Irish Potato Harvesting, Curing and Storage: Stop Rot, Greening and Early Sprouting

A potato farmer in Nyandarua or Meru can grow a good crop and still sell it for less than it cost to produce. The usual story goes like this: the crop is dug in a hurry the moment a broker arrives, tubers are thrown into a heap in the sun, skins rub off in the bag, and within two weeks the bruised ones are rotting and the rest are sprouting or turning green. The farmer has no choice but to sell at the lowest price of the season, alongside everyone else who did the same.

Irish potatoes are alive after harvest. They breathe, they lose water, they heal wounds and, sooner or later, they sprout. Every step between the field and the buyer either works with that biology or against it. Getting it right does not need a cold room. It needs well-timed harvesting, gentle handling, a short curing period, and a dark, cool, airy place to keep the crop.

This guide covers the post-harvest chain for ware potatoes (potatoes for eating) and seed potatoes on a Kenyan smallholding: when and how to harvest, how to cure, how to stop greening and early sprouting, how seed storage differs from ware storage, and how to bag and weigh for sale. Dormancy periods and storage life vary by variety and conditions; your seed supplier, county extension officer or KALRO can advise for your variety.

Start Before the Harvest: Let the Skins Set

A newly formed potato has a thin, loose skin that rubs off at the slightest touch. Skinned patches lose water fast, darken, and give rot organisms an easy way in. The skin firms up, or "sets", in the weeks after the leaves and stems (the haulm) die back.

On a crop that is dying back naturally, wait until the haulm has yellowed and dried. Where the crop is still green at the planned harvest date, many farmers cut or slash the haulm at ground level and leave the tubers in the soil for about two weeks before digging. This is called dehaulming. It stops tuber growth at the size you want and gives the skins time to set. It also helps where late blight is on the leaves, because removing infected foliage before harvest reduces the chance of spores washing down onto the tubers. Remove the cut haulm from the field rather than leaving it in heaps on the ridges.

Test before you dig the whole field. Lift a few tubers from different parts of the plot and rub the skin firmly with your thumb. If it slides off easily, the skins are not ready; wait several more days. If it holds, you can harvest.

Harvesting Without Bruising

Bruises and cuts made at harvest are the main source of storage losses, and most of them can be avoided.

Dig when the soil is workable, not waterlogged. Tubers dug from mud carry heavy soil into storage and are more easily damaged. In the short rains, pick a dry spell and dig in the cooler morning.

Dig from the side of the ridge. Drive the jembe or digging fork into the soil at the side of the ridge, well away from the stem, and lever the ridge over, rather than chopping straight down onto the tubers. A lighter jembe gives more control for this than a heavy one.

Handle tubers like eggs. Do not throw them into buckets or bags from a height, do not stand on full bags, and do not drop full bags off a lorry. Every impact causes internal bruising that shows up later as dark patches inside the tuber.

Keep them out of the sun. Tubers left in the open after digging heat up and start to turn green within hours to a day or so. Gather them into a shaded heap or under cover as you go, and cover them with sacks or leaves if you cannot move them straight away.

Grade as you pick up. Separate tubers into at least three lots: sound tubers for sale or storage, damaged or diseased tubers for immediate home use or disposal, and (if you keep your own seed) well-shaped seed-sized tubers from healthy plants. Never mix cut or rotting tubers into the storage lot.

Curing: The Two Weeks That Decide Storage Life

Even careful harvesting leaves some small cuts and scuffs. Potatoes can heal these by forming a corky layer over the wound, a process called curing or wound healing. Healed tubers lose less water and resist rot far better in storage.

To cure, keep the sound, graded tubers for roughly one to two weeks in a dark, well-ventilated place at moderate temperature, with enough humidity that they do not shrivel. In practice, on a Kenyan farm this usually means a shaded, airy store, a well-ventilated room, or heaps covered with dry grass or sacks under a roof, never under plastic sheeting, which traps heat and moisture. Spread them in shallow layers rather than deep heaps so air can move around them. After curing, sort once more and remove any tuber that has started to rot.

Freshly dug potatoes sold immediately do not need curing. Any potatoes you intend to hold for more than a couple of weeks do.

Greening and Sprouting: The Two Clocks in the Store

Greening. When tubers are exposed to light, they produce chlorophyll and turn green. The green colour is harmless in itself, but it comes with a rise in glycoalkaloids (mainly solanine), natural compounds that taste bitter and can cause illness if eaten in quantity. Green tubers lose value at the market, and green parts should be cut away generously or the tuber not eaten. The cure is simple: keep ware potatoes in the dark from the moment they are dug to the moment they are sold. Even the light in a market stall turns tubers green over a few days, which is one reason buyers prefer them fresh from dark storage.

