag, sit the tubers rose end up about 10cm apart, then cover with another 10–15cm. Water lightly and keep the bag somewhere sheltered. First earlies can go out from mid-March if you can protect them; otherwise wait until early April.

Now comes the part that decides your crop. When shoots reach 15–20cm, add compost to bury them, leaving just 3–4cm of leaf showing. Repeat two or three times until the bag is nearly full. This is the step people skip and it costs them dear: buried stems throw out extra tubers, and any tuber pushed near the surface turns green and becomes inedible. Stop when the compost is within 5cm of the rim.

Watering, Feeding and Frost

Bag potatoes dry out fast, so water thoroughly once shoots break through and keep the compost evenly moist from the moment flowers appear — that is when tubers are swelling. A dry spell followed by a drenching gives you split or hollow potatoes, so be consistent. Soak until water runs from the base, preferably in the morning. Saucers can help in a heatwave, but tip them out if rain is forecast.

From the first open flowers, feed every fortnight with a high-potassium tomato feed. Go easy on nitrogen or you will get towering haulms and precious few spuds. Tall stems in a light bag catch the wind, so move bags against a wall or weigh the base down. If frost is forecast, drape fleece over the foliage or wheel the bags into a porch — a late May frost can blacken the lot overnight.

In a wet summer, watch for blight: brown patches spreading through the leaves. Cut the haulm off at ground level, wait a fortnight, then harvest. The tubers will store far better for it.

Harvesting and the Compost Afterwards

First earlies are ready about 10–12 weeks after planting, usually when the flowers open. Have a feel down the side of the bag and lift a few new potatoes, or tip the whole lot out onto a sheet. Second earlies take 13–15 weeks and maincrops 18–22, once the foliage yellows and dies back. If you plan to store them, leave the tubers on the surface for an hour so the skins set.

The spent compost is still worth having — spread it on borders or use it for next year's containers, though not for potatoes again. Give the bags a scrub, let them dry and store them flat. Come next spring you will be ready to start the whole happy cycle again.