white potatoes – Sustainable Market Farming

What Makes Potatoes Sprout?

“White” or Peruvian potatoes (sometimes called Irish potatoes) are stem tubers in the nightshade family; sweet potatoes are root tubers in the Morning Glory family. This article is about Peruvian potatoes, not sweet potatoes.

Curing potatoes

Potatoes are cured enough for storage when the skins don’t rub off. It’s best to leave the potatoes in the ground for two weeks after the tops die, whether naturally or because of mowing, if you want them to store. When the potatoes are harvested after the skins have toughened, there will be less damage during harvest. Curing allows skins to harden and some of the starches to convert to sugars. These changes help the tubers to store for months.

When potatoes first go into storage, they are still “alive” and respiring, and need fresh air frequently. They will heat up if left closed in, and could develop black centers, where the cells die from lack of oxygen.

Storing newly harvested potatoes

For the first two weeks after harvest, the root cellar or other storage space will need 6-9 hours of ventilation every two or three days. The temperature goal is 60°F–75°F (16°C–24°C), with 95% humidity. Ventilate when the temperature is 0–20F (0–11C) cooler than your goal: in the daytime if nights are too cold and days are mild; at night if nights are mild and days too warm. If it is very damp in there, ventilate more.

Two weeks after harvest, sort all the potatoes. By this time, any which are going to rot have likely started doing so. Restack, remembering to keep airspace between the crates and walls. For weeks 2–4, the temperature goal is 50°F (10°C) and fresh air is needed about once a week.

Potato crates in our root cellar.
Photo Nina Gentle

Long term potato storage

After week 4 in winter, cool to 40°F (5°C); in summer, below 50°F (10°C). Ventilation for air exchange is no longer needed, as the tubers have become dormant. The final long-term storage conditions are cool and fairly moist, 40°F–50°F (5°C–10°C), 85%–90% humidity—a root cellar is ideal. Below 40°F (5°C) some starches convert to sugars, giving the potatoes a bad flavor and causing them to blacken if fried. Try hard to avoid having the cellar cool down, then warm up. That causes the potatoes to sprout.

Pre-sprouting seed potatoes

Potatoes have a dormant period of 4–8 weeks after harvest before they will sprout. The warmer the conditions after dormancy ends, the quicker they will sprout. If you want potatoes to sprout during the dormant period, trick them by refrigerating for 16 days, then pre-sprouting them in the light.

We routinely “chit” or pre-sprout our seed potatoes before planting. Bring the seed potatoes into a warm, well-lit room around 65°F–70°F (18°C–21°C) and set them upright in shallow boxes, rose end up, stem (belly-button) down, for 2–4 weeks in spring, 1–2 weeks in summer. For summer planting, store your seed potatoes in a cool place at 45°F–50°F (7°C–10°C) until 2 weeks before your planting date, then sprout them.

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

The effects of ethylene

Ethylene is a naturally occurring, odorless, colorless gas produced by many fruits and vegetables, but it can also be produced by faulty heating units and combustion engines. Propane heaters should not be used, as propane combustion produces ethylene. Incomplete combustion of organic fuels can result in the production of carbon monoxide, ethylene and other byproducts. Do not use any unvented hydrocarbon fuel heaters near stored produce.

Ethylene is associated with ripening, sprouting and rotting. Some crops produce ethylene in storage—apples, cantaloupes, ripening tomatoes, already-sprouting potatoes all produce higher than average amounts. Chilling, wounding and pathogen attack can all induce ethylene formation in damaged crops.

Some crops, including most cut greens, are not sensitive to ethylene and can be stored in the same space as ethylene-producing crops. Other crops are very sensitive and will deteriorate in a high-ethylene environment. Potatoes will sprout, ripe fruits will go over the top, carrots lose their sweetness and become bitter.

Summary: Potatoes are more likely to sprout if they are more than 4–8 weeks after harvest; in the light; near fruits, vegetables, flowers or malfunctioning propane or natural gas heaters that produce ethylene; too warm, or warm after being cool. Potato sprouts are toxic, see my earlier article.

Starting sweet potato slips

Growing sweet potato slips from roots planted in flats of compost.Credit Kathryn Simmons — Growing sweet potato slips from roots planted in flats of compost.
Credit Kathryn Simmons

The weather outside is still full of rain, but I’m optimistically starting growing our own sweet potato slips. Surely it will dry out by May?

Sweet potatoes are related to morning glory, in the genus Ipomoea. They are not yams, even though they are often called yams! True yams are a tropical species of tuber (genus Dioscorea). Sweet potatoes are roots, not tubers, and will not even cross with yams. So forget yams. Unlike white potatoes, which have the annual plant sequence of vegetative
growth, flowering and dying back, sweet potato plants would continue growing forever if the weather was warm enough.

