Why Grow Potatoes in Bags at All?

If you've only got a patio, a balcony or a narrow strip of paving, potatoes can seem like a crop for people with allotments. They're not. A sturdy potato bag — usually a woven plastic or heavy-duty fabric sack of around 35 to 40 litres, with handles and drainage holes — will happily raise two or three plants, and you can shuffle it about to catch the sun or tuck it against a warm wall when frost threatens.

There's a practical bonus, too. Because you grow in fresh compost and can move the bags, soil-borne problems are far less likely to build up, and slugs are easier to spot. Best of all, you can start a few bags every fortnight and have new potatoes from early summer well into autumn.

Choosing Tubers, Bags and Compost

Buy certified seed potatoes rather than using supermarket spuds. They're grown to be disease-free and sold in sensible quantities. For bags, first earlies and second earlies give the best return because they mature quickly. 'Rocket' and 'Swift' are fast and dependable, 'Charlotte' is a superb salad potato, and if you fancy a maincrop, choose a blight-resistant variety — our damp summers are unforgiving.

• The bag: 35–40 litres, drainage holes in the base, and roll-down sides if you can find them. They make earthing up and rummaging for early spuds far easier.
• The compost: peat-free multi-purpose is fine. Avoid garden soil — it's heavy, drains poorly in a bag and can carry pests.
• The feed: a handful of balanced granular fertiliser or fish, blood and bone stirred through the compost at planting.
• The quantity: three tubers per bag for first earlies, two for maincrops. Any more and they'll compete for water and give you less.

You'll get through roughly 50 litres of compost per bag once earthing up is finished, so buy a little more than you think you need.

Chitting: Getting a Head Start

About six weeks before you plan to plant — late February is about right for most of the UK — stand your tubers upright in an old egg box or seed tray, with the rounded end carrying the most "eyes" facing upwards. Put them somewhere cool, light and frost-free. An unheated bedroom windowsill is ideal.

You're after short, stubby, dark green shoots rather than long pale ones, which mean the spot is too warm or too dark. When the shoots reach roughly 1–2cm, rub off all but the strongest three or four. That's the whole job. Chitting isn't essential, but it usually brings the harvest forward by a fortnight, which matters when you're trying to beat blight later in the season.

Planting in Layers and Earthing Up

Start with about 10cm of compost in the base of the bag. Sit the tubers on top, shoots uppermost, evenly spaced, then cover with another 10cm. Water gently and stand the bag somewhere sunny and sheltered.

As shoots push through, add more compost to bury roughly two-thirds of each stem, leaving the top few centimetres of leaf showing. Repeat each time they break the surface until the bag is about three-quarters full — usually three or four top-ups. This is what gives you a long stem with tubers forming all the way up it rather than one measly layer at the bottom. Stop filling at three-quarters so there's still room to water properly.

Keep fleece or an old cloth to hand. A late frost will blacken the tops overnight. If it happens, don't panic — just wait for fresh growth to come through.

Watering, Feeding and Everyday Care

Bags dry out fast, especially on a sunny patio, and dry compost is the single most common reason for a disappointing crop. Check daily in hot weather by pushing a finger in to the second knuckle. Aim for consistently moist, never waterlogged. Little and often beats a flood every few days.

Once the plants begin to flower, switch to a high-potash liquid feed every fortnight — the same sort you'd give tomatoes. Watch for slugs grazing young shoots, and for blight during humid spells: brown patches spreading inwards from the leaf edges. If blight strikes, cut the foliage off at compost level and leave the tubers in the bag for two or three weeks before lifting, so the skins firm up.

Harvesting Your Crop

First earlies are usually ready 10–12 weeks after planting, often when the flowers open. Don't tip the whole bag out at once. Unroll the side, feel about for a few tubers, then roll it back up. The rest carry on growing, and you get that unbeatable moment of scraping new potatoes straight into the pan.

Second earlies take about 13–15 weeks; maincrops 18–22 weeks, once the foliage has yellowed and died back. Leave maincrops for a fortnight after the tops go over so the skins set, then tip the bag onto a sheet. Brush off the compost and store sound tubers in a paper sack somewhere cool, dark and frost-free. Any with cuts or holes should be eaten first.

When you're finished, tip the spent compost onto the borders or into the compost bin, scrub the bag and store it dry for next year. Then start another one — there's usually still time for an autumn crop.