Potatoes in raised bed gardening: plant, hill, and harvest

Potatoes (Solanum tuberosum) do well in raised beds because the loose, deep soil lets the tubers form freely and makes harvest far easier than digging compacted ground. You plant seed potatoes a few inches deep in spring, mound soil or straw over the stems as the plants grow, keep the bed evenly watered, and pull the crop by hand once the tops die back. The hilling step is the key to a heavy harvest, because potatoes form along the buried stem, and burying more stem gives you more potatoes.

The first time I grew potatoes in a raised bed, I was amazed at the harvest. In the ground the year before, I had sliced half my crop in two with a digging fork and spent an afternoon hunched over hard clay. In the bed, I simply pulled each plant up by the stems and ran my hands through the loose soil. Whole, clean tubers came up by the handful. I have grown potatoes in beds every year since.

Why potatoes suit a raised bed

Potatoes need three things to crop well: loose soil, steady moisture, and room to spread. A raised bed gives you all three. The soil stays loose because you never walk on it, so the tubers expand without hitting compacted ground. The bed drains well, which prevents the rot that ruins potatoes in waterlogged clay. And the depth gives the roots and tubers room to develop.

The warmer spring soil in a raised bed also matters. Potatoes can go in earlier because the bed thaws and warms ahead of the surrounding ground, which buys extra growing time in a short season. In my zone 5b garden, the soil in a raised bed is workable 7 to 10 days before the surrounding ground, and that is enough time to push an early variety to harvest before fall frost. Early varieties planted in a warm bed can be ready by midsummer.

The biggest practical advantage is harvest. Digging potatoes from heavy ground is hard work, and you always lose some to the fork. In a raised bed, you lift the plant and sift the loose soil with your hands. Almost nothing gets damaged, and the job takes minutes instead of an afternoon.

Choose and prepare seed potatoes

Plant certified seed potatoes rather than ones from the grocery store. Seed potatoes are grown to be disease free, while store potatoes are often treated with chlorpropham or another sprout inhibitor and may carry disease into your bed. Garden centres sell them in spring by the variety.

Choose early varieties for the quickest harvest in a short season, or maincrop varieties if you have a longer window and want potatoes to store. Early types like ‘Yukon Gold’ (70 days) and ‘Red Norland’ (75 days) are ready in about 10 to 12 weeks. Maincrop types like ‘Kennebec’ (90 days) and ‘German Butterball’ (95 days) take longer but yield more and keep better through winter.

A few weeks before planting, set the seed potatoes in a cool, bright spot to sprout, a process called chitting. Stand them with the end that has the most eyes facing up, in an egg carton or seed tray at 50 to 60 degrees F. When the sprouts are about 1/2 inch (1.3 cm) long and sturdy, the potatoes are ready to plant. Chitting gives the plants a head start of about 10 days and a slightly earlier crop. Cut large seed potatoes into pieces of about 2 ounces (57 g) each, each with at least two eyes, and let the cut surfaces dry for a day before planting to form a protective callus.

Plant at the right depth and spacing

Plant seed potatoes about 4 inches (10 cm) deep and roughly 12 inches (30 cm) apart, in rows about 15 inches (38 cm) apart in a wide bed. Set each piece with the sprouts facing up. In a 4 by 4 foot (1.2 by 1.2 m) bed, that works out to around 10 to 12 plants.

Do not plant the potatoes at the full depth of the bed right away. You want room above them to hill soil over the stems as they grow. Starting them shallow and burying more stem over the season is what produces a heavy crop. Cover the seed potatoes with 4 inches (10 cm) of soil and water them in.

Wait until the soil has warmed to at least 45 degrees F and the danger of hard frost has mostly passed. In zone 5b, that usually means mid to late spring, around late April to mid May. Potatoes can take a light frost on emerging shoots, but a hard freeze will set them back. If a late frost threatens after the shoots are up, mound a little soil over them or cover the bed for the night with a row cover or an old bedsheet.

Hill the plants as they grow

Hilling is the step that separates a small harvest from a heavy one. As the potato plants grow and reach about 8 inches (20 cm) tall, mound soil, compost, or straw up around the lower stems, leaving the top 3 to 4 inches (7.5 to 10 cm) of leaves exposed. The buried section of stem produces new stolons that swell into tubers, so the more stem you bury over the season, the more potatoes each plant makes. Trials at the University of Maine and elsewhere have shown that hilling at least twice during the season increases total yield by 30 to 50 percent compared to a single early hilling.

Hill the plants two or three times as they grow, every 2 to 3 weeks, until the bed is mounded up and the foliage fills the space. Hilling also keeps developing tubers covered. Potatoes exposed to light turn green and produce solanine, a glycoalkaloid that tastes bitter and can cause illness in quantity, so keeping them buried is essential. If you run short of soil, straw works well for the upper layers and is easy to pull back at harvest.

Water steadily for a good crop

Potatoes need steady moisture, especially once the plants flower and the tubers are sizing up. Dry spells during this period give you small, cracked, or knobbly potatoes. A raised bed drains well, which is good for avoiding rot, but it also dries faster than open ground, so water regularly through summer.

Aim to keep the soil evenly moist at 1 to 1.5 inches (25 to 38 mm) of water per week, watering deeply once or twice a week rather than a little each day. Mulching the bed with 2 to 3 inches (5 to 7.5 cm) of straw helps hold moisture and keeps the soil cooler, which potatoes prefer. Soil temperature above 75 degrees F (24 C) slows tuber formation, so the cooler root zone under a straw mulch matters in mid-summer. Stop watering once the tops begin to yellow and die back, which signals the tubers are mature and the skins are setting for storage.

Harvest by hand

You can harvest new potatoes, small and thin-skinned, a couple of weeks after the plants flower. Reach into the soil at the edge of a plant and feel for tubers, taking a few without disturbing the whole plant. New potatoes do not store well, so eat them within a few days.

For a full harvest, wait until the foliage has yellowed and died back, then lift the whole plant. In a raised bed, this is the easy part. Grip the stems near the base and pull the plant up, then sift the loose soil with your hands to find the rest. Let the harvested potatoes dry on the soil surface for an hour, then move them out of the light. Cure maincrop potatoes for 2 weeks at 50 to 60 degrees F (10 to 15 C) in a dark, airy place to toughen the skins before storing.

Deal with common potato problems

A few problems show up often enough that it helps to know the signs. Common scab (Streptomyces scabies) gives the tubers rough, corky patches on the skin. It is more common in dry soil and higher pH above 6.0, so steady watering and avoiding fresh lime or wood ash in the potato bed both reduce it. The potatoes are still fine to eat once peeled, but bad scab spoils their look.

Late blight (Phytophthora infestans) is the more serious problem, a fungal-like oomycete that browns and kills the foliage in wet, humid spells and can rot the tubers. Good spacing for airflow, watering at the base rather than over the leaves, and choosing blight-resistant varieties like ‘Defender’ or ‘Elba’ all help. If blight strikes, cut off and remove the affected foliage to slow it, and harvest the tubers once the tops die back. Clemson HGIC 2217 also notes that the same pathogen affects tomatoes, so do not plant tomatoes where potatoes grew the previous season.

Hollow centers and cracked tubers usually trace back to uneven watering, a fast burst of growth after a dry spell. Steady moisture through the season prevents both. Most potato troubles in a raised bed come down to water and airflow, and a well-spaced, evenly watered bed avoids the worst of them.

Variety and yield at a glance

Store your harvest well

A good harvest is worth storing properly so it lasts through winter. New potatoes do not keep and should be eaten within a few days, but maincrop potatoes store for months if cured and kept right. After lifting, let the tubers dry on the surface for an hour, then cure them for 2 weeks at 50 to 60 degrees F (10 to 15 C) in a dark, airy place to toughen the skins.

Once cured, store potatoes somewhere cool, dark, and dry, between 38 and 45 degrees F (3 to 7 C). Light turns them green, warmth makes them sprout, and damp rots them, so a dark, cool, ventilated spot is ideal. Check the stored potatoes now and then and remove any that have started to rot before they spoil the rest. Stored this way, a summer harvest from a raised bed feeds you well into winter, often 3 to 5 months for maincrop types.

Rotate to keep the soil healthy

Do not grow potatoes in the same bed year after year. Potatoes are prone to soil-borne diseases like common scab and late blight, which build up when the crop returns to the same ground repeatedly. Clemson HGIC 2217 recommends a 3-year rotation for solanaceous crops, which means waiting three years before planting potatoes in the same bed again.

Avoid following potatoes with their relatives, especially tomatoes and peppers, which share the same diseases. A simple rotation puts potatoes in one bed, then follows with beans or leafy greens the next year, which helps keep the soil clean. The fresh, well-drained soil of a raised bed already starts potatoes off with fewer disease problems than open ground, and rotation keeps it that way over the years.

Sources: Clemson Cooperative Extension, HGIC 2217 Tomato Diseases & Disorders (notes on Phytophthora infestans, scab, and rotation intervals); University of Maine Cooperative Extension, potato variety trials.