You can grow great potatoes, but if the harvest, postharvest and storage conditions aren’t right, they won’t live up to their storage potential. In this article, I’ll discuss how to know when to harvest potatoes and keep them in the best possible condition for eating and storage. In an article in the January 2022 magazine, I’ll discuss green sprouting and planting potatoes.

 

Potato development stages

First, the plant produces roots, stems and leaves. This vegetative stage lasts 30 to 70 days. Next, comes a two-week period of tuber formation and stem-branching. All the tubers (potatoes) that will grow on a plant are formed in those two weeks. 

Then, the tubers grow larger, but don’t increase in number. When the leaves start to turn pale, the plant has finished its leaf-growing stage and will be putting energy into sizing-up the tubers under the ground. Two or three weeks after flowers appear (if they do), the baby potatoes will be 1 to 1.6” (2.5–4 cm) across. Potatoes dug during this tuber-sizing-up period will be “new” potatoes without the thick skins necessary for storage. 

Finally, the tops yellow and start to die. No more regular sizing-up is possible. Avoid irrigating at the end of the growing period or the potatoes may develop hollow heart, make knobby secondary growths or even crack. For maximum yield, wait until the tops are completely dead. Harvesting is also easier if the vines are well dead.

In the two weeks after vines die, the potato skins thicken, the potatoes become more resistant to scrapes and bruises, and become higher in dry matter. These changes make them suitable for storage.

If you want to hasten storable potatoes, mow the vines and wait two weeks or so. To test for storability, dig up two potatoes and rub them together, or rub them firmly with your thumb. If the skins rub off, wait a couple of days before trying again. If the skins are strong, go ahead and harvest. 

Early harvest

Harvest for immediate use anytime you’d like after the tubers reach a big enough size. Sometimes there are reasons to terminate potato growth early. You might do this if you have a fast crop turnaround after spring-planted potatoes, such as we used to do when following spring potatoes with fall cabbage and broccoli. 

 

Picking up, sorting and crating potatoes. Photo by Nina Gentle.

 

Another reason to bring potatoes to a rapid end is to save your crop if it gets late blight late in the season after tubers have formed. Mow the foliage, rake it off and dispose of it, leaving the field untouched for two weeks before harvesting. Late blight can travel down the stems from infected leaves into tubers, rotting them. Cutting off foliage before the rot has made it into the tubers is one way to salvage a potato crop soon after blight has been detected. Late blight moves fast, and if infected potato vines are left to dry down in the field as normal, it can result in total crop loss. 

Disposing of large amounts of blighted foliage is no easy task. If left to rot in a pile, it can continue releasing late blight spores and infect other crops. Digging a big hole and burying it is probably best.

Late Blight (Phytophthora infestans) is by far the worst disease to afflict potatoes. The disease is caused by a species of a fungus-like oomycete (water-mold) that blows in on the wind. It is worse in warm, wet weather with cool nights. Initial symptoms are water-soaked spots on the leaves. These expand into gray-black scorched areas, sometimes with a dotted white mold growth, especially on the underside of the leaves. Cut stems reveal a dark circle of infected tissue. 

 

Here is our Cecchi and Magli potato digger in action in very clean conditions. Photo Lori Katz

 

The disease spreads rapidly, turning plants black as if badly frosted and can kill an entire planting in 10 days unless stopped by hot dry weather. It spreads via cull piles, nightshade plants and petunias — it needs live plant material to survive. The best defense is to always remove volunteer nightshades from your fields and compost or bury all crop debris. 

 

Preparing for potato harvest

In Virginia, we plant in March and June and harvest in July and October. For our unmulched March-planted July harvest, we mow two weeks before our planned harvest to fit a tight crop turnaround. In hot weather the cut tops and weeds dry up. If more weeds grow, we mow again the day before harvest. For our fall-harvested crop, planted and mulched in June, we need to remove the mulch after mowing and before the harvest. This is a slow job, but necessary, and the mulch makes a good addition to our compost pile. If it looks like rains will delay the harvest, or nights are forecast to be very frosty, we delay de-mulching until the day before harvest.

