Dakota Gardener: Sowing vegetable seeds during a fickle spring

Deciding the optimal time to sow vegetable seeds in the garden is never easy in the northern Plains. Back-to-back blizzards in the west, flooding in the east, and 50-degree temperature swings make this year more challenging than most.

The calendar approach to gardening may not work for direct sowing seeds. Gardening calendars are based on the average last frost date. The averages are calculated over several decades. The weather appears to be departing from the average this year.

A more accurate approach is to use soil moisture and soil temperature to guide seed planting decisions.

The most important rule is don’t plant when the soil is still sopping wet. If soil sticks to your shovel when you insert it into the garden bed, you know it is too wet. The danger is that working in the garden will compact the soil and destroy soil structure. Instead, wait until the soil is drier.

Then, use soil temperature to guide your planting decisions. The earliest vegetable crops such as lettuce and spinach have a minimum soil germination temperature of 32 to 35 degrees Fahrenheit.

However, the minimum temperature is not necessarily the optimum germination range. Lettuce, for example, will take 49 days on average to germinate at 32 F and is quite susceptible to seed rot at that temperature. In contrast, lettuce seed will germinate in 15 days at a soil temperature of 41 F.

Lettuce and peas should be planted when the average soil temperature reaches 40 F or higher. Likewise, carrots, radishes and spinach have an optimum germination range that starts at 45 F, while beets, Swiss chard and onions prefer 50 F.

Warm-season crops such as beans and cucumbers should not be sowed until the soil temperature reaches at least 60 F and the danger of frost is past.

Sweet corn is fussy about germination temperatures. If the seed has been treated with fungicide, it can be sowed at a soil temperature between 55 and 60 F. Untreated corn seed should not be planted until the soil teaches at least 65 F.

Inexpensive soil thermometers can be purchased from local garden centers or a meat thermometer can be used. Make sure to ask before you use your spouse’s expensive meat thermometer in the garden! The thermometer should be inserted 2 inches into the soil and temperatures taken for three consecutive days and averaged.

Alternatively, the North Dakota Agricultural Weather Network features a map displaying the average soil temperature of locations across North Dakota, western Minnesota and eastern Montana.

Many gardeners transplant warm season vegetables into the garden such as tomatoes and peppers. Don’t be in a rush to transplant these tropical species. Wait until the soil temperature is 65 to 70 F and the air temperature at night is consistently above 45 or 50 F.

For more information about gardening, contact your local NDSU Extension agent. Find the Extension office for your county at https://www.ndsu.edu/agriculture/extension/county-extension-offices.