Potato Research on Harvest and Storage

Last week I mentioned that while researching potato yield figures, I found an interesting publication, The Potato Association of America, Commercial Potato Production in North America 2010. I’ve been reading that and learning more about potatoes. Here I’m going to focus on harvest and storage, because that’s the bit we’re currently challenged by. I also learned more about planting in hot weather, but that’s for another time.

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. We have grown Red Pontiac, Yukon Gold and Kennebec here, mostly. They all seem to be determinate varieties. I only just learned there are determinate (varieties with naturally self-limiting growth, generally “early” varieties) and indeterminate varieties (such as “Russet Nugget,” “Nicola,” “German Butterball” and “Elba”). The distinction is explained in Potato Bag Gardening. Growers using towers, grow bags, and cage systems want indeterminate potatoes, which continue to produce more layers of tubers on the stems as they are progressively covered with more soil. Growers wanting a fast reliable crop in the field mostly choose determinate types, which grow as a bush, then flower and die. The Wild Woolly Web does seem to have some contradictory statements about which varieties are determinate and which indeterminate, and some dedicated container growers make assertions not supported by experienced commercial growers. So Reader Beware! I trust Extension and here’s a link to their Ask an Expert page on potato types, and the University of Maryland Extension Home and Garden Info Center Potatoes.

Whether the vines die naturally at the end of their lifespan, or they die of disease, or the frost kills them, or you kill them yourself by mowing or flaming, the potatoes will store better if you then wait 2-3 weeks before harvesting. The potato skins thicken up (becoming more resistant to scrapes and bruises) and the potatoes become higher in dry matter. Harvesting is easier if the vines are well dead. We generally bush-hog ours. Decades ago, in England, we had late blight in the middle of the season, and we cut the tops off and made a very smoky bonfire. (I wouldn’t participate in that much air pollution nowadays!) After waiting for a couple of weeks for the late blight spores to die, we dug the potatoes. The idea was to prevent spores getting on the tubers. As I remember, it all worked out OK.

If at all possible, harvest when the soil moisture is 60-80% of field capacity. Not too dry, not too wet. This reduces damage from scraping. If using a digger, don’t set it digging too deep, or too much soil will be damped on the harvested potatoes.

Tuber temperature will also impact bruise and rot susceptibility. Ideally soil temperature will be 45-65F (7-18C).  Because soil temperature lags 3-4 hours behind air temperature rise each day, in cold weather, try to harvest around 6 pm or a bit later. In hot weather, harvest in the morning.

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.

When harvesting in summer, we stack the crates of potatoes under a big tree overnight to lose some of the field heat before moving them to the root cellar early next morning. Potatoes you take from storage can be no better than the quality of the potatoes you put into storage!

The first part of the storage period is the curing. The potatoes are still actively respiring, so they need a good oxygen supply. Failure to ventilate the cellar enough can lead to Black-heart, where the inner tissue of the potatoes dies and turns black. During the curing period, the skins further toughen up, and cut surfaces and superficial wounds heal over, enabling long term storage. The temperature should be as close to 50-58F (10-14.4C) as you can get. The lower end of the range is best for fresh eating (as opposed to junk food manufacture). Hotter temperatures will promote more rot, and age the potatoes faster, leading to early sprouting. Relative humidity should be 90%, but not 100%! If there is too much condensation, use a fan and open the cellar doors, when temperatures are closest to the goal. Curing takes 10-14 days.

We find that a single thorough sorting after 14 days 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. They don’t need as many air changes as during curing, but if the cellar is too warm, you will need to aerate more. The temperature during the storage period should be 40-50F (4.4-10C), and closer to the lower end of the range is best. Constant temperatures or a steady decline is the goal, not dramatic fluctuations. Humidity should still be 90-95%, to keep weight loss to a minimum.

Potatoes have a natural dormancy of 60-130 days (depending on the storage temperature). After that period, they will start to sprout. Some plant extracts, including clove oil, can add 20-30 days storage, and will then need to be reapplied. I do not know anything about this myself, and do wonder how you remove the clove flavor from the potatoes!

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Speaking Events

I have two speaking events coming up in September

2019 Mother Earth News Fair Pennsylvania.

September Friday 13- Sunday 15, 2019

Location: Seven Springs Mountain Resort, 777 Waterwheel Dr., Seven Springs, Pa. 15622

I am giving two 60 min workshops

Lettuce Year-Round on Friday 9/13 12.30-1.30 pm at the Grit Stage

This presentation includes techniques to extend the lettuce season using row covers, cold frames, and hoop houses to provide lettuce harvests in every month of the year. The workshop includes a look at varieties for spring, summer, fall, and winter. Pam Dawling considers the pros and cons of head lettuce, leaf lettuce, baby lettuce mix, and the newer multileaf types. She also provides information on scheduling and growing conditions, including how to persuade lettuce to germinate when it’s too hot.

Cool Season Hoophouse Crops on Saturday 9/14 3.30-4.30 pm at the Building and Energy Stage

Learn how to fill your hoop house with productive food crops in the cool seasons. Pam Dawling discusses suitable crops, cold-hardiness, selecting crops, calculating how much to harvest and how much to plant, crop rotation, mapping, scheduling, seasonal transitions, succession planting, interplanting, and follow-on cropping.

Book-signing at the Bookstore Saturday 4.30-5 pm. Buy new books at the Bookstore and bring your grubby used copies to be signed too!

Demos at New Society Publishers booth, of tomato string-weaving and wigglewire system for fastening hoophouse plastic to framework
Friday 3 -3.30 pm, 4.30-5 pm; Saturday 10-11 am, 1.30-2.30 pm; Sunday 9-10 am. 1-2 pm, 3.30-4 pm

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Heritage Harvest Festival, Monticello, Charlottesville, VA

September Friday and Saturday 21-22, 2019

Buy tickets online

Workshop with Ira Wallace

10:30 – 11:30 am Saturday in the Heritage Tent

Winter Gardening: No Tech to High Tech 

Learn tips on growing cold-hardy vegetables (not only kale!) out in the open and with varying degrees of protection from rowcovers, low tunnels, coldframes and hoophouses (high tunnels). We’ll consider crop choices, planting dates and harvesting so there’s always something to eat for everyone from winter market gardeners to small backyard growers. We’ll explain ways to maximize production with succession planting and follow-on cropping.

 No extra fee for the workshop, included with the price of general admission

Booksigning: SATURDAY, SEPT. 21^st, 11:45am – 12:15pm, MONTICELLO SHOP TENT (WEST LAWN). Buy new books at the Bookstore and bring your grubby used copies to be signed too!