Growing Potatoes

Potato plant emerging in spring.
Photo Kathryn Simmons

This is the second part of a monthly series on potatoes. Last month I talked about Planting potatoes. Now they are in the ground, we turn our attention to growing healthy plants and doing all we can to maximize the yield.

Frosted potato plants May 10., after two nights at 30F and 29F. I hope this didn’t happen to you! If you know a frost is coming, try to get the potatoes hilled, which protects more of the stem and leaves under the soil. This increases the likelihood the plants will be able to grow back.
Photo Pam Dawling

Potato development stages and crop requirements

  1. First the plant produces roots, stems and leaves. This vegetative state lasts 30–70 days. Bigger plants have more yield potential, so the goal for this stage is to produce robust large plants. Vegetative (leafy) growth of potatoes is favored by warm, 80°F (27°C) moist weather, but tuber growth is favored by cooler soil conditions of 60°F–70°F (15.5°C–21°C). This combination can be achieved either by planting in spring, when the soil temperature lags behind increasing air temperatures and is still cool enough for tuber formation, or by adding organic mulches to keep the soil cool if planting in early summer.
  2. Tuber formation (a two-week process) and branching of the stems comes next. All the tubers (potatoes!) that will grow on that plant are formed in those two weeks. The number of tubers produced per plant depends on hours of daylight, temperature and available water in that short period of tuber initiation. Watering stimulates the production of more tubers. 5 gal/yd2 (22.8 l/m2) is a good amount to supply when tuber formation begins. Short day length is optimal, with a night temperature of 54°F (12°C). If temperatures at night are 68°F (20°C), initiation will be reduced; and at 84°F (29°C), will be inhibited. High nitrogen also inhibits initiation. During this stage, leaf growth continues. Flowering can happen too, but it’s not essential, so don’t worry if you get few or no flowers. Hilling adds soil to the stems, encouraging stem growth and providing sites for tubers to form.
  3. Third, 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. Adequate water and nutrients are important during this critical stage which lasts until the plant reaches maturity for that variety, up to 90 days. Try to ensure at least one inch (2.5 cm) of water per week, up until two weeks before harvest. Avoid very uneven watering, or overwatering, as hollow heart could result. The size of the tubers depends on various growing conditions. Two or three weeks after flowers appear (if they do), the baby potatoes will be 1–1.6″ (2.5–4 cm) across. The best temperature is around 65°F (18°C), and I’ve read that potato size decreases by 4% for every Fahrenheit degree (7% per Celsius degree) above the optimal. Spacing is another factor — we got large potatoes one summer because we had poor emergence and therefore wide spacing! The heat of the summer didn’t stop them.
Three flat-tined potato forks to the left, four round-tined digging forks to the right. Flat tines do less damage, usually.
Pam Dawling

Potatoes dug during this tuber-sizing-up period will be “new” potatoes, and not have the thick skins necessary for storage. If you dig your potatoes during this stage (or “snitch” some, leaving the rest of the plant growing), you will be happy short-term, but your final total yield will be less than if you grew all the tubers to full size. If you planned for this, and grew plenty, you will be happy now, and happy later!

  1. Finally, the tops naturally yellow and die. The skins of the tubers thicken, which makes them suitable for storage. No more growth is possible. Sometimes there are reasons to terminate potato growth early – see Early Harvest below for more about this.

Hilling potatoes

Start hilling (pulling soil up over the plants in a ridge) when the plants are 6” (15 cm) tall. Hill again two or three weeks later and two more weeks after that, if the plant canopy has not already closed over, making access impossible. Hilling also provides an opportunity for dealing with weeds, so if possible do this task in sunny breezy weather which won’t let the weeds re-root.

Rows of potatoes after hilling.
Photo Wren VIle

On a small scale, use a rake or standard hoe to pull soil up from the side of the row opposite to where you are standing. If you are sharing the job, one person can work each side of the row at the same time. If you are alone, turn round and work back when you get to the end of the row. Don’t be tempted to twist your arms around and move the soil up the side nearest you. You will damage your body by this distortion of your spine and shoulders!