Clean and air the root cellar or storage shed and warm it to 70°F (21°C). Don’t start with a cold building or you can have problems with condensation from the potatoes.

 

Harvesting your potatoes

Be sure to have enough crates, buckets, totes, gloves and workers. If possible, harvest when the soil moisture is 60 to 80 percent of field capacity. Not too dry, not too wet. This reduces damage from scraping. When you have choice in the matter, try to harvest potatoes from relatively dry soil, so they are less likely to grow mold. Ideally, the soil temperature will be 45° to 65°F (7° to 18°C).  Because soil temperature lags three to four hours behind the air temperature rise each day, in cold weather, try to harvest late in the day, but with time to finish before dark. In hot weather, harvest in the morning as early as possible. 

Tuber temperature will also affect bruise and rot susceptibility. Do not harvest when tuber temperatures are below 45°F (7°C) or above 85°F (30°C). 

We use a Checchi and Magli single-row side delivery SP100 harvester. (Their website has videos.) It does a good job in clean soil and an excellent job in fairly dry, clean soil but gets stuck if we have a lot of organic material on the soil (weeds or mulch). The 1-row Potato Digger from US Small Farm Equipment, which a neighboring farm bought, has the same challenge. If using a digger, don’t set it too deep or too much soil will be dumped on the harvested potatoes.

During harvest, someone walks alongside the tractor with a long-handled hook/claw tool, to clear blockages and hook any potatoes from the path of the tractor wheels. The rest of the crew follows, picking up and sorting the potatoes. If the potatoes are wet, we leave them a couple of hours to dry. We sort the damaged ones into “farm use” buckets and crate up the good ones. We try not to leave any potato parts in the field to reduce the chance of spreading diseases.

 

Only store sound potatoes

Potatoes you take from storage can be no better than the quality of the potatoes you put into storage. Damaged and poor quality vegetables never store well, nor do very small potatoes. Don’t try to store diseased potatoes. For information on potato diseases, The Potato Association of America has a guide at https://tinyurl.com/4p9hu3as.

Black Scurf or Stem Canker fungus (Rhizoctonia solani) is worst in cold wet soils. Red-brown stem lesions develop into cankers. The infection can spread to the tubers, which then become cracked and misshapen, and may have dead tissue at the stem end. There may be firm black sclerotia (small dried reproductive bodies) on the tuber. 

Always handle storage crops gently. When freshly harvested, potatoes are tender, breathing vegetables. Avoid bruising, internal damage without breaking the skin, by not dropping potatoes more than 6” (15 cm) or throwing them toward a container. Don’t bang them to knock off extra soil. In hot weather we aim to work until done and not leave any potatoes in the field baking for long. In cold weather we aim to get done before nightfall and not have any freeze overnight. 

Yields average 150 lbs/100ft (223 kg/100m) and 200 lbs/100ft (300 kg/100m) or more is a good yield. Double this is possible. We like to get a rough idea of yield as we pick. A 5-gallon bucket and square plastic “milk” crate hold about 30 lbs each (14 kg). 

 

Cooling before storing

When harvesting in summer, we stack the crates of potatoes covered with a tarp under a big tree overnight to lose some of the field heat before moving them to the root cellar early next morning. For the fall harvest, if the weather is chilly, we take the crates straight into the root cellar. 

 

Post-harvest two-week curing

Potatoes are one of those vegetables that need to cure (before storage) in conditions that are different from those needed for storage. Potato curing takes about two weeks at a surprisingly warm temperature: 60°F–75°F (15.5°C–24°C) and 95 percent humidity. Not 100 percent humidity, though. During the curing period, the skins toughen up more, and cut surfaces and superficial wounds heal over, enabling long-term storage. Wounds in the skin will not heal below 50°F (10°C). Curing also allows some of the starches to convert to sugars. Remember to keep potatoes in the dark while curing as well as during storage.