The mystique of sweet potato slips

Sweet potatoes are not grown from seed or from replanted roots, but from “slips,” which are pieces of stem with a few leaves, grown from a mother root. We used to buy bare-root sweet potato slips to plant, believing growing our own would be very tricky. The collapse of our supplier and our desire to have organic plants (plus a need to reduce our expenses one year), pushed us into growing our own. We had some problems initially, so I can warn you about how not to do it. Now we have a system we really like, and we’ve found several advantages of homegrown slips over purchased ones.

With purchased slips, we had to specify a shipping date months ahead, then hope the weather sprites would be kind. We had to jump to when the plants arrived, and get them all in the ground pronto, to keep them alive as best we could (because their roots needed moisture). We accepted as normal a certain amount of drooping. We can have late frosts, spring droughts or El Niño wet springs, and climate change is only adding to the uncertainty. With homegrown slips we can delay planting if that seems wise; we can plant them in stages rather than all on one day. The transplants don’t wilt. We can grow them big and plant them with three to five nodes underground, giving more chance of survival in heat or frost. We can keep some spares on hand to replace casualties. The sturdy plants get off to a strong start, which could be an even bigger advantage further north where the season of warm-enough weather is on the short side for a 90–120-day plant. And we are self-reliant — we never have to spend money on them.

Timing

Figure out your ideal planting date and work back to find your starting date. Planting is usually done about two weeks after the last frost. The soil temperature should reach at least 65°F (18°C) at 4″ (10 cm) deep on four consecutive days. For us, that’s around May 12. It takes eight weeks to grow the slips, and the roots produce more slips if conditioned for two weeks (or even four), before you start to grow slips. So start ten to twelve weeks before your planting date. We now start March 1. Here’s where I made my first big mistake — following directions written for much further south, I tried to start growing slips in mid-January. Dismal fight against nature!

Testing (Optional)

If you want to get the best yields from your mother roots, first test the roots in a bucket of water — the ones that float are said to yield better and produce better-flavored roots. Next, test for viral streaking — also known as color breaks or chimeras, where paler spots or radial streaks appear in the flesh — and discard roots with pale spots or streaks wider than a pencil lead. Cut a thin slice from the distal end of each root — the stringy root end, opposite the end that was attached to the plant stem. All the sprouts will grow from the stem end, so don’t cut there! If you can’t tell the difference between the ends, you can ignore this step and plan not to propagate your own slips for more than a couple of years (so the virus load doesn’t get too high). Or if you are a home gardener dealing with a small crop, you could keep the slips from each root separately and cut up the mother root before planting and then discard the slips from streaked roots.

Conditioning (Also Optional)

Put the chosen roots in flats, boxes or trays, without soil, in a warm, moist, light place for two to four weeks. Ideal conditions are 75°F–85°F (24°C–29°C) at 95% humidity. This can double or triple the number of sprouts the root will produce in a timely manner. We use our germinating chamber, which is an old glass door refrigerator heated by a light bulb. See the photo above. Conditioning after testing allows the cut surfaces to heal before they are covered by compost. The environment for sprouting the roots is similar, so you can probably use the same location.

Sprouting

Set up a place with light, humidity and ventilation at 75°F–85°F (24°C—29°C) and with about 12″ (30 cm) of headroom. Plant the selected roots almost touching each other, horizontally in free-draining light potting compost in flats or crates. Water the boxes and put them to sprout. Once again, we use our ex-fridge germinator. Using boxes is much more manageable than having the roots loose in a big coldframe. Indoor spaces are much easier to heat than the great outdoors! Boxes can be insulated and put on a bench at a decent working height, with lights or heat lamps over them. Keep the compost damp, and if your planting medium is without nutrients, give liquid feed occasionally once sprouting starts. For small quantities of slips, it is possible to sprout the potatoes half-submerged in water, either in trays of water or by suspending a sweet potato impaled on toothpicks, resting on the top of a glass of water. For larger quantities I recommend our method.

Cut sweet potato slips put in water to grow roots.Credit Kathryn Simmons — Cut sweet potato slips put in water to grow roots.
Credit Kathryn Simmons

Cutting and spotting the slips
After 5–7 days, the roots will begin to produce slips. Ideally, wait until the slips are 6″–12″ (15–30 cm) tall with 4–6 leaves beginning, then cut them from the root and stand them in water. If necessary, cut them a bit shorter. Some people pull or twist the slips from the roots, but this could transfer diseases by bringing a small piece of the root with the sprout. I cut the slips daily, bunch them in a rubber band and stand them in a small bucket of water. The slips will grow more roots while they are in water for several days, which seems to be an advantage. Once a week I spot (plant) the oldest, most vigorous slips (with good roots) into 4″ (10-cm) deep wood flats filled with compost. The spotted flats require good light in a frost-free greenhouse and sufficient water. If you are two weeks away from your planting date and are short of slips, you can take cuttings from the first flats of slips that were spotted, to make more. The slips planted in flats become very sturdy, allowing flexibility about planting dates and a longer slip-cutting season. About ten days before planting, start to harden off the flats of slips by reducing the temperature and increasing the airflow. It’s also possible to skip the spotting stage and transplant the slips outside directly from the water, but I don’t think this is as good as spotting them into flats of good compost for a few weeks.

