Gardening in Minnesota: zones, frost dates and when to plant

North and south

UMN Extension’s “A Minnesota guide to garden timing” says southern Minnesota gardeners can often plant one to two weeks earlier than northern gardeners. Up north, soils warm more slowly and the risk of frost lasts longer. Compare the frost dates for your city in the table above, and expect a few warm April days to be followed by freezing nights and cold soil.

Moisture also varies across the state. NOAA’s state climate summary reports average yearly precipitation ranging from about 23 inches in the far northwest to more than 35 inches in the southeast. Nearly two-thirds of it falls in the May-through-September growing season. Drought still comes now and then, when dry air moves in from the interior of the continent. A county map in the NOAA summary shows that from 1995 to 2012, declared extreme heat events were more frequent in central and southern counties than in the north.

What limits gardeners here

The short season is the main constraint. UMN Extension’s “Planting the vegetable garden” guide calls transplanting a key advantage because it gives crops a head start. Tomatoes, peppers, eggplant and other warm-season crops will not mature if you sow them directly in the garden.

Spring itself is tricky for cool-season crops. UMN Extension compares a Minnesota spring to a Mediterranean winter, the climate where brassicas, lettuce and spinach come from. Planted too late, cabbage may never form a head and radishes may never bulb, because the plants don’t get enough cool weather. Too much nitrogen can cause the same problem. Early summer heat makes spinach and lettuce bolt and turn bitter.

Planting by soil temperature

UMN Extension’s timing guide sorts crops by the soil temperature they need, measured about 4 inches deep:

• 40-50°F: peas, spinach, lettuce, radishes, carrots, beets, kale, Swiss chard, onions from sets or transplants, and potatoes.
• 55-60°F: broccoli, cabbage, cauliflower and Brussels sprouts. These handle cold above ground but root better in warmer soil.
• 65°F or warmer, after frost danger is past: tomatoes, peppers, cucumbers, squash, pumpkins, beans, corn, basil and melons.

UMN sources differ slightly here. The “Planting the vegetable garden” guide gives 50 to 60°F as the soil range for starting warm-season crops, while the timing article uses 65°F for the heat-lovers. Use the warmer figure for tomatoes, peppers and melons.

Air temperature matters too. UMN notes that a light frost (32°F) damages tender plants and a hard frost (28°F or colder) kills most warm-season crops. In farm country, the guide offers a simple cue: when local soybean growers are planting, conditions are usually right for most garden crops.

Getting more out of a short season

UMN Extension suggests several ways to gain time:

• Warm the soil early. Raised beds and black plastic help. Tilling in late fall lets you plant earlier the next spring.
• Use hot caps. Set them over the soil a week before sowing cucumbers, pumpkins and watermelons. Hot caps also let transplants go out a week or two early, but vent paper caps so plants don’t overheat.
• Transplant brassicas. Transplanted brassicas mature before peak heat and stand up better to flea beetles and cabbage maggots than direct-seeded ones. Radishes, carrots and beets don’t transplant well and should be sown in place.
• Plant in succession. Spread cool-season plantings over several dates, and pick varieties labeled for spring, fall or heat tolerance to match each sowing.
• Build simple protection. Cold frames, low tunnels covered in 2-6 mil plastic, and row covers add warmth. In a cold frame, wait for about 50°F soil before sowing lettuce.
• Sow a fall crop. Leaf lettuce, radishes and kohlrabi can be sown again in August for fall harvest.

Soil testing

UMN Extension recommends a soil test before you fertilize. The test shows whether you need lime, nitrogen, phosphorus or potassium. When you read the report from the U of M Soil Testing Laboratory, choose a fertilizer close to the recommended N-P-K ratio without going over on phosphorus, since compost adds phosphorus too. UMN notes that nitrogen deficiency is common in vegetable gardens.