How to grow potatoes

A seed potato is a tuber grown specifically for planting and certified free of the virus and disease load that builds up in potato stock over generations. It is not a special kind of potato — it is an ordinary one with paperwork, and the paperwork is the entire value.

Supermarket potatoes are a poor substitute for two reasons. They are frequently treated with sprout suppressant, so they sit and do nothing. And they carry no assurance about virus, which accumulates invisibly, reduces yield year on year, and stays in your soil.

You can save your own tubers from a healthy crop and many people do — a potato is propagated by cloning, so a saved tuber is genetically identical to its parent and the variety persists unchanged. The catch is the same one: disease clones along with it. Save from visibly healthy plants, and buy in certified stock every few years to reset.

Choose by group rather than by name. First earlies are ready in around ten weeks and are dug small and waxy; second earlies follow; maincrop takes the longest, yields most, and is the group you store. In a short or blight-prone season, earlies are the safer bet — they are often out of the ground before blight weather arrives.

Chitting is sprouting the tubers before they go in the ground. Stand them in a cool, light, frost-free place — egg boxes are the traditional rig — with the end carrying the most eyes uppermost, and leave them for a few weeks.

Light is what matters. Sprouts grown in the light are short, stubby, green or purple and tough; sprouts grown in the dark are long, white and fragile, and they snap off when you plant. That difference is the whole technique.

It buys a couple of weeks, which matters most for earlies and least for maincrop. It is genuinely optional — an unchitted potato grows perfectly well — so treat it as a way of getting ahead rather than a requirement.

Solomon takes the same approach for dry gardening and pushes it further: he presprouts in daylight at room temperature for weeks, then plants one whole medium potato every 18 inches, barely covered.

Plant when the soil has begun to warm, about two weeks before your last frost. Frost kills the foliage but not the tubers, so a late frost on emerged shoots costs you a fortnight of growth rather than the crop.

This is the operation with no equivalent elsewhere in the garden, and the one that decides how much of your crop you can actually eat.

Potatoes form on stems, not on roots — on short underground shoots called stolons that grow from the buried stem above the seed tuber. The more buried stem there is, the more places tubers can form. Earthing up creates that stem.

It does a second, more urgent job. Any tuber exposed to light turns green, and green means solanine, which is bitter and genuinely toxic. This is not a cosmetic problem and it cannot be peeled away reliably. Every tuber must stay covered, all season.

So: when the shoots are about eight inches high, draw soil up around them from both sides of the row until only the top few inches show. Repeat once or twice as they grow, until you have a ridge. Straw or a thick mulch does the same job if you would rather not move soil, which is what growing potatoes under straw amounts to.

If you are growing in a bag or a tub, the same rule applies: start with a few inches of compost and keep adding as the shoots rise.

Late blight is the same organism that takes tomatoes, and it is the reason potato growing here has a deadline. It needs prolonged leaf wetness with mild temperatures — a Pacific Northwest late summer — and it moves fast: brown-black patches on leaves and stems, a distinctive musty smell, and a canopy that collapses within days.

The danger is that it travels down. Spores wash off the foliage into the soil and rot the tubers in the ground or, worse, in store, where one infected potato takes a sackful with it.

The response is decisive and works: the moment blight is established, cut all the foliage off at ground level and remove it. Do not compost it. Then leave the tubers in the ground for two or three weeks before lifting. That gives the spores on the soil surface time to die and lets the skins set. Cutting the tops early costs you some yield and saves the crop.

Prevention is variety choice and timing rather than spraying. Grow earlies that finish before blight weather, choose blight-resistant maincrops, space for airflow, and earth up well — a deep ridge is a physical barrier between spores and tubers.

Scab, by contrast, looks alarming and barely matters: rough corky patches on the skin, worse in dry alkaline soil, and entirely peelable. Never lime ground destined for potatoes; they want it slightly acid.

Earlies are ready when the plants flower — dig a single plant to check rather than guessing, and take what you need. They do not store; they are dug and eaten.

Maincrop is ready when the foliage yellows and dies back. Then wait another two or three weeks before lifting. That is when the skins set, and skin set is the whole difference between potatoes that store until spring and potatoes that rot in a month. A tuber whose skin rubs off under your thumb is not ready.

Lift on a dry day, as the photograph at the top of this page shows, and leave the tubers on the surface for a few hours to dry. Then cure them somewhere dark and airy for a week or two at cool room temperature, which lets minor damage heal over.

Store in the dark, cool but frost-free, in paper or hessian sacks that breathe. Never plastic, and never the fridge — cold converts starch to sugar and makes potatoes sweet and prone to darkening when cooked.

Check the sack occasionally. One rotting tuber will take the rest with it, and that is the ordinary way a good crop is lost after it has been successfully grown.