For the very first episode of The joe gardener Show that’s dedicated solely to growing potatoes, I couldn’t have asked for a better guest than Jim Gerritsen, the founder of Wood Prairie Family Farm in Bridgewater, Maine, where he has grown potatoes organically for 45 years. Jim generously shares everything to know about growing epic potatoes, from before you plant to storing the harvest.
Wood Prairie Family Farm is a certified organic potato seed grower in Northern Maine. Jim points out that about 70 years ago, Maine was the No. 1 potato-producing state in the country, and Aroostook County was the center of the “potato empire.” Potatoes have been grown in the area for more than 200 years, since the first European settlers arrived. Still today, and for more than a century, Maine has continued to be the leader in growing seed potatoes.
“There’s just no better place on earth to grow potatoes than Aroostook County, Maine,” Jim says proudly. “There’s a tremendous reservoir of knowledge of how to grow potatoes here.”
Jim is a third-generation farmer — his father and grandfather were apple farmers in Yakima, Washington. He started Wood Prairie Family Farm when he was 21 years old. The farm is 115 acres, half field and half woods, which he says is typical for Maine. About four years ago, his son Caleb took over, and Jim says now he and his wife, Megan, are his son’s farmhands.
Wood Prairie Farm has been certified USDA organic since 2002 and is also certified by the Maine Department of Agriculture for certified seed, a designation that ensures seed crops are free of disease and damage. A blue tag is affixed to bags of seed potatoes to denote this certification, which Maine, New York, Wisconsin Michigan, North Dakota, Minnesota, Montana and Washington all have a version of.
Advantages of Organic Seed Potatoes
Seed potatoes grown conventionally have been treated with fungicide prior to planting, given synthetic fertilizer, treated with systemic insecticide, and treated with systemic fungicide on the foliage. As Jim puts it, they have been pampered.
But an organic seed potato hasn’t been treated with chemicals, and that means it’s a case of “survival of the fittest.” Organic seed potatoes are simply the most resilient, with more vigor and better disease resistance. Grown in good soil, they will grow healthy plants that don’t need chemical inputs to thrive, and the tubers will be more nutrient dense.
Why Maine Is Conducive to Growing Potatoes
Maine has two things that make it such a great place for growing potatoes, the climate and the soil. Jim says Maine benefits from being far north, where it doesn’t get all that hot in the summertime. There is also a good distribution of rainfall, and cool nights with sunny, warm days. The soil is sandy and well-drained, which is perfect for potatoes.
Potatoes are a cool-season crop, so further south, the growing season is shorter. When temperatures are steady in the mid-90s, potato plants will die down and won’t produce well. But in Maine, potatoes can be grown through summer and harvested in late September and October.
Wood Prairie Family Farm’s fall-harvested seed potatoes go into underground storage — 38 degrees with high humidity — and are then shipped out of storage for the next 10 months, through the Fourth of July.
Seed Potatoes Vs. New Potatoes
Seed potatoes are the little tubers that farmers and gardeners alike plant to grow a new crop of potatoes. To make seed potatoes, farmers kill the top growth of potato plants before the tubers have grown to full size, so the tubers remain small. They are left in the ground for a few weeks to allow the moisture content in the skin to decrease. That toughens up the tubers so they can stand up to the rigors of mechanical harvesting and long-term storage.
Seed potatoes differ from “new potatoes.” New potatoes still have green foliage, which has not been killed, cut or removed. The potatoes are immature and tender — a thumb rub will take the skin right off. Jim says the beauty of having a home garden or going to a farmers market is the ability to eat true new potatoes, as opposed to those in the grocery store that were mechanically harvested and marketed as “new.”
In Year in the Life of a Seed Potato Farmer
The growing cycle of seed potatoes at Wood Prairie Family Farm starts in mid-April. It takes about a month to warm up seed potatoes for planting, so that’s when they take out about 25,000 or 30,000 pounds of seed potatoes to “green sprout.”
When the weather has dried out and the soil is 50 degrees at 7 a.m. at a depth of 4 inches, the conditions are right for planting. This is usually May 15 to May 18. It then takes a few weeks to get everything planted, wrapping up in early June. The challenge then becomes keeping weeds and insects out, which involves flame weeding and cultivation for weeds and hand-removal for pests.
In August, crews “kill down” the fields to get the potatoes ready for harvest. As organic farmers, they use propane torches to kill the top growth of the plants. Mechanical beaters are another organic option, though the beaters risk spreading mosaic virus. (Non-organic farms use Diquat herbicide or sulfuric acid to kill down fields.)
“The best seed potatoes are killed somewhat immature,” Jim explains. “You’re going to have more youthful vigor in the seed tubers, lower physiological age, and there’s a direct correlation to yield in the next generation.”