At the next scale up, use a rototiller with a hilling attachment, or perhaps a wheel hoe with a hiller, if your soil and stamina allows. We have used a semi-manual planting method, making single furrows with our BCS walk-behind tiller, planting by hand in the furrows, then using the tiller again to cover the seed pieces and hill. Nowadays we use a tractor-mounted furrower that can make two furrows in each pass, and disks turned inwards in pairs to ridge the soil.

Alternatives to hilling potatoes

If you can’t hill, you can increase the effective depth of planting by covering the rows with thick straw or hay mulch. This is easiest to do immediately after planting, before the plants emerge. We don’t mulch our spring-planted potatoes because we want the soil to warm up some from its winter temperatures.

Potato plant emerging through hay mulch in early July.
Photo Pam Dawling

When we plant in June, we cover the seed pieces, then hill, then unroll round bales of spoiled hay immediately, like wall-to-wall carpeting. We choose this method to help keep the soil cooler through the summer. In warm conditions, deeper planting, hilling and thick organic mulches all help keep the plants cooler, as does irrigation.

Weed control for potatoes

Potatoes are sometimes said to be a “cleaning” crop, as if they did the weeding themselves. Not so! Any cleaning that takes place is a result of cultivation. As with many plants, the initial growth stage is the most critical for weed control. Hilling in sunny weather can deal with lots of weeds in a timely way, especially if the machine work is followed up by the crew passing through the field hoeing. Organic mulches also reduce weeds. Potatoes later in life produce a closed canopy that discourages more weeds from growing until the tops start to die. Mary Peet reports that potato yields were decreased 19% by a single red root pigweed per meter of row left in place for the entire season.

Flame weeding can be used for potato plants while they are small, as well (as for carrots and beets before emergence as we see here).
Photo Brittany Lewis

In wet weather it can be impossible to hill when you’d like to, and this is where flaming can save the day. Flaming is not an alternative to hilling, but it can be a way to buy time and deal with rampant weeds if the soil is too wet to hill. Potatoes may be flamed at 6″–12″ (15–30 cm) tall. Beyond that, flaming is not recommended. See ATTRA for more on flame weeding. Flaming when the potatoes are less than 8” (20 cm) tall is also an effective control measure for Colorado potato beetles (More on pests next month). Choose a warm sunny day when the pests are at the top of the plants. Flaming can kill 90% of the adults and 30% of the egg masses, according to ATTRA.

Early harvest of potatoes

Sometimes there are reasons to terminate potato growth early. If you need storable potatoes, cut, flame or mow the tops of the plants, and wait two weeks for the skins to thicken up. 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. You might do this if you have a fast crop turnaround after spring-planted potatoes, such as we used to do when following our spring potatoes with our fall cabbage and broccoli. Another time I’ve brought potatoes to a rapid end was in England, when we got Late Blight. We cut and removed the diseased tops (so no spores went down into the soil), and were able to salvage the potatoes two or so weeks later. Back then, the recommendation was to burn the green tops. This would probably not be recommended these days. It made for a smoky fire we kept going for several days. Terrible air pollution!

For the earliest possible crop in a dry climate (but not the highest yield), plant “old” seed (ones with lots of hairy sprouts) in early spring, hold off on watering until the tubers are marble size, then give a single good watering at 5 gal/yd2 (22.8 l/m2).

While it’s tempting to dig up potatoes early, the yields will be higher if you wait till the tops die.
Photo Kathryn Simmons

Next month I will write about potato pests and diseases.

Planting potatoes

Planting potatoes.
Photo Wren Vile

Planting potatoes

This is the first of a monthly series on growing potatoes, a dietary staple. Later parts will be

  • Part Two: Growing potatoes (May)
  • Part Three: Colorado potato beetle (and maybe other pests) (June)
  • Part Four: Harvesting potatoes (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.

