The potato (Solanum tuberosum) is a nightshade, kin to tomatoes and peppers, grown from pieces of tuber instead of seed. That decides most of how you grow it.
Grown for its tubers; the plant flowers above ground. Photo: George Chernilevsky · Public domain, via Wikimedia Commons.
Certified seed potatoes from a garden center or seed company. A bag from the grocery store won’t do.
A spot that hasn’t grown potatoes, tomatoes, peppers or eggplant in the last three years
A hoe or spade for hilling, and some straw to mulch with afterwards
A garden fork for digging, and a cool, dark place to keep the crop
Potatoes are one of the most generous things you can grow. You put a few pieces of tuber in the ground in spring, dig at the end of summer, and come up with a pile of potatoes you grew yourself. Most of the skill is in a handful of decisions made before and around planting.
What kind of plant is Solanum tuberosum?
Solanum tuberosum is the ordinary potato. It belongs to the nightshade family (Solanaceae) along with tomatoes, peppers, eggplant and tomatillos. That family tie matters more than it sounds, because those crops share diseases, which is why they share a rotation.
The part you eat is a stem. A potato tuber is a swollen underground stem that forms at the tip of a runner growing out from the piece you planted. The plant does flower, and it sometimes sets small potato-like fruits with seed inside, but that seed won’t grow true to the variety, so potatoes are grown from tubers.
Can I plant potatoes from the grocery store?
Because the new plant grows straight from a piece of the old tuber, whatever the old tuber carried comes along with it. That’s why the start matters.
Potatoes are grown from certified seed potatoes (disease-free seed tubers from a certified source). Don't plant saved tubers or grocery-store potatoes. Large seed tubers are cut into blocky pieces, each with at least one eye, and the cut surfaces left to dry to a tough protective skin before planting. Grocery-store potatoes may be treated to stay dormant and can carry disease that persists in the soil.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: Potato is propagated vegetatively (the crop grows from a piece of tuber), so any virus or disease in the seed tuber is passed straight to the new plants and can remain in the soil for years; certified seed is inspected, disease-free stock, which is why saving one's own tubers or planting grocery potatoes risks introducing disease. Each planted piece must carry an eye, the bud the new shoot grows from. A freshly cut piece exposes moist flesh that storage and soil rots colonise; holding the cut pieces a few days until the surface dries and forms a corky protective layer (suberises) seals that wound before planting. Grocery tubers may additionally be treated with a sprout inhibitor, so they are slow to grow.
Evidence: checked against 2 sources in the research-based literature.
When buying seed tubers, ask for certified seed as this will help reduce the potential for introducing disease into the garden
Remedy
Buy certified seed potatoes. Cut large ones into blocky pieces of about two ounces, each with at least one or two eyes; leave small tubers whole. Spread the cut pieces at room temperature (about 60-70 °F) for a few days until the cut surface has dried and formed a tough skin, then plant. Do not plant grocery-store potatoes (possible sprout inhibitor, and disease that lingers in the soil) and do not replant your own saved tubers year to year (virus and disease build up).
A large seed potato can be cut into blocky pieces of about 40–60 g1½–2 ounces, each with at least one good eye. Small seed potatoes can go in whole. Leave cut pieces somewhere at room temperature for a few days, until the cut face has dried to a tough skin, which helps the piece resist rot once it’s planted.
For an earlier start, you can pre-sprout (“green sprout”) whole seed potatoes. Keep them in the dark at room temperature until they sprout, then move them into the light so the sprouts stay short and sturdy. Cut and plant as usual.
When and how deep do I plant potatoes?
Potatoes are half-hardy. They’ll take a light frost, so they don’t have to wait for the tomatoes’ frost-safe date, and the planner puts them on the early window.
Cold-hardy annuals can be sown or transplanted before the last spring frost (hardy crops about three weeks before, half-hardy about two weeks before). Tender crops wait for the frost itself.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: Cold hardiness. Hardy crops tolerate hard frost and half-hardy crops tolerate light frost, so they can occupy the ground weeks before frost-tender crops are allowed in. The offset is measured from the last spring frost.
Evidence: checked against 2 sources in the research-based literature.
Soil temperature matters as much as the calendar. Plant once the soil has warmed to at least 7°C45°F. Seed pieces planted into a long, cold spell sit there and can rot, and hard frosts set the young plants back.
Plant the pieces cut side down, 8–13 cm3–5 inches deep and about 25–30 cm10–12 inches apart, in rows 75–90 cm30–36 inches apart. Closer spacing gives you more, smaller potatoes; wider spacing gives fewer, bigger ones.
Potatoes come in early and late types. Early ones give you tender new potatoes, and later ones give you the potatoes that keep for months. Growing one of each spreads the harvest out. The planner knows three types: first early (new potatoes), second early, and maincrop (for storage). Tell it which one you’re growing and it narrows the harvest window to that type. The second-early and maincrop day counts are estimates, so take those two as a guide.
Why do potatoes turn green, and how does hilling stop it?
New potatoes form close to the piece you planted, near the surface, and a tuber that catches light turns green. Hilling means drawing soil up around the stems to cover them.
Potato tubers that catch light while growing (wherever soil settles or washes away and shallow tubers are left uncovered) turn green and become bitter and toxic, so the crop must be kept covered with soil.
This one is about eating safely, not how the plant grows. The planner suggests it.
How it works: Tuber tissue exposed to light produces chlorophyll and, in that same tissue, synthesises the toxic glycoalkaloids solanine and chaconine, so soil mounded over the developing tubers keeps light off them and prevents both the greening and the toxin from forming.
