Soil temperature matters

By Marianne Lepa

Your gardening skills improve year by year. Each season brings the opportunity to learn something new. The importance of soil temperature for germinating seeds and setting out plants is one of those advanced skills you do eventually pick up.

You probably already know that different seeds require different temperatures to germinate. Some seeds, like radish, lettuce and peas, prefer cold wet soil to germinate, while other seeds, like beans, corn and squashes, need a toasty warm soil with good drainage to get going. Transplants also have certain soil temperatures they need to find food and grow well. The air temperature is important, but the soil temperature will determine whether your plant grows quickly or fails to thrive.

For warm weather crops, if the soil is too cold, seeds won’t germinate and could rot. Tomatoes, peppers, eggplant take too long to mature from seed in our climate, so they generally go into the ground as a transplant. If the soil is too cold for them, they won’t grow, they just wait patiently until the soil warms up. But that could affect the ultimate yield at the end of the season.

The simplest way to determine soil temperature is to stick your hand in it. Poke your fingers into the soil right up to your palm. Are you fingers freezing, just cold, or do they feel nice and warm? You can also use a food thermometer to find out if your soil is cold or warm. Take the temperature 10-15 cm deep. For warm weather crops, the soil should be 18-24C and for cold weather crops, the soil should be 8-10C

On the farm we used to determine the soil was ready for tomatoes and squash when you could sit comfortably on it with no pants! Farmers don’t actually do that (at our place, anyway) but the hand up to the palm method will tell you if you’d be happy sitting bare bottomed on that soil for very long.

Cold season vegetables are:
  • Asparagus
  • Lettuce
  • Beets
  • Carrots
  • Turnips
  • Radishes
  • Cabbage
  • Collards
  • Kale
  • Spinach
  • Swiss chard
  • Arugula
  • Broccoli
  • Brussels Sprouts
  • Cauliflower
  • Kohlrabi
  • Potatoes
  • Chives
  • Leeks
  • Parsnips
  • Turnips

Seeds of these veggies will germinate in cold soil and you can transplant seedlings of these vegetables into cold soil.

Warm season vegetables typically grown here are:
  • Corn
  • Cucumbers
  • Summer squash
  • Winter squash
  • Tomatoes
  • Sweet peppers
  • Hot peppers
  • Eggplant
  • Melons
  • Watermelon

These seeds and plants need soil that is pleasantly warm and well drained.

While mulch will slow down the soil’s ability to warm up, covers can warm soil quickly. If it’s been a cold spring, like this year, and you want to warm up your soil, black or clear plastic covers will increase the solar energy directed into the ground. But be ready to protect warm season crops from cool nights and potential frosts if you speed things up.

You’ll notice that I am not giving you dates to plant out. The soil temperature will depend on the weather, how much snow there was in winter and how much rain there was in spring. Dry soil warms faster than wet soil. Sunny days will heat soil quicker than cloudy or overcast days. Check your soil temperature before putting any plants or seeds into the ground.

Comments are closed.