Potatoes are good food

Potatoes are a rewarding crop to grow, with a lot more flexibility about planting dates than the traditional instruction to plant on St Patrick’s Day might have you believe. As Carol Deppe points out in The Resilient Gardener (which I wrote about here), potatoes provide more carbohydrates per area than any other temperate crop, and more protein per area than all other crops except legumes. Many people are surprised to learn this. A 2,000-calorie all-potato diet contains considerably more protein than a 2,000-calorie all-rice diet. Potatoes contain 10.4 grams of protein per 100 grams dry weight, and are a good source of vitamin C and carbohydrates. Carol Deppe, has written a very interesting article The 20 Potato a Day Diet versus the Nearly All Potato Winter about the nutritional and gastronomic wonders of potatoes.  It will inspire you to grow and eat more potatoes!

The short version

Obtain your seed potatoes and set them to pre-sprout for 2-4 weeks. Then figure out where they’re going to grow and prepare the soil. Then plant and, before they emerge, figure out what to do next.

Potatoes emerging in spring.
Photo Kathryn Simmons

Potato planting dates and temperatures

Potatoes are a cool-weather crop, but the tops are not frost tolerant. A good guideline for suitable spring planting conditions is three consecutive days with a temperature at a depth of four inches (10 cm) exceeding 43°F (6°C). Some growers wait for soil temperatures to reach 50°F (10°C) before planting. A traditional phenology sign is that the daffodils should be blooming. The spring planting is usually timed with the goal of having most of the shoots emerge after the frosts. A light frost will only nip the tops of the leaves and do no real damage (the plants will regrow), so a small risk is worth taking. It takes a temperature of 29°F (–2°C) to kill the shoots, and even then regrowth is possible.

The practice of hilling soil over most of the leaves once the plants are six inches (15 cm) tall will protect against frost. So if you have plants growing and a frost is predicted, a hilling that day may save them. In the fall, frosts will kill the foliage and growth will stop, so late plantings should be timed to get the tubers to maturity before the expected frost date. Some late varieties do not bulk up until the last moment, so if you are pushing the late end of your planting season, plant early varieties or fingerlings. (“Early” = fast-maturing)

Here in central Virginia, we plant our first crop in mid-March, about four weeks before our last spring frost, and plant a second crop in mid-late June, which allows three and a half to four months before our average first frost date. We could plant any time mid-March to mid-June and harvest mature potatoes. In summer, the ideal soil temperature is 60-75°F (15-24°C). It’s possible to pre-irrigate to lower the soil temperature in summer.  If we wanted to, we could plant a fast-maturing variety in July.

If you want to plant at “unusual” times of year, you may need to plan ahead, buy your seed when it’s available and store it in a cool dark place below 50°F (10°C), such as a refrigerator, until you need it. Many suppliers only ship in March and April. Growers in zones 8–10 may need to buy their spring seed potatoes in the previous fall. We buy our seed potatoes for the June planting in April, before local suppliers sell out of spring stocks. An advantage of summer planting is that the harvested crop need only be stored from October or November, not over the hotter months.

A grower specializing in many kinds of fingerlings might want to plant once a month during their season, for a continuous supply of fresh new potatoes. Growing in a hoophouse offers another option for growing for a late market, for example new potatoes for winter holiday dinners.

Planting seed potatoes by hand.
Photo Ira Wallace

Dormancy

Potatoes have a dormant period of four to eight weeks after harvest before they will sprout, so if you plan 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 sixteen days, then pre-sprouting them in the light for two weeks. Apples, bananas or onions will help them sprout by emitting ethylene.

Potato planting quantities

If using 10″ (25 cm) spacing, we buy enough to plant 16–17 lbs/100′ of row (around 1.2 kg/10 m). 12″ (30 cm) spacing is more common, providing bigger potatoes than at 10″ (25 cm), although yields may be lower. For 12″ (30 cm) spacing, the recommendation is to allow 10–12 lbs/100′ (7–9 kg/10 m). In practice, we need a higher seed rate, maybe 15 lbs/100′ (11 kg/10 m).

