Techniques
Chitting
Letting seed potatoes sprout short, sturdy green shoots before planting, to give them a head start and an earlier crop.
Chitting simply means encouraging your seed potatoes to grow short shoots, called chits, before they go into the ground. It is one of the easiest jobs in the gardening year and a lovely sign that the season is getting going.
To chit, stand each seed potato with its "rose end" pointing upwards. The rose end is the one with the most little eyes (dormant buds) clustered together; the other end is where it was attached to the parent plant. An empty egg box is the classic holder because it keeps each tuber upright and tidy, but a seed tray or shallow box works just as well.
Put the box somewhere cool, bright and frost-free, such as a windowsill in an unheated spare room, a porch, or a light shed. Light is important: it keeps the shoots short and tough. Too much warmth and not enough light, and you get long, pale, weak shoots that snap off easily when you plant.
What good chits look like
After around six weeks you are aiming for shoots about 1.5–2.5cm long that are stubby, dark green or purplish, and firm. Short and sturdy is exactly right. If shoots are long, white and floppy, your spuds have been too warm and dark, so move them somewhere cooler and lighter. Some gardeners rub off all but three or four of the strongest shoots per tuber for fewer, larger potatoes, but you can happily leave them all on for a heavier crop of smaller ones.
When to start in the UK
Begin in late January to February. That gives the chits time to form before the soil is warm enough and the heaviest frosts have eased, usually March for first earlies and April for second earlies in much of the country. Chitting itself does not protect against frost, so still wait for the right planting window, and have a cloche or some fleece handy if cold nights threaten the emerging foliage.
Which potatoes benefit most
Chitting gives the biggest head start to first and second earlies, where the goal is an earlier harvest of new potatoes. Maincrop varieties are in the ground for far longer and have plenty of time to bulk up anyway, so chitting them makes little difference; many growers simply plant maincrops straight from the bag.
Whether or not you chit, the real work begins once shoots appear above the soil. Keep an eye on growth and start earthing up to protect new tubers from light and frost.
In a UK garden
In the UK, start chitting in late January or February so first earlies are well sprouted and ready to plant out once the worst frosts have passed in March or April.
Example
Stand seed potatoes rose-end up in an old egg box on a cool, bright windowsill, and in about six weeks they grow short green shoots ready for planting.