How do you stop homegrown potatoes from sprouting and going green in storage?

Potatoes stored incorrectly either sprout within weeks, rot from inside, or turn green and become mildly toxic — three separate problems with three different causes. Understanding what triggers each lets you address them before they waste your harvest. A properly stored mainscrop potato harvest will stay in good condition from October through to March or April, giving you six months of home-grown eating without refrigeration or processing.

Why potatoes turn green — and whether they're safe to eat

Green colour in potatoes is caused by exposure to light, which triggers the production of chlorophyll and also the alkaloid solanine. Solanine in small amounts causes a bitter taste; in large amounts it can cause nausea. Potatoes that are only slightly green — a small patch on the surface — are safe to eat if you cut away the green and any flesh beneath it. Potatoes that are extensively green throughout should not be eaten. The solution is simple: store potatoes in complete darkness. Paper sacks, hessian bags, wooden boxes with a lid, or any opaque container all work. Never store potatoes in clear plastic or where daylight reaches them.

Curing potatoes before storage

Curing is a brief period after harvest where the potato skin thickens and toughens, healing any minor cuts or abrasions from lifting. Without curing, damaged skin is an entry point for rot that will spread through stored tubers. After lifting, lay the potatoes out in a single layer in a cool, dark place for ten to fourteen days. A temperature of 10–15°C is ideal for curing. After this period the skins will be noticeably tougher and the potatoes are ready for long-term storage.

Ideal storage conditions

Potatoes store best at 4–7°C — cooler than a domestic fridge, warmer than freezing. Below 4°C the starches convert to sugars and the potato tastes unpleasantly sweet when cooked. Above 10°C sprouting accelerates and quality drops quickly. A frost-free outbuilding, shed, or basement that stays cool through winter is ideal. The storage space should also be slightly humid — around 85–90% relative humidity — so the potatoes don't desiccate and shrivel. Storing in hessian or paper sacks, rather than plastic, maintains the right humidity balance while allowing airflow.

Potatoes must not freeze. A single hard frost will turn stored potatoes to a watery mush that cannot be salvaged. If your storage area risks freezing, insulate with old blankets or sacking.

Why potatoes sprout in storage

Sprouting is triggered by warmth, light, and the natural dormancy period ending. Maincrop varieties harvested in autumn have a natural dormancy period of several weeks, after which they will sprout given any opportunity. Keeping them cool (below 7°C) and in total darkness delays sprouting significantly. Storing potatoes near apples also accelerates sprouting — apples produce ethylene gas that breaks dormancy. A lightly sprouted potato with short, white sprouts is still fine to eat after removing the sprouts; a potato with long green sprouts has lost significant nutritional value and is beginning to soften.

Sorting and checking during storage

Check stored potatoes every two to three weeks. Remove any that are soft, shrivelled, or showing signs of rot. A rotten potato has a distinctive unpleasant smell and often collapses under light pressure. Remove it immediately — the bacterial rot it harbours will spread to adjacent tubers. Sorting promptly at each check is what separates a harvest that lasts five months from one that succumbs to progressive rot.