Be sure to provide good ventilation during curing: the root cellar will need six to nine hours of ventilation every two or three days. The potatoes are still actively respiring, so they need a good oxygen supply. Potatoes will heat up if left closed in. Failure to ventilate enough can lead to Blackheart, where the inner tissue of the potatoes dies from lack of oxygen and turns black. 

Ventilate when the temperature is 0-20°F degrees (0-11°C degrees) cooler than your goal. Air in the daytime if nights are too cold and days are mild and at night if nights are mild and days too warm. Try hard to avoid having the cellar cool down and then warm up. That causes the potatoes to sprout. Keeping root cellar temperatures within a narrow range takes human intervention, or sophisticated thermostats and vents. Sometimes night ventilation offers drier air than you can get in the daytime. If there is too much condensation, but the temperature is okay, use a fan and open the cellar doors when temperatures are close to the goal.

Expect a small percentage of your crops to go bad in storage. It’s not a sign of failure, just a reminder that life has limitations. 

 

Ethylene

Potatoes are very sensitive to ethylene and will sprout in a high-ethylene environment. Ethylene is a naturally occurring, odorless, colorless gas, generally associated with ripening, sprouting and rotting. Chilling, wounding and pathogen attack can all induce ethylene formation in damaged crops. Some crops, such as ripening fruits, produce ethylene in storage. Don’t be tempted to set that bargain box of very ripe bananas you bought on the way home near anything you don’t want to sprout or ripen further. Propane heaters and combustion engines produce ethylene. Be careful if using your garage to store potatoes. Do not use any unvented hydrocarbon fuel heaters near stored produce. 

 

Root cellar ecosystem 

We have no automated ventilation, or even ventilation ducts. We simply leave the door open at night when we want to cool it down, or in the daytime in winter, choosing a time when the forecast temperature is in the range we’re aiming for. Yes, mice do come in the open door. We encourage black snakes to live in our cellar, to keep the mice under control. 

How do we encourage snakes? I mean we don’t drive them out, and if we need to, we move one or two in there. This can be a bit unnerving, as the cellar is dark. We chose not to have a light, as leaving it on by accident could cause a lot of potato greening before we noticed our mistake. We have developed a special door-opening technique so we can co-exist with the snakes, who like to hang out on the top of the doorframe. We unlatch the door, open it a crack, then bang it closed, before opening it fully. Any resting snakes have by then dropped to the floor where we can see them and avoid them. No snakes have been hurt in this process! People who don’t like snakes will be really motivated to fit a rodent-proof vent system.

 

Long-term potato storage

After two weeks of curing, we sort through the whole storage crop for rot. We find that this single thorough sorting can remove almost all of the storage problems that are going to happen. The rate of potato deterioration drops right down after the first two weeks. Not sorting at this point lets rots spread. Re-stack the crates, remembering to keep airspace between the crates and walls. 

 

Black snakes do a good job of removing rodents in our root cellar. Photo Nina Gentle.

 

After the curing period, the potatoes become more dormant and do not respire so actively. Fresh air is needed about once a week in weeks two to four, after which air exchange is not needed as the tubers have become dormant. If the cellar is too warm, you will need to ventilate to lower the temperature. 

The relative humidity should be 90 to 95 percent in weeks two to four, to keep weight loss to a minimum. After week four, long-term storage conditions are fairly moist, 85 percent to 90 percent humidity.

Once potatoes are more than a month from harvest, the temperature should be 40°to 50°F (4.4° to 10°C), and closer to the lower end of the range is best for long-term storage. In summer we work hard to reduce the temperature to 50°F (10°C) for long-term storage, but in the winter we can easily reach 40°F (4.5°C). Constant temperatures or a steady decline is the goal, not dramatic fluctuations, as these can cause stress and physiological aging, which leads to sprouting. Hotter temperatures promote more rot, and age the potatoes faster, also leading to early sprouting. 

Potatoes do not do as well under refrigeration. Below 40°F (5°C) some starches convert to sugars, giving the potatoes a bad flavor and causing them to blacken if fried.

 

Potato storage sites

Potatoes have specific storage conditions. With a good in-ground root cellar, potatoes can be stored for five to eight months, but other options can also work. A max-min thermometer will help you keep the storage space in the right range.

Plan your storage sites, buy a thermometer for each site, and gather suitable containers. Clean and prepare your storage space before going out to do a big harvest. Containers should rest on shelves, pallets or blocks of some kind, and not be set on bare concrete floors. This helps improve ventilation and reduce condensation.

For traditional storage without refrigeration, potatoes (and most other root crops) store best un-washed (less wrinkling), though this can make them harder to clean later. If you might not be able to keep temperatures low enough, choose stackable crates rather than closed bags. The packing of your containers should allow for airflow, but you don’t want the produce to shrivel up, so be observant. 

 

Basements and root cellars

Traditional root cellars are made by excavating a large hole, lining it with block or stone-work walls, casting a well-supported and well-insulated concrete roof, then covering the top with a big mound of soil. The more modern version is to construct an insulated cellar in the basement of a building such as a CSA distribution barn or your house. See Mike and Nancy Bubel’s book Root Cellaring for drawings and instructions. 

Provide wide doorways with ground-level access if possible (roll that garden cart right in). Good lighting and drainage are important, so you can see if everything is storing well, or hose the shelves and floor down if it isn’t. Mouse-proofing is worth considering upfront. Our 10’ x 11.5’ (3 x 3.5 m) cellar will hold 360 crates with an ample central path. That’s 10,800 pounds (4900 kg), or around 5 tons.

 

Dormancy

Potatoes have three types of dormancy. Endodormancy is the period immediately following harvest, during which the potato is incapable of sprouting. The exact time depends on the variety. I used to think I knew it was four to eight weeks, but the more I learn, the less sure I am. If you planned to dig up an early crop and immediately replant some of the potatoes for a later crop, it won’t work. Get around this problem by refrigerating them for 16 days, then pre-sprouting them in the light for two weeks. 

Ecodormancy is when environmental conditions prevent sprouting, e.g., storage at low temperatures. A three-degree (F) temperature difference can mean a 20-day difference in dormancy break. Dormancy break is a complicated process involving five major plant hormones, as well as temperature. Dormancy can be 20 to 35 percent shorter at 48°F (9°C) compared to 42°F (6°C). This is why we warm seed potatoes to pre-sprout them before planting. Apples, bananas or onions will help them sprout by emitting ethylene. The total dormant period may be four to 12 weeks, or even as long as 18 weeks, depending on the storage temperature. After that period, they will start to sprout. 

Paradormancy in potatoes refers to the sprouting of the dominant eye preventing others growing. Environmental conditions can also affect paradormancy. If your livelihood depends on growing potatoes, you could benefit from studying this aspect. 

 

Avoid sprouting 

To avoid sprouting, keep the potatoes below 50°F (10°C) once they are more than a month from harvest, avoid excess moisture, and avoid “physiological aging” of the potatoes, caused by stressing them with fluctuating temperatures, among other things. If eating potatoes do start to develop sprouts, it’s a good idea to rub off the sprouts as soon as possible, because the sprouting process affects the flavor, making them sweet in the same way that low temperatures do. Sprouting also produces ethylene, which encourages yet more sprouting.

Potatoes are more likely to sprout if: they are more than four to eight weeks after harvest; in the light; near fruits, vegetables and flowers; or malfunctioning propane or natural gas heaters that produce ethylene; too warm, or warm after being cool. Potato sprouts are toxic.

 

Pam Dawling works in the 3.5 acres of vegetable gardens at Twin Oaks Community in central Virginia. Her books, Sustainable Market Farming: Intensive Vegetable Production on a Few Acres, and The Year-Round Hoophouse, are widely available, including at sustainablemarketfarming.com, or from Growing for Market. Her weekly blog is on her website and on facebook.com/SustainableMarketFarming.