Let's Talk Soil Temp

by - September 08, 2009


Every spring, as soon as the sun begins to warm