Seed Potatoes: Chitting, Cutting and Drying Before Planting

Why not to plant potatoes from the kitchen

Potatoes left in the cupboard will sprout, but they are the wrong thing to plant. Certified seed potatoes are sold with certification that they have been inspected for virus and scab, so the risk of introducing disease to your plot has already been reduced for you.

Eating potatoes are often treated to delay sprouting, so they come up unevenly. More importantly, a virus infection is invisible from the outside: the plants look normal, the yield quietly drops, and the disease stays in your soil for years afterwards. Seed potatoes cost very little. This is not the place to economise.

The certificate comes with the bag. Keep it until harvest and you can check the variety and the supplier afterwards, which makes comparison next year possible.

Choose the variety for the dish and the season

Grown exactly the same way, a floury variety breaks down and goes fluffy while a waxy one holds its shape through cooking. In a home garden you will be happier choosing by what you want to cook, and by whether you are planting in spring or late summer, than by yield figures.

Only varieties with a short dormancy can be planted for an autumn crop. Do not assume seed sold in spring can be replanted later in the year.

Chit the tubers to get short, fat sprouts

Three to four weeks before planting, stand the seed potatoes in a single layer somewhere cool and light. At 59 to 68F (15 to 20C) in good light the sprouts stay short and come dark green and stubby rather than long and pale. This is called chitting.

Left in the dark, the sprouts come white, long and brittle, and they snap off as you plant. The replacement sprouts are weaker, emergence is about a week later, and the harvest is late by the same margin.

1. Where to stand them Somewhere with direct light and no rain, such as a porch or a cold greenhouse. Avoid anywhere that freezes at night, and lay a sheet of newspaper over them on a cold night.
2. How to lay them out One layer, not stacked, with the end that has the most eyes uppermost. Swap them around after ten days or so and they chit evenly.
3. When they are ready Sprouts a quarter to half an inch (5 to 10 mm) long, dark purple or green. Plant before they get past three quarters of an inch (2 cm).

Cut them, or plant them whole

Aim for pieces of about one and a half to two ounces (40 to 60 g). Smaller than that and there is not enough stored food to carry the early growth, so the stems come thin with few leaves. Much larger and you simply get more sprouts without a proportional increase in yield.

Small tubers that can go in whole have no cut surface at all, so there is almost no risk of rot. That suits a first attempt, and it suits a late summer planting when the soil is warm. The cost per pound is higher.

Cut lengthways from the top, then dry the faces

A potato has a scar at the end where it joined the stem, and an opposite end where the eyes cluster. The sprouts near that end are the strongest. Cut across the tuber and you end up with a piece that has almost no eyes, so always cut lengthways, from the eye end towards the stem scar.

A freshly cut, wet face against cold soil rots easily. Stand the pieces cut side up somewhere shaded and airy for two or three days, until the surface has dried and formed a thin skin, then plant.

Dusting the cuts with wood ash is a traditional practice. It draws out moisture and adds a little potassium, but a thick coating on a wet face actually slows the drying. Use it thinly, and only once the face is half dry.

1. Disinfect the knife Flame the blade or wipe it with alcohol between every tuber. Virus travels in the sap on the cut, and this alone breaks the chain.
2. Which way to cut Stand the tuber with the eye end up and cut straight down in one stroke. Divide so each piece has two or three eyes and they come up evenly.
3. When to cut You can cut on planting day, but cutting two or three days ahead and drying the faces clearly reduces rot.
4. Where to dry them Somewhere shaded, airy and out of the rain, cut side up in a single layer. In direct sun only the surface shrinks and the inside stays wet.
5. How long it takes Two or three days in mild weather, about four in cold weather. When the face no longer feels damp to a finger, it is ready.

If a wet spell means you cannot dry them, do not force it. Buying smaller tubers that can go in whole is a perfectly good decision.

For a late summer planting, do not cut at all

A late summer planting goes in while the soil can still be above 77F (25C). In those conditions bacteria get into the cut before it dries and the tuber collapses into rot. For an autumn crop, plant small tubers of one to two ounces (30 to 50 g) whole.

If all you have is large tubers, cut them and dry them in the shade for a full week until the faces are brown and hard. Even then the risk is higher than in spring, so plant a few extra.

Only short dormancy varieties suit an autumn crop. Tubers you lifted in early summer are still dormant and will not sprout if you replant them.

When nothing comes up, or the tuber rots

Before you dig anything up, count the days. At soil temperatures around 45F (7C) emergence can take three to four weeks, and a surprising number of people dig up seed that was about to come through.

After thirty days, dig to find out. A tuber still white and firm means the soil was too cold. One that has gone black and soft has rotted. Either answer tells you whether to replant this year or fix it next year.

1. The cut-off for replanting Replanting is worth it up to about six weeks after your normal planting date. After that, accept fewer plants and grow the ones you have larger.
2. Not repeating it After a year with a lot of rot, switch to small tubers you can plant whole. With no cut surface, the route in disappears.

Only dig one plant at the end of the row to check. Digging the lot dries out seed that was still alive.