Bridgewater, Maine, is one of the last areas in the United States that still closes schools for three weeks starting in mid-September for the fall potato harvest. “We get students that help us, as well as our own kids,” Jim says.
Fulfilling orders and shipping begin as soon as the potatoes are taken out of the field. The first orders come from Gulf Coast states — Florida, Texas, Alabama, Georgia, Mississippi, Louisiana — and Southern California, where winters are mild enough to successfully grow a fall potato crop.
Any potato, once harvested, requires a dormancy period of four to eight weeks before it will sprout and grow. Jim says the most dormant variety he’s ever come across is Red Cloud, which is bred in Nebraska and takes about two extra weeks to sprout — which makes it a great variety for storing. On the other end of the spectrum are Russian banana fingerling and dark red Norland, which will readily break dormancy in warm fall conditions.
To encourage these first-shipped potatoes to sprout, they are treated with organic gibberellic acid derived from seaweed. This is done up until about Thanksgiving, after which the potatoes from storage will have no trouble sprouting without treatment.
Early March is peak time for shipping, when Wood Prairie Family Farm works seven days a week to keep up.
“Green Sprouting” Potatoes
The first step to planting seed potatoes comes a month before actually putting the potatoes in the ground. Jim says to warm the seed potatoes up to about 75 degrees while keeping them in the dark. He recommends setting the potatoes in a warm spot, like on top of the refrigerator, for two to three days.
Once dormancy is broken this way, the potatoes can be kept at 55 degrees in the light. This will cut back the respiration rate so the seed potatoes will be primed for planting and will not lose moisture and dry out. Jim notes that potatoes are living organisms that need oxygen, so monitoring the temperature and keeping respiration down is important for storage.
If a seed potato has not broken dormancy before planting, it could become a meal for pathogenic fungi — especially if there are heavy rains after planting. To ensure potatoes have broken dormancy, Jim says to plant a seed potato only after a sprout is coming from the eye. A sprouted potato has broken dormancy and has a much better chance of survival.
Pre-sprouting potatoes is known as “green sprouting” or “greening” in the United States, and as “chitting” in Europe. This can be done under any light: sunlight, fluorescent, incandescent, LED.
Seed potatoes and their sprouts turn green in light, hence the name green sprouting. If you have ever lost track of a potato in your cellar for a long time, you likely found it with long, white sprouts. The white sprouts are in search of light. But under light, sprouts will be green and compact, making them less likely to break off during planting.
When to Plant Seed Potatoes
Seed potatoes may be planted in spring as soon as the soil temperature at a depth of 4 inches has reached 50°F. The depth is important because that’s how deep potatoes are planted. Jim measures at 7 a.m. Any later in the day, after the sun has been up for some time, the temperature reading may be artificially high.
It’s important to wait for the soil to be 50° because at any temperature lower than 45°, wounds on tubers will not heal and callous over. Also, though potato foliage killed by a late-spring hard frost will resprout, seed potatoes will be killed if the ground itself freezes.
Jim recalls the old-timers in Maine who said to plant potatoes when the last of the snow in the shade of the woods has melted. That’s usually in mid-May near him, which coincides with when the soil temperature has reached 50° — so those old-timers really knew their stuff! Another method he’s heard of is to watch for when dandelions bloom. “Nature has ways of helping us along and teaching us what the right time to do activities in the garden is,” Jim says.
In the North, potatoes may be planted later as long as the chosen varieties have time to mature before the first frost in fall. Jim knows gardeners in the Maritimes of Canada who wait until July 1 to plant so they miss the Colorado potato beetles. Growers in warmer climates don’t have that option.
In the South, with its shorter growing season for potatoes, plant a short-season variety as early in spring as the conditions allow, with no delay. Growing in hot soil can cause net necrosis, the symptoms of which are unsightly specks and streaks in raw tubers, and hot weather brings other problems as well.
How to Plant Seed Potatoes
Prepare for planting potatoes by green sprouting, or use this accelerated method: A week or two before planting, take potatoes out of cold storage and warm them up to encourage sprouting. Once sprouted, get them under light so the sprouts stay compact.
A tuber the size of a hen’s egg should be cut in half before planting. Any smaller, and it should be left whole. Any bigger, cut it into any many sections as are equal in size with half an egg. Every cut piece should weigh approximately 1.5 ounces
In sandy soil, gardeners can get away with cutting seed potatoes and then planting them right away. But with clay soil, which holds water next to seed pieces, let the cuts callous over for three or four days before planting.
And another trick is to dip the exposed cut in agricultural lime — either calcium lime or magnesium lime. Lime will desiccate the cut, making it less hospitable for pathogenic fungi. A side benefit is that potatoes love calcium, and lime provides it to them.