Varieties

The many varieties of potatoes are generally divided into four categories.

  • Early potatoes take 55–65 days from planting to harvest — the more famous ones include Yukon Gold, Irish Cobbler, Red Pontiac and Caribe.
  • Mid-season potatoes mature in 70–80 days, and include Kennebec, Katahdin, Desiree and Yellow Finn.
  • Late-maturing varieties take a full 85–120 days to mature and include Russet Burbank, Butte and Green Mountain.
  • The fourth category is fingerling potatoes, which are small, attractive and have a high market value. They are prolific and no harder to grow than other potatoes.

Farms that are not certified organic have the option of buying non-organic seed potatoes locally, which saves money on shipping. Be sure, though, to buy seed potatoes that are certified disease-free. Late blight is a disease not worth risking. Some growers buy “B” potatoes that are small enough to plant without cutting. For most growers, “B” potatoes are not available, and we settle for larger seed potatoes, which have fewer eyes for the weight than small ones do, and need to be cut into pieces before planting.

Pre-sprouting potatoes

Pre-sprouting, also called chitting or green-sprouting, is a technique to encourage seed potatoes to start growing sprouts before you put them in the ground. It’s not essential, but advantages are:

  • getting an earlier start on growth in the spring;
  • being less dependent on outdoor weather conditions;
  • giving the potatoes ideal growing conditions early on and so increasing final emergence rate;
  • bringing harvest forward 10–14 days;
  • increasing yields by optimizing the number of sprouts per plant;
  • making the cutting of seed potato pieces easier (the sprouts are more obvious than eyes);
  • enabling cover crops or food crops to grow longer before the land is needed for the potatoes;
  • giving you the chance to prepare and irrigate the soil as needed before planting.

To start the sprouting process, bring seed potatoes into a warm well-lit room, around 65°F–70°F (18°C–21°C), and set them upright in shallow crates or boxes, rose (eye) end up, stem (belly button) end down, for 2–4 weeks in spring, or 1–2 weeks in summer. If you have no space or time for chitting, warming the potatoes for a couple of weeks (maybe even just a couple of days) will be beneficial. Some people like to warm the potatoes in the dark for two weeks, then spread them out in the light for the last two weeks before planting. I don’t know if the two-part process offers advantages, because I’ve never tried it. In the light, the growing shoots will grow green and sturdy, not leggy and fragile. Make sure the potatoes have a moist atmosphere so they don’t shrivel while they are sprouting. At this point don’t worry if a few sprouts break off; more will grow later.

In spring, the sprouts will grow considerably faster with indoor warmth than they would if planted unsprouted in cold ground, where they could take as long as four weeks to appear. Once planted, chitted potatoes will emerge sooner, and more evenly, which is always reassuring, and the weed competition will not be as fierce. Fewer seed pieces will die before emerging. And if weather prevents soil preparation when you had planned, just wait and know that your plants are growing anyway.

For summer planting, encourage sprouting success by storing seed potatoes in a cool place like a refrigerator, at 45°F– 50°F (7°C–10°C) until two weeks before planting time, then sprouting and cutting them. This encourages the lower eyes as well as those at the rose end to sprout. For warm-weather planting, one sprout per seed piece is usually sufficient. Tubers with many sprouts can be cut into many seed pieces, which can save money.

Cutting seed potato pieces.
Photo Kati Falger

Cutting potato seed pieces

Before planting, cut the seed potatoes (unless already small) into chunks about the volume of a ping-pong ball and weighing 1–2 oz (30–60 g) each, with the smaller fingerlings at 0.7–1 oz (20–30 g). Within a reasonable range, the size of the seed piece has little effect on the final yield, so long as it doesn’t shrivel before growing, and has enough food reserves to get the stem up into the sunshine. Cutting large potatoes is more economical than planting them whole.

For cold-weather planting early in the year, aim for two sprouts per piece, which allows one for insurance if the first one gets frosted off after emergence. For warm-weather plantings, one sprout per piece is enough. Extra sprouts can be rubbed off when planting. Planting seed pieces with too many sprouts will cause only small potatoes to grow, as each stem is effectively a single plant and will be competing with the others for light and nutrients. Also, overcrowding can force tubers up through the soil, and they will turn green if they reach the surface.

