Soil Temperature: Why is it important?

Hey All! It’s that time of year to start delving into the nitty gritty of starting seeds and what our plants need to be healthy. One of the most important things for a successful garden is soil temperature. And that’s all because of a little thing called germination. 

 What is Germination?

Germination is the process of seeds sprouting and developing into new plants. In order to have successful germination, your soil must be at a temperature between 40-90 degrees Fahrenheit, depending on what you’re growing, with a germination sweet spot between 60-80F.  If the soil is too cold, your seeds may rot before they germinate and if your soil is too hot, the seeds will go dormant or even die.

 Soil Temperature & Germination Rates

  • Soil thermometers can be purchased online or from many garden supply stores locally. They generally run between $10-$20 and are invaluable for helping you get a jump on spring planting.  Just stick it straight into your soil, turn it on, and a reading will pop up in a few seconds.
  • NOTHING will germinate in soils less than 32F and most plants prefer soil temps to be between 65F-85F.
  • While cold hardy plants like peas, lettuce, spinach, brassicas, and some root veggies will germinate in soils as cold as 35-40F, they will take a lot longer to grow than if the soil was warmer.  At 40F, it will take a pea seed an average of 36 days to germinate, but at 77F, it will take that same pea seed only 6 days to emerge.
  • To get a jump on spring planting, you need to employ strategies to warm up your garden soils!
  • Check out this handy soil temperature chart for common garden crops: https://extension.oregonstate.edu/sites/default/files/documents/12281/soiltemps.pdf

 How to Achieve Correct Soil Temperature OUTDOORS:

  • Use raised beds. This could be wood or even just hilling up the soil in your planting area by a few inches.  This has the added bonus of making your soil drain better.
  • Cover planting beds with some clear plastic for 2-3 weeks. Remove the plastic to plant and cover your rows with frost cloth to speed up germination.
  • Row covers once seeds have germinated. Medium weight frost cloth over wire hoops will help keep seedlings and your soil warmer during temperature swings in the early spring. 
  • Mulch can help keep soils cool in the summer, but it can delay spring planting. Remove any winter mulch from your beds while seeds are germinating.  Once they are up, mulch around the plants to help reduce weeds and protect seedlings from any late frosts.

 How to Achieve Correct Soil Temperature INDOORS:

  • Keep the room your seed trays are in set at 72F and have light trained on them for 14-18 hours per day. 
  • For plants like tomatoes and peppers that prefer even warmer temps, you can purchase a seed tray heat mat for around $30 that will keep them at around 80-85F.
  • Cover those babies up while the seeds are germinating. This helps to entrap heat and moisture to keep seedlings comfortable. A plastic dome over your tray works, but you can use a plastic bag or even damp newspaper in a pinch.  Just be sure to check your trays every day and remove the cover at the first sign of seedlings emerging. 

Remember, if at first you don’t succeed, try try again!  Early spring plantings outdoors can fail due to weather and indoors due to all sorts of problems.  The more you try, though, the better you’ll get at keeping seedlings alive, so don’t give up!