How to Measure Soil Temperature for Planting
By: Julie Day
Different veggies germinate at different temperatures.
Whether you’re planting seeds or targeting weeds, it’s important to check your soil temperature before beginning. Even the best-planned garden project can fall flat if temperatures are not appropriate for the occasion! For example, did you know that you should:
- Plant spring bulbs when the soil temperature drops below 60° F.
- Apply crabgrass control in spring, when soil temperatures reach 55° F for 4-5 days in a row.
- Plant cool-season grass seed once soil temperatures are in the 50s F.
- Give your new shrubs time to grow roots before soil temperatures fall below 40° F.
- Be very careful when starting vegetable seeds, since germination temperature is vital to the seeds’ success and every vegetable is different.
A soil thermometer is a budget-friendly addition to your garden toolbox.
You can purchase a simple soil thermometer at your local garden center for just a few dollars. The most economical ones are glass bulb thermometers with a strong metal point. However, any thermometer will do, as long as it measures temperatures down to freezing (medical thermometers usually don’t go low enough).
How to Measure Your Soil Temperature
- Measure the Right Depth: If you are planting seeds or new plants, take your measurement at the recommended planting depth. If you’re measuring for a mixed garden, check at least 5-6 inches deep.
- Make a Pilot Hole: Use a screwdriver to make a pilot hole so that you don’t break your thermometer by pushing it into hard soil.
- Follow Directions: Refer to your thermometer package for specific instructions. With most glass bulb thermometers, make sure it is firmly touching the soil, and allow a few minutes for the temperature to register.
- Provide Shade: If the sun is bright, shade the thermometer with your hand to keep the reading accurate.
- Multiple Measurements: Take a reading in the morning and late afternoon, then average the two numbers. If you’re seeding a lawn, take readings on all four sides of your house, since some areas warm more quickly than others.
- Check Reading: To double-check, refer to these handy Soil Temperature Maps from Greencast for a comparison with your soil reading.
Make a pilot hole with a screwdriver to measure the right depth.
Garden Vegetable Seed Germination Temperatures
The soil temperature for planting vegetables should be:
- 40° F or warmer: Lettuce, kale, peas, spinach.
- 50° F or warmer: Onions, leeks, turnips, Swiss chard.
- 60° F or warmer: Broccoli, cabbage, cauliflower, carrots, beans, beets.
- 70° F or warmer: Tomatoes, squash, corn, cucumbers, melons, peppers.
The seed germination temperature is often much warmer than the plant’s growing temperature. Once established, many veggies can handle much cooler air temperatures as long as the soil is warm enough. To get a head start on spring planting, plant seeds indoors or use plastic row covers to warm the soil more quickly.
Check out these charts for vegetable seed germination:
- When Is It Warm Enough to Plant? (Gardener’s Supply Company)
- Soil Temperatures for Vegetable Seed Germination (Alabama Extension)
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