Cutting does “age” the seed, leading to weaker sprouts, and the final plant size will be smaller and the plants will die sooner. The total yield will be lower (although earlier) than from “younger” seed. Young unsprouted seed potatoes can be cut and then held at about 50°F (10°C). We often keep ours at 65°F–70°F (18°C–21°C). Older seed should not be kept above 45°F (7°C). Since sprouting ages the tuber, temperatures should be lower for seed that has already sprouted.

We usually cut our seed 1-3 days before planting. Varieties like Atlantic and Kennebec have slow healing abilities, and are best cut ahead of time. Up to 14 days ahead of planting is OK for cutting pre-sprouted potatoes. Unsprouted potatoes can be cut as much as a month ahead, although my choice would be to sprout them for at least two weeks and then cut pieces. It is more challenging to cut unsprouted potatoes, because there’s no knowing which eyes will actually sprout. I think cutting immediately before planting only works in warm dry conditions, as the unhealed surfaces can rot in cool wet conditions. Delayed emergence and patchy stands are signs of planting the seed in soil that was too cold, too wet or even too dry. Erratic and slow plant growth interferes with timely hilling; smaller plant canopies offer less weed competition.

Cutting seed potato pieces.

Make clean cuts with a sharp knife, aiming for blocky pieces about 1–2 oz (30–60 g) each. Avoid cutting thin slices or slivers, as these may dry out and die rather than grow. The cuts should not be too close to the eyes. Reject any potatoes with no sprouts. Some people cut their potatoes a few days ahead of planting and put the pieces back into the crates to allow the cut surfaces to heal over. For large quantities you may need several layers deep. If so, use fans to keep a good air circulation. Relative humidity of 85 – 95% is needed to promote healing and avoid dehydration. Some people coat the cut surfaces with sulfur or bark dust to help suberization (toughening of the cell walls).

See the University of Maine Extension Service Bulletin #2412, Potato Facts: Selecting, Cutting and Handling Potato Seed for lots of details. These drawings come from their bulletin.

Seed potato pieces after pre-sprouting for planting.
Photo Kati Falger

Planting your potatoes

Potatoes need to have a good final depth of soil and/or organic mulch above the seed piece. All the new potatoes grow from the stem that grows up from the seed piece. None will grow below the seed piece, so be sure to plant deep enough and hill up and/or lay on thick organic mulch to provide plenty of space for your crop.

Row spacing of 32″–45″ (80–115 cm) is common, with in-row spacing of 10″–15″ (25–38 cm). In early spring, when the soil is cold — if you want fast emergence and can hill up two or three times — you could plant shallow: as little as one inch (2.5 cm) deep in the North and four inches (10 cm) deep in the South. This technique helps avoid Sclerotinia problems. When the chilliness of deeper soil is not an issue, plant deeper, especially if your chances to hill might be restricted (for instance, by too much rain).

Dig furrows (by machine or by hand) at the chosen depth, normally 4-6” (10-15 cm). Add compost if possible. Plant the potatoes, sprouts up. Take care not to bruise the seed pieces when planting. If you are planting by hand, have some kind of measure – your foot, a stick or the width of the crate. Cover with at least 2” (5 cm) of soil. Later more soil will be piled up against the stems, in the process called “hilling”.

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

When plants are 8-10” (20-25 cm) tall, they need hilling. I’ll cover this more fully next month.

An alternative planting method for those with lots of organic mulch, is to set the potatoes on the surface of the (loose, not compacted) soil, and cover with 12” (30 cm) of loose straw or hay.

Potatoes can be grown in containers, such as drums, barrels, large bags. I don’t recommend stacked tires as these often contain too much toxic dust and particles.

Also see Harvest to Table How to Grow Potatoes

Spring potato rows, in need of hilling.
Photo Wren Vile