The seed pieces should be planted between 2 and 4 inches deep. Any deeper risks the tubers sitting in wet soil.
Shallow tubers at 2 inches deep will have the benefit of soil that is more drained of water. But in the South, Jim recommends 4 inches or even 6 inches deep with straw mulch on top of the soil to keep the tubers cool and to protect the tubers from sunlight, which will turn them green. (Green potatoes have solanine, which is mildly poisonous, and they will be bitter.)
Hilling Up Potatoes
“Hill up” potato plants by adding soil around the stem once the top growth is 4 to 6 inches above the soil. This will prevent sunburn and the hills will drain water more readily. Straw, peat moss or leaves topping the hill will both prevent sunburn and rot down, improving the tilth of the soil and adding organic matter, benefits that compound every season.
Most modern varieties of potato set tubers deep in the hill, but older potato varieties will grow tubers up the stem, which makes hilling up all the more important when growing heirlooms.
“Potatoes are gluttons,” Jim says. They like plant food and a lot of water — and will reward gardeners with big tubers in return. Potatoes require calcium, potassium and phosphorus, but not too much nitrogen, which contributes to foliar growth and delays tuber growth.
Fishmeal is a good potato fertilizer, and Wood Prairie Family Farm sells a proprietary organic fertilizer formulation made just for growing potatoes.
Potato Pest and Disease Issues
Colorado Potato Beetle – Jim says the best control for Colorado potato beetle is handpicking at least twice a week. What’s great about hand-picking is the bugs will never develop chemical resistance. For large-scale issues, there’s Entrust, an organic insecticide containing spinosad, which insect populations can develop a resistance to after repeat applications.
Potato leafhopper – Historically, these were not a problem in Maine, but due to climate change potato leafhoppers arrived two decades ago and show up in the field two out of every three years. Potato leafhoppers are fast, tiny, hard-to-see bugs. They show up in warm weather. If a control method is necessary, Wood Prairie Family Farm uses PyGanic 5.0, an organic, broad-spectrum contact insecticide made from pyrethrin, which is derived from Chrysanthemum.
Potato late blight – This is the pathogen that caused the Irish potato famine of the 1840s. It occurs in moist weather when fungal spores are present. Because winds can blow late blight spores up to 50 miles, blight spreads readily. Certified organic farmers may use copper-based fungal controls for late blight. The hitch is that it must be applied prophylactically — sprayed on foliage before infection.
Potato scab – Scab is typically introduced to soil on seed potatoes. Minimize scab by having adequate phosphorus and mycorrhizal fungi. Scab likes alternating wet and dry conditions and raw horse manure, so let horse manure rot down into compost before using as a fertilizer and keep the garden watered. Farmers used to beat scab by lowering the pH of the soil, making it more acidic, but then acid-tolerant scab emerged. Jim notes that scab is really a cosmetic problem — scabby potatoes are still edible. In fact, they are drier and will more readily absorb butter.
How to Harvest Potatoes
Potato tubers are edible from the time they are pea-sized. When potato plants blossom — in colors that range from white to blue to red and pink — the tubers will be ping-pong ball size. Then about six weeks after flowering, the foliage dies back and the tubers will reach their full size
Potatoes can stay in the ground for four to six weeks after the top growth has died. In this time, the skins dry out and toughen up, making the tubers better suited for storage and less prone to bruising. However, if there are heavy rains or the ground is set to freeze, harvest them sooner.
If just looking for a couple of potatoes to have with dinner, reach gently into a hill and feel around for large potatoes, pull them out, and leave the rest of the plant to continue growing. This can lead to a higher yield from the plant, since the whole plant isn’t killed at the same time.
To take a whole plant out of the ground, loosen up the soil with a garden fork and lift by the dead foliage. Dig around gently for any potatoes left behind.
How to Store Potatoes
In planning to store potatoes throughout fall and winter, hold off on harvesting until four to six weeks after the foliage has died naturally. That way, the tubers have had adequate time to toughen up.
Potatoes store best in cool, soil-like conditions: 38 degrees, dark and high humidity. Potatoes can store for 10 months in those conditions.
If you haven’t already listened to my conversation with Jim Gerritsen on growing potatoes, you can do so now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
What’s your preferred method for growing potatoes? Let us know in the comments below.
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
joegardener Online Gardening Academy™: Three popular courses on gardening fundamentals; managing pests, diseases & weeds; and seed starting.
joegardener Online Gardening Academy Master Seed Starting: Everything you need to know to start your own plants from seed — indoors and out.
joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
Disclosure: Some product links in this guide are affiliate links, which means we would get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us. None of the items included in this list have any bearing on any compensation being an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Exmark, Greenhouse Megastore, High Mowing Organic Seeds, Territorial Seed Company and Wild Alaskan Seafood Box. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.