Planning to grow potatoes again

Planning to grow potatoes again 

Potato plant emerging in spring.
Photo Kathryn Simmons

This is the last part of a monthly series on growing potatoes, a dietary staple.

I have a whole chapter about potatoes in Sustainable Market Farming, where the basics of potato growing can be found. Below are more details about growing potatoes that you may not have wanted or needed this year. Use your own records and this information to plan for bigger harvests, at times of year suited to your farm.

Varieties

Some varieties store better than others, so advance planning will help achieve good results. Scrutinize the small print in the seed catalogs before your next seed order.

Covering a row of seed potato pieces aligned under a rope.
Photo Ira Wallace
Photo Ira Wallace

Potato types: determinate and indeterminate

We have mostly grown Red Pontiac, Yukon Gold and Kennebec. They all seem to be determinate varieties — they grow as a bush, then flower and die. I only learned this year that 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 Internet 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.

Crop rotation, including cover crops

This is very important for potatoes, which are nightshades like potatoes, peppers and eggplant. Colorado potato beetles emerge from the soil in spring and walk (they don’t fly at this stage) towards the nearest nightshades they can detect. Give them a long hike! A distance of 750′ (230 m) or more from last year’s nightshade plots should keep them away. A three- or four-year rotation out of nightshades in each plot is ideal.

Suitable cover crops before potatoes include brassicas (which can help reduce root knot nematodes and Verticillium), Japanese millet (which can reduce Rhizoctonia) and cereals in general. Beware beets, buckwheat and legumes such as red and crimson clovers, and some peas and beans, as these can host Rhizoctonia and scab.

Late corn undersown with oats, mowed high in October to deal with weeds, and the ex-sweet potato patch sown in winter wheat and crimson clover. Credit Ezra Freeman

In our ten year crop rotation, our March-planted potatoes follow a winter of oats and soy (which winter-kill in our zone 7a climate). This cover crop is undersown in our late sweet corn about 30 days after sowing. Our June-planted potatoes follow a winter cover crop mix of winter wheat or winter rye and crimson clover. This mix is sown in early-mid October after our middle planting of sweet corn. (Yes, we risk the clover.) We had read that potatoes are said to do well after corn, so when we set up our crop rotation, that’s what we did. I have no scientific proof that the assertion is true, but we often have good potatoes, so at least it does no obvious harm!

Fall broccoli transplanted after July potato harvest, and undersown with a mixed clover cover crop.
Photo Nina Gentle.

After harvesting our March-planted potatoes in mid-July, we regularly did a fast-turnaround and transplanted our fall broccoli and cabbage in late July. We undersowed that with a clover mix 4 weeks after planting the brassicas. We kept the clover mix for an all-year Green Fallow, right round until the February a year and a half later. This fast-turnaround was a bit nerve-wracking, so we no longer do that, simply following the potatoes with the clover mix, while transplanting the brassicas in another plot.

After harvesting our June-planted potatoes in October, we sow winter wheat or winter rye with crimson clover or Austrian Winter Peas, depending when we are ready to sow. (Wheat and clover if by 10/15, rye and peas if later)

Preparing the Soil

A bed ready for tilling after mowing the cover crop and spreading compost.
Photo Pam Dawling

Potatoes benefit from generous amounts of compost or other organic matter (they use 10 tons/ac, 22,400 kg/ha) and will grow in soils with a pH of 5.0–6.5. They use high amounts of phosphorus (P) and potassium (K), and need adequate soil levels of iron and manganese. They are less affected by low levels of copper and boron. Hay mulch can be a good source of K. As Carol Deppe points out, potatoes will still produce an OK crop in poor soil, where you might not be able to grow much else. See the ATTRA publication Potatoes: Organic Production and Marketing.

Dormancy

See Part 5, Storing Potatoes, for an introduction to this topic. When potatoes sprout and whether they grow one or more sprouts, can be controlled by manipulating the storage conditions.

