Garden Tips: Is the soil warm enough to plant seeds?

Marianne C. Ophardt, WSU Benton County ExtensionMarch 7, 2014 


Choose vegetable garden plants carefully, and then pare back. (Algerina Perna/Baltimore Sun/MCT)


It's March, we are setting the clocks ahead this weekend and the daytime temperatures have reached above 50 degrees, but winter may still have a few last gasps before we can say spring has arrived and planting can start.

St. Patrick's Day is a traditional day for some to plant potatoes and peas, but smart gardeners wisely check the soil temperature before planting their vegetable seed. If the soil is too cold, the seeds will sit there and may rot before sprouting and growing.

To check the soil temperature, invest in a soil thermometer. You can find one for about $10 to $15 at a local garden store or from an online garden supply company. Take the soil temperature in mid-morning by inserting the thermometer's probe 2 inches into the soil for small-seeded crops (such as lettuce) and 4 inches into the soil for large-seeded crops (such as squash and beans). The probes of some of the soil thermometers have markings that indicate inches to make this easier.

Seeds of early spring cool-season crops can be planted when the soil temperature is 40 degrees or above. This includes lettuce, peas, kale, radishes, arugula and spinach. When the soil reaches 50 degrees, plant seeds of leeks, onions, Swiss chard and turnips. Wait until it reaches 60 degrees for planting beans, beets, broccoli, Brussel sprouts, cabbage, carrots and cauliflower, and 70 degrees for cucumbers, squash, melons and corn. The soil temperature should be consistent for several days before deciding these optimum temperatures have been reached.

Seed potatoes are best planted when the soil temperature is 45 degrees or above and daytime temperatures are consistently in the 65 degree range and nighttime temperatures in the 55 to 65 degree range.

If you are anxious to plant, warm up the soil faster by covering the garden with a sheet of clear plastic. To keep the wind from wreaking havoc with the plastic, lay it out smoothly and then pull it taut, firmly burying all the edges in trenches.

If you choose to keep the plastic in place, you can plant seeds and transplants by making holes in the plastic, but weeds will grow profusely under the plastic. In addition, the clear plastic will heat the soil to plant damaging or stressful levels during the sunny, hot part of summer unless your garden plants are big enough to shade the plastic by then.

Clear plastic works better than black plastic for warming the soil because it allows sunlight in during day and then traps heat that builds, much like a greenhouse. I recommend warming the soil with clear plastic, but removing it before planting. Gardeners also find that the soil in raised beds warms faster, and situating your garden so it receives full sun and faces south will also help.

The last average date of frost for our area is May 1. Keep in mind that tomatoes, peppers, eggplants, beans, cukes, squash and melons will need protection if frost is in the forecast. Row cover fabrics can provide several degrees of protection.

-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

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