This time of year I get anxious to plant my vegetable garden. I look out at the sunny days and think that I should be working in my plot; but sunny, warm days can be deceiving! I need to take a step back and consider other important environmental factors before I begin.
Soil temperature is probably the most critical indicator of when to plant. I have found out the hard way that seeds do not germinate—and transplants won’t grow—if I plant when the soil is too cold. A soil thermometer is now an essential component in my gardening toolbox.
Factors that affect soil temperature measurement:
Time of day
- Soil temperatures tend to be coolest between 6 and 8 a.m. in the morning, so use temperatures at this time as a guide as for planting early season crops.
- In the heat of summer, soil temperatures are highest between 3 and 5 p.m., so use temperatures at this time as a guide for planting late season crops.
Depth of thermometer
- Take soil readings for seeds at a depth of 1 to 2 inches.
- Take soil readings for transplants at a depth of 4 to 6 inches.
How to measure soil temperature:
- Push the thermometer into the soil to the depth of planting.
- In dense soil, you can use a screwdriver to make an initial hole to the right depth to avoid bending your thermometer.
- Follow the directions that came with the thermometer as to how long to leave the thermometer in the soil to get an accurate reading.
- Record the temperature and then move it to another part of the garden.
- Record measurements from 3 to 4 different places in your planting plot.
- Take soil temperatures daily for a few days and average the temperatures to be sure your soil is the right temperature for planting
Soil temperatures vary for each type of vegetable plant!
A list of soil temperature data for some vegetable seeds’ germination:
- Lettuce: minimum 32℉; optimum range 60 to 75℉
- Peas: minimum 40℉; optimum range 65 to 75℉
- Snap beans: minimum 60℉; optimum range 75 to 85℉
- Pumpkins: minimum 60℉; optimum range 85 to 95 ℉
If you check the soil temperature rather than the air temperature or only the calendar date, you can increase your chance of your seeds sprouting and thriving!
Article by Bob Wardwell, Extension Master GardenerSM Volunteer
Interactive Soil Temperature Map for North Carolina: https://climate.ncsu.edu/map/
Soil Temperature Conditions for Vegetable Seed Germination:
https://sacmg.ucanr.edu/files/164220.pdf
Range of possible planting dates: https://content.ces.ncsu.edu/western-north-carolina-planting-calendar-for-annual-vegetables-fruits-and-herbs