Evidence: checked against 2 sources in the research-based literature.
Green skin occurs when potato tubers experience exposure to light
Remedy
Hill the crop. Once the stems are roughly 8-12 inches (20-30 cm) tall, draw loose soil up around the base of the plants to cover most of the stem and bury any shallow tubers, and repeat two or three times through the season so the tubers stay covered. Green tubers are bitter and should not be eaten; the green signals the toxin is present and is not reliably removed by peeling, so the defense is to keep light off the tubers in the first place, in the ground and again in dark storage.
Start hilling when the stems are about 30 cma foot tall, and hill once or twice more as they grow. By the end of the season you’ll have added about 15–20 cm6–8 inches of soil along the row. Some gardeners plant in a shallow trench instead, then fill it back in as the plants grow. Once you’ve finished hilling, a thick straw mulch keeps the soil cool and moist.
If a potato does turn green, cut the green parts away before cooking. Don’t eat potatoes that are green over much of their surface.
Water them steadily
Potatoes want even moisture, especially while the tubers are forming. If they run short early on, you can get knobby or hollow potatoes, and too much water causes its own problems. Soak the bed thoroughly once or twice a week, aiming for about 2.5 cman inch of water a week including rain. Sandy soil needs watering more often. Water at the base of the plants. Wet foliage spreads fungal disease. The watering guide covers the rest.
When do I dig potatoes?
New potatoes are ready about 7–8 weeks after planting, usually once the plants have flowered. Dig gently with a garden fork so you don’t bruise or spear them.
For potatoes you want to keep, wait until the tops have died back, then leave the tubers in the ground about two more weeks so their skins toughen. Dig on a dry day, and don’t dig when the soil is wet. Every cut or bruise is a way in for rot. Brush off loose soil, but don’t wash them.
Before storing them, let the potatoes cure for about two weeks somewhere dark and airy so the skins set and small nicks heal. Then store them cool, dark and unwashed. The harvest and keeping guide covers curing and storage in detail.
Why did my potatoes get scab, and where do they go next year?
Common scab leaves rough, corky patches on the skin. It’s ugly, but a scabby potato is perfectly good to eat. Scab gets worse as soil becomes less acid, which is why the usual garden habit of liming works against potatoes.
Limed ground is the wrong ground for the next potato crop, because common scab worsens as soil pH rises and lime raises it.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: Common scab (Streptomyces scabies) infects young tubers through the lenticels and is favored as soil goes less acid; at about pH 5.2 and below it is largely suppressed. Lime raises pH and its effect lasts years, so ground limed for a crop that wants sweet soil carries a scab-prone pH into the seasons that follow.
Evidence: checked against 5 sources in the research-based literature.
Lime will also raise the pH of soil. A soil test will tell you how much lime to add.
Remedy
This ground has been limed, and potatoes want it sour. Scab is skin-deep and the tubers still eat, but where it has been a problem, put the potatoes on ground you have not limed, choose a scab-resistant variety, keep the soil evenly moist for the four to nine weeks after planting while the tubers set, and never lime or add wood ash where potatoes are going next. A later pH reading at or below about 5.2 on this ground would quiet this. We will not give a sulfur rate to get there, because the amount is your lab's to give: ask it for its sulfur recommendation for your soil.
The sources agree that potatoes like acid soil, but they don’t agree on one number. One source puts the target lower wherever scab keeps coming back, and another explains why: the right number depends on whether your variety resists scab. So Milpa keeps its potato pH range marked as an estimate. The same goes for feeding. Potatoes are hungry, but how much to add is a question for a soil test.
Blight is the other disease that comes back with the crop. Late blight survives the winter in potatoes left in the ground. It doesn’t survive in the soil itself.
Late blight comes back from last year's potatoes. It does not come back from the soil. Tubers left in the ground carry it through winter as volunteers, so ground that grew potatoes last season needs its volunteers pulled before this year's tomatoes or potatoes go in.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: Phytophthora infestans needs living tissue to overwinter. Infected tubers left in the ground, in a compost heap that does not freeze, or in a cull pile survive and sprout as volunteers that carry the blight into the new season; tomatoes do not carry it, because frost kills the whole plant and the seed is clean.
Evidence: checked against 2 sources in the research-based literature.
P. infestans can overwinter in Minnesota if protected in potato cull piles
Remedy
From April to June, walk last year's potato ground and any compost or cull pile, pull every volunteer potato and put it in the bin. Don't compost it. Then plant.
If you skip this, you lose real yield. The planner warns you.
How it works: Alternaria solani (early blight) and Septoria lycopersici overwinter on tomato debris and solanaceous weeds; a 2-3 year break plus debris removal starves this inoculum. Fusarium has a very narrow host range, so rotating away from the host is effective. Verticillium is the exception. Its microsclerotia persist ~10 years and it infects 300+ host families, so a garden-scale rotation does not control it.
Evidence: checked against 5 sources in the research-based literature.
Verticillium wilt tends to affect a wide range of hosts and can infect many unrelated plants. Fusarium spp., on the other hand, have very narrow host ranges.
Remedy
Plant nightshades in different ground, or wait until 3 years have passed since this ground last held them. Clear and remove the old debris either way, because the inoculum overwinters on it.
Three years without potatoes, tomatoes, peppers, eggplant or tomatillos is the goal. If your beds are too few for that, the crop rotation guide helps you plan around it.
Further reading
Good places to read more about the technique on this page. The grade on each card comes from that card’s own sources.
