To force potatoes inside, or to grow potatoes in bags in your small garden...

Plant 2 tubers in an inside-out compost bag or extra thick bin liners in the greenhouse in February or March (or in August if you want potatoes for Christmas), without bothering to chit. Inside out, the bags are black and absorb any heat going. I use 'International Kidney' and 'Belle de Fontenay' for forcing, or 'Charlotte' for Christmas.

Roll down the sides of the compost bags to about half their height, make a few holes in the bottom of the plastic for drainage and fill the bag to about the depth of 30cms. Use one third molehills (or a soil based compost such as John Innes no 2) and two thirds compost. Avoid mushroom compost with potatoes as the lime in it promotes the proliferation of scab.

Earth from molehills will give you lovely crumbly loam where the moles have done lots of the hard work for you. They create the most delicious, friable grass-free soil from a depth usually below the worst of the weed seed.

Put in two tubers per bag and bury them in the 30cm of soil/compost mix and back fill another 15cms or so on top. Water them in well. Put your sacks somewhere bright, frost free and a little warm. Within 3 weeks or so, they will have begun to shoot.

Keep the compost damp, but not sopping wet. Once the shoots are about 15cm, roll up the edges of the bag a few turns and fill up to that level with more soil/compost mix. Carry on earthing them up, bit by bit every couple of weeks, until they reach nearly the top of the bag. Then allow the shoots to come up to flower, which should be in May, and you can start to harvest.

Harvesting tips

If you have several people to feed at once, you can turn out a whole bag at a time. It's easiest to do this into an empty wheelbarrow, but with all varieties we grew, the flavour was better when harvested and eaten straight away, not stored. Some of the sugars in the tubers convert to starch when stored so the flavour gradually disappears.

For this reason, it's worth perfecting your potato milking technique: cut off a corner of the bag and put your hand in from the bottom. Harvest and eat only what you need for that meal. You can then water the haulm from above and if you have not disrupted the root system too much, it should continue to grow.