Pre-sprouting and Cutting Seed Potatoes

Reader Contribution by Pam Dawling
1 / 3
2 / 3
3 / 3

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

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. If you have suddenly decided to grow potatoes this year, buy your seed potatoes asap and set them to pre-sprout (chit), while figuring out where they’re going to grow and preparing the soil. Then plant and, before they emerge, figure out what to do next.

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!

Potato Planting Dates

Potatoes are a cool-weather crop, but the tops are not frost tolerant.

A traditional spring phenology sign is that daffodils should start blooming before you plant potatoes. 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 tops and growth will stop, so late plantings should be timed to get the tubers to maturity before the expected frost date. 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. 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)

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

Pre-sprouting seed 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;

  • making up for a late start in getting seed potatoes;
  • 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 and the size of the potatoes by reducing the number of stems 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. 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, 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 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. Planting seed pieces with 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.

Photo by Kati Falger. Cutting seed potatoes into planting pieces.

Cutting Seed Potato Pieces

Cut the 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. We usually cut ours 1-3 days before planting. Up to 14 days ahead of planting seems to be OK. I think cutting immediately before planting only works in warm dry conditions, as the unhealed surfaces can rot in cool wet conditions. For cold-weather planting early in the year, have 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.

Cut the seed potatoes into chunks about the volume of a ping-pong ball and weighing 1–2 ounces (30–60 grams) each. There is no yield advantage in using seed pieces bigger than this. Make clean cuts with a sharp knife, aiming for blocky pieces about 1–2 ounces (30–60 grams) 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. See the University of Maine Extension Service Bulletin #2412, Potato Facts: Selecting, Cutting and Handling Potato Seed for lots of details and helpful drawings of cut pieces.

Photo by Kati Falger. Potato seed pieces curing after pre-sprouting and cutting.

Planting 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. Potato plants need hilling when they are 8-10” (20-25 cm) tall.

Row spacing of 32–45 inches (80–115 cm) is common, with in-row spacing of 10–15 inches (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 inches (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”.

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.

Pam Dawlinghas worked at Twin Oaks Community in central Virginia for more than 27 years, growing vegetables for 100 people on 3.5 acres and training many members in sustainable vegetable production. She is the author ofSustainable Market FarmingandThe Year-Round HoophousePam often presents workshops at MOTHER EARTH NEWS FAIRs and at sustainable agriculture conferences. She is a contributing editor with Growing for Market magazine, and a weekly blogger onSustainableMarketFarming.com. Connect with Pam on Facebook, and read all of her MOTHER EARTH NEWS postshere.


All MOTHER EARTH NEWS community bloggers have agreed to follow our Blogging Guidelines, and they are responsible for the accuracy of their posts.