Sprouting. After harvest, each potato goes through a period of dormancy during which it will not sprout even in good conditions. The length of that dormancy depends mainly on the variety, and warmer storage shortens it. Shangi, the most widely grown variety in Kenya, is well known among farmers and researchers for its short dormancy, which is part of why it sprouts quickly in storage and is usually sold soon after harvest. Varieties with longer dormancy can be held longer. Once sprouting starts, the tubers lose water and quality, and buyers mark them down.

Cool storage slows sprouting. In much of the Kenyan highlands, a well-shaded, ventilated store keeps tubers cooler than the outside daytime air, especially if it is opened to the cool night air and closed during the hot afternoon. Do not try to store potatoes in a household fridge for long; very low temperatures make tubers taste sweet and fry dark. Chemical sprout suppressants exist, but only use a product registered for that use in Kenya, and follow its label and your agronomist's advice.

Storing Seed Potatoes: Diffused Light Stores

Many Kenyan farmers keep part of their crop as seed for the next season. Seed stored in a dark heap tends to produce long, pale, brittle sprouts that break off when the seed is handled and planted, which delays and weakens the new crop.

The diffused light store (DLS), promoted in East Africa by the International Potato Center (CIP) and national research and extension programmes, solves this. Seed tubers are spread in shallow layers, one to a few tubers deep, on slatted wooden shelves in a building with a roof that keeps out direct sun and rain, and open or netted sides that let in indirect light and air. Under diffused light, the tubers turn green and produce short, thick, green sprouts that survive handling and give a stronger start. The greening also makes the tubers less attractive to some pests. Because greened tubers are not for eating, keep seed and ware lots clearly apart and label them.

A small diffused light store can be built from local poles, timber off-cuts and a roof of iron sheets or thatch. Keep shelves off the floor, leave space between shelves for air and for inspection, and screen the sides against rodents and chickens. Use only healthy-looking tubers from healthy plants as seed, and remember that farm-saved seed builds up diseases over successive seasons. Certified or quality-declared seed from a recognised multiplier gives a cleaner start; your extension officer can point you to sources in your county.

Store Pests and Store Hygiene

The potato tuber moth is the best known storage pest of potatoes in Kenya. Its larvae tunnel into tubers, leaving them unsaleable, and infestation often begins with eggs laid on exposed tubers in the field or at harvest. Keep tubers well covered with soil during the season (good ridging helps), do not leave dug potatoes lying in the field overnight, and keep the store clean.

Before the new crop comes in, remove every old tuber, sprout and piece of rotten potato from the store, sweep it out and repair screens. Check stored potatoes every week or two, removing any soft, rotting, wet or tunnelled tubers before they affect their neighbours. A single rotting tuber in a sack can spread rot to the rest. If you consider any storage protectant, use only one registered in Kenya for stored potatoes and follow the label; tubers for eating must never be treated with products meant for seed.

Bagging, Weighing and Selling

For years, Kenyan potato farmers lost money to the oversized "extended" bag, packed well above its rim and sold at a single bag price whatever it really weighed. Kenya's Irish potato regulations under the Crops Act set a maximum package of 50 kilogrammes, aimed at ending that practice, though enforcement has varied from county to county. Whatever the rules in your market, the answer for the farmer is the same: know the weight of what you sell.

Weigh each bag as you fill it, write the weight on a tag, and compare any per-bag offer with the going price per kilogramme. A farmer who knows that a "bag" offered by a broker actually holds well over 50 kilogrammes can negotiate, or walk away. A hanging scale or platform scale that reads true is enough; check it with a known weight before the selling season, as our weighing scales guide explains.

Use sacks that breathe. Potatoes sealed in plastic sweat and rot quickly. Woven or mesh bags that let air through are better for short holding and transport, and potatoes for longer storage are better kept loose in shallow, ventilated heaps or in crates than in tightly packed sacks. Stack bags so air can move between them, and keep them shaded during transport.

Potato Post-Harvest Checklist

Tools for Harvest and Sale, With Prices

These items appear on the Fuga price list at the time of writing; check the site or call a branch for current prices and stock.

Potatoes share many principles with other fresh produce, and our guide to cooling and storing fresh produce explains when investing in cooling makes sense. If your farm also grows grain, our grain storage guide covers the other side of the store. For a weighing scale, jembes or buckets for the harvest, talk to any Fuga branch in Eldoret, Nairobi or Nakuru, or WhatsApp 0716 961 018.

Frequently Asked Questions