This is the fourth part of a monthly series on growing potatoes, a dietary staple.
PART ONE: Planting potatoes (April)
PART TWO: Growing potatoes (May)
PART THREE: Potato pests and diseases (June)
PART FOUR: Harvesting potatoes (This one, July)
PART FIVE: Storing potatoes (August)
PART SIX: Planning to grow potatoes again (September)
I have a whole chapter about potatoes in Sustainable Market Farming, where most of this information can be found.
See Root Crops in June for info on digging up new potatoes, if you can’t wait for them to mature! Harvest for immediate use anytime you’d like after the tubers reach a big enough size.
Preparing for potato harvest
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. 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, and to harvest for storage, wait until the tops are completely dead.
In England, we planted in spring and harvested in October, waiting for the frost to kill the vines. In Virginia, we plant in March and June, harvesting 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 the 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.
Varieties such as Red Pontiac, Yukon Gold and Kennebec are determinate — they grow as a bush, then flower and die. If you have indeterminate varieties such as Russet Nugget, Nicola, German Butterball or Elba, you will need to kill the vines. I’ll come back to the topic of determinate and indeterminate varieties in Part Six, Planning to grow potatoes again (September). You can bring about an early vine death by mowing or flaming. This will also remove weed growth that could interfere with your digging equipment.
Whether the vines die naturally at the end of their lifespan, or they die of disease, or the frost kills them, or you do it yourself by mowing or flaming, it helps storability to wait 2-3 weeks more before harvesting to allow the skins to toughen up. They become more resistant to scrapes and bruises) and the potatoes become higher in dry matter. Harvesting is also easier if the vines are well dead. Test by digging up a sample and rubbing the skins. When the skins don’t break, the potatoes are storable.
Be sure to have enough crates, buckets, totes, gloves and workers. Clean and air the root cellar and warm it to 70°F (21°C).
Harvesting your potatoes
If possible, harvest when the soil moisture is 60-80% of field capacity. Not too dry, not too wet. This reduces damage from scraping. Ideally the soil temperature will be 45°-65°F (7°-18°C). Because soil temperature lags 3-4 hours behind 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. It does a good job in clean soil and an excellent job in clean fairly dry 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 they are wet, we leave them to dry for a short time. 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.
When freshly harvested, potatoes are tender, breathing things. Avoid bruising, which is damage that does not break the skin, by not dropping potatoes more than 6” (15 cm), or throwing them towards 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!
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. Potatoes you take from storage can be no better than the quality of the potatoes you put into storage!
Potato harvesting raises rocks to the surface, so we try to find time soon after the potato harvest to collect them for use in road repairs and construction.
Yields are likely to be 150 lbs/100ft (223 kg/100m); 200 lbs/100 ft (300 kg/100m) or more is a good yield; double this is possible. In my book Sustainable Market Farming, in the potato chapter, I made a mistake and gave these as pounds per acre, which would be a miserable yield! 5 gallon buckets and square plastic “milk” crates hold about 30 lbs each (14 kg)
Post-harvest two week curing
After harvest, potatoes need to cure for two weeks at a surprisingly warm temperature: 60°F–75°F (15.5°C–24°C), and 95% humidity. While curing, the root cellar will need 6-9 hours of ventilation every two or three days. The potatoes are still actively respiring, so they need a good oxygen supply. Failure to ventilate enough can lead to Blackheart, where the inner tissue of the potatoes dies and turns black.
Potatoes will heat up if left closed in. Ventilate when the temperature is 0-20 F° (0-11 C°) cooler than your goal: air in the daytime if nights are too cold and days are mild; at night if nights are mild and days too warm. Try hard to avoid having the cellar cool down, then warm up. That causes the potatoes to sprout. If there is too much condensation, use a fan and open the cellar doors, when temperatures are closest to the goal. During the curing period, the skins toughen up more, and cut surfaces and superficial wounds heal over, enabling long-term storage.
Sorting and preparing for long-term storage
After two weeks, 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. Not sorting at this point lets rots spread.
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 2-4, after which air exchange is not needed. Relative humidity should be 90-95%, to keep weight loss to a minimum, but not 100%! If the cellar is too warm, you will need to ventilate to lower the temperature.
Once potatoes are more than a month from harvest, the temperature, should be 40°-50°F (4.4°-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 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 have a natural dormancy of 60-130 days (depending on the storage temperature). After that period, they will start to sprout.
I will address long-term storage and preventing sprouting in Part 5 next month.
The Potato Association of America, Commercial Potato Production in North America 2010.
2016 Organic Production and IPM Guide for Potatoes from Cornell has a big focus on dealing with diseases and pests.