Gardening in North Dakota: zones, frost dates and when to plant

A continental climate, east to west

North Dakota lies in the northern Great Plains, where the moist eastern United States gives way to the semiarid West. NOAA’s state climate summary points out that the state sits far from any ocean’s moderating influence and has no mountains to the north to block arctic air. The result is large temperature extremes. The northeast is the coldest corner in both January and July, and the south runs warmer. Temperatures of 100°F or higher occur nearly every year, mostly in the drier southwestern and south-central parts of the state.

Moisture also changes across the state. Annual precipitation runs from less than 16 inches in the northwest to about 24 inches in the southeast. Most of it falls in late spring and early summer, when thunderstorms peak. The most severe storms can bring hail and damaging straight-line winds.

Plant by soil temperature

NDSU Extension horticulturist Esther McGinnis argues in her Dakota Gardener column on sowing during a fickle spring that calendar dates, which are built on decades-long frost averages, can mislead in a given year. Blizzards, floods and 50-degree temperature swings are all possible in a northern Plains spring. Check soil moisture and temperature instead.

• Don’t work wet soil. If soil sticks to your shovel, wait. Working it then compacts the bed and destroys its structure.
• Lettuce and peas: sow once the average soil temperature reaches 40°F. Lettuce can germinate in colder soil, but it is slow and prone to seed rot.
• Carrots, radishes and spinach: their best germination range starts at 45°F.
• Beets, Swiss chard and onions: 50°F.
• Beans and cucumbers: at least 60°F, and only after frost danger has passed.
• Sweet corn: fungicide-treated seed at 55 to 60°F; untreated seed only once the soil reaches 65°F.
• Tomato and pepper transplants: wait for 65 to 70°F soil and nights consistently above 45 to 50°F.

To measure, insert a soil thermometer 2 inches deep and average readings over three days. The North Dakota Agricultural Weather Network also maps soil temperatures across the state. Use the frost dates for your city above alongside these soil readings.

A small garden that works

NDSU Extension’s vegetable guide Pickin’ Patch (H1405) favors a small, well-kept plot over a large, weedy one. It recommends a spot with six to eight hours of sun, well-drained soil away from low spots, and shelter from wind, which lets plants grow faster. Start tomatoes, peppers, cabbage, broccoli and cauliflower indoors, then harden them off with gradually more sun and wind for at least two weeks before planting out.

The guide sorts crops by cold tolerance:

• Tolerate a little freezing: broccoli, cabbage, onions, peas, radishes, rutabagas and spinach.
• Tolerate only very light frost: beets, carrots, cauliflower, lettuce, potatoes and Swiss chard.
• Killed by any frost: beans, cucumbers, melons, peppers, pumpkins, squash and tomatoes.

Gardens need about 1 inch of water a week. When rain falls short, make up the difference with morning watering, ideally through a soaker hose rather than a sprinkler. To save space, the guide suggests bush squash, bush cucumbers and compact tomatoes.

Varieties tested by North Dakota gardeners

NDSU’s Home Garden Variety Trials send seed to gardeners across the state, who grow the varieties under home conditions and rate them. In the 2023 results, ‘Sweetness’ stood out as an early sweet corn. Most gardeners preferred ‘Xtra Tender 274A’ among midseason super sweets, and ‘Troubadour’ beat ‘Kate’ among late varieties. ‘Bristol’ was the better slicing cucumber, and ‘Green Light’ excelled among snack cucumbers. Both ‘Narvik’ and ‘Naval’ early Nantes carrots grew well, and ‘Naval’ yielded more at more sites. ‘Detroit Dark Red’ and ‘Sweet Dakota Bliss’ beets both rated very highly. The same report describes a cold April and wet May in 2023 that delayed planting, and drought near the Canadian border.

Alkaline and salty soils

NDSU’s Evaluating, Preparing and Amending Lawn and Garden Soil (H1325) notes that most North Dakota topsoil formed under prairie grasses and is dark and rich in organic matter. Subsoils, however, may hold accumulated salts and carbonates. North Dakota soils are often quite alkaline, which ties up iron. Low spots where water ponds and the surface dries to a white crust may signal sodium problems. Those spots need better drainage before any amendment will help, and raised beds are often a better choice. For clay, add compost, peat moss or composted manure, never sand, which turns clay into something like cement. Because heavy manure use can build up phosphorus and lead to iron and zinc deficiencies, test before adding compost.

For testing, collect six-inch-deep samples from three to six spots and mix them. NDSU Extension agents can help, and many county offices have sample bags and probes. NDSU directs samples to the University of Minnesota Soil Testing Lab or a private lab such as AGVISE Laboratories. Its soil guide adds that locally calibrated tests give more reliable recommendations for North Dakota conditions than home kits.