Once a week we plant the rooted slips in flats.Credit: Kathryn Simmons. — Once a week we plant the rooted slips in flats.
Credit: Kathryn Simmons.

From Sustainable Market Farming, (c) Pam Dawling, New Society Publishers 2013

Home from CFSA, Superstorm Sandy

Beauregard - 6 Slips — Beauregard sweet potato
Photo Southern Exposure Seed Exchange

Superstorm Sandy didn’t do us much damage, luckily. It’s been raining for 40 hours, but we’ve only got 2.8 inches so far and it looks like it’s going to clear up later today. Despite my worries about the broken hoophouse windows blowing in and us losing our newly re-plasticked hoophouse, it didn’t happen. We didn’t have any really high winds, and we didn’t even lose power, but of course we did all the prep work.

Georgia Jet - 6 Slips — Georgia Jet
Photo Southern Exposure Seed Exchange

Yesterday we re-stacked our sweet potatoes which had finished curing (the skins don’t rub off any more). We moved them into a wire rodent-proof cage, and close-stacked them, taking away all the sticks that spaced the boxes during curing. I haven’t got numbers for the total yield yet, but it comes to 96 boxes. The Georgia Jet produced 42 boxes and the Beauregard only 32 from the same length row. Our two heirloom varieties produced three boxes each. We don’t expect many of them, but we are keeping the varieties alive, because genetic diversity is important and who knows what secret virtues these varieties have?

We also bravely spent time in the rain, digging drainage ditches to reduce the impact of the hurricane. They seem to have worked quite well. And we draped the soggy rowcovers over the frost tender crops, in anticipation of freezing conditions.

While I was away at the CFSA Conference, the crew harvested the white potatoes. We got a good yield (also no numbers yet), but we got a disappointingly large number of greened potatoes. (Green from being exposed to the light.) I think the reason is that our new experimental tractor-mounted furrow-making disks don’t make furrows as deep as we need. The walk-behind BCS furrower on the rototiller made adequate furrows, but not as good as the old Troybilt furrower. This flags a need to research better gear before March.

I had a great time at the CFSA Conference. I think there were about 700 people there. About 70 came to my workshop Growing Great Garlic, on Saturday afternoon. They were very appreciative, and I managed OK without my notes! It’s not as bad as it sounds – I had a slideshow and had practiced quite a few times, and knew it better than I would have guessed. Somehow I couldn’t get my notes on the laptop screen without them also appearing on the big screen along with lots of clutter. This flags a need to find out before my three workshops at the Southern Sustainable Agriculture Working Group conference in Little Rock, Arkansas, January 23-26. Busy, busy.

The conference was very well organized and the food was spectacular – mostly local and sustainably grown. I had the chance to attend several workshops by other people. Tony Keinath, the vegetable pathologist at Clemson University, talked on Sustainable and Organic Approaches to Managing Cucurbit and Tomato Diseases – a very well-prepared and information-packed session. I feel in a better place to tackle next year’s plagues now. I was struck by the fact that he had seen NO benefits of using Oxidate, the hydrogen peroxide disease control product.

A workshop I found particularly valuable was Laura Lengnick‘s presentation “Is Your Farm Climate Ready?” She is doing valuable work to help farmers get ready for climate variability. She is one of the main authors of a USDA ARS report Climate Change and Agriculture: Effect and Adaptation. Its publication date is November 14 2012. She also spoke at the August 2012 symposium of the Ecological Society of America, Climate change impacts on agricultural systems:

She suggests viewing climate change as yet another production risk to assess and prepare for. The vulnerability of your farm has two components: exposure and adaptive capacity. As far as vulnerability, the most immediate key exposure is water issues (too much and too little). Rising air temperatures, including night temperatures, more extreme temperatures provide threats and some opportunities. Increasing CO₂ levels will provide some positive effects such as faster crop growth. As far as adaptive capacity, the main feature of that aspect is our personal capacity to respond and plan. Laura Lengnick says “Greater attention to climate as critical for decision-making is expected by future generations of producers.” We need to start with ourselves.

Baby ginger, ready to be eaten, pickled, candied, frozen.
Photo East Branch Ginger

Next I attended a workshop by Susan Anderson of East Branch Ginger, and learned so much about how to do the best by this crop, that I am looking forward to an even bigger harvest next year. This year we harvested 165 pounds, and saved 65 pounds as seed stock, so we can plant a bigger patch in next year’s hoophouse.

Harvested baby ginger, about 6 months old
Photo East Branch Ginger

Meanwhile I’ve finished my next article for Growing for Market. My working title is Knowing When to Take Action. It’s the third part of my series on being a resilient farmer. This article includes scouting and monitoring for pests and diseases; using pest and disease forecast services; and being prepared for the effects of extreme high and low temperatures. When is it time to cut your losses? A big part of the article is a table of soil temperatures to help when deciding planting and harvesting dates.