For extra-early spring planting, aim to sprout relatively few eyes per potato, so that relatively few shoots will grow and the seed pieces will be big enough, with enough nutrients for the plants. Do this by priming the seed potatoes at 65°F (18°C) until the eyes at the rose end just start to sprout. Store at 45°F (7°C) until two weeks before planting time, then finish the sprouting in warmth and light. The early sprouting of the rose-end eyes suppresses the sprouting of the other eyes. If needed, break off extra sprouts before planting.

To avoid sprouting, keep the potatoes below 50F (10C) 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 will produce ethylene, which will encourage more sprouting.

Physiological age of seed potatoes

Seed potatoes can act differently depending on their “physiological age.” The warmer the conditions are after dormancy ends, the quicker the sprouts grow and the faster the tubers “age.” When we buy seed potatoes the storage conditions they have already received are beyond our control. As a guide, the length of the longest sprout, and the number of sprouts are measures of physiological age (if the sprouting has taken place in the light). Varieties do not all show these effects to the same degree.

Don’t let your potatoes sprout in storage
Photo Jesse Strassburg

Deliberately adjusting storage temperatures is a way of manipulating the physiological age, in order to get higher yields or earlier maturity. To age seed potatoes, buy the seed in late fall or early winter before they break dormancy and store them rose (eye) end up in daylight at 50°F (10°C) until just before the planting date. In spring, reduce the temperature just before planting, to minimize the thermal shock from the cold soil.

Physiologically “young” tubers will have just one or two sprouts, due to apical dominance (when the leading bud inhibits the other eyes from developing shoots). The plants will have fewer stems, leading to fewer, but larger, potatoes. They will need longer to grow, and so give a later harvest. If you hurry and dig them early, you will only get low yields.

“Middle-aged” tubers give the best yields (27% higher than young or old tubers). “Middle-aged” seed potatoes have multiple short sprouts, without the hairy look of “old” ones. The pre-sprouting instructions given in Part One: Planting potatoes aim to produce “middle-aged” seed.

Physiologically “old” seed potatoes will have many “hairy-looking” branched sprouts, coming from eyes all over the potato. These potato plants emerge faster and start tuber formation sooner. The final plant size will be smaller (because the shoots are weak) and the plants will be more susceptible to drought and die sooner. Because the tubers do mature quickly, they may be good if you seek an early harvest, or are planting a fall crop a bit too close to the frost date. The total yield will be lower (but earlier) than from “younger” seed.

See the University of Maine Extension Service Bulletin #2412, Potato Facts: Selecting, Cutting and Handling Potato Seed  Their drawings are reproduced here. Also see Know and Grow Vegetables by Salter, Bleasdale, et al. for more on this complex topic.

More POTATO resources

The University of Maryland Extension Home and Garden Info Center Potatoes

University of Maryland IPM series on potatoes, which is a troubleshooter sheet on causes and solutions for problems

Cornell 2016 Organic Production and IPM Guide for Potatoes (104 pages)

The Potato Association of America, Commercial Potato Production in North America 2010. (90 pages)

The Government of Canada Biofungicides provide Post-harvest Disease Protection in Potatoes

Other related Blog Posts

Potato Research on Harvest and Storage

What Makes Potatoes Sprout, Nov 2017

How to Deal with Green Potatoes (one of my most-read blog posts!)

Green Potato Myths and 10 Steps to Safe Potato Eating in Mother Earth News

Harvesting potatoes

Harvesting potatoes and sorting storable from others.
Photo Nina Gentle

Harvesting potatoes

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.

Picking up lifted potatoes and crating them.
Photo Nina Gentle

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.

Potato harvest in progress. Note the line up of garden carts to haul away the bounty. Photo Nina Gentle

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).

To use our potato digger we have to remove the mulch first.
Photo Twin Oaks Community

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!

Chain of people moving crates of potatoes from the truck to stack under a big pine tree.
Photo Nina Gentle

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 potatoes in early November.
Photo Wren Vile

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.

Resources

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.

Don’t let your potatoes sprout in storage (more next month)
Photo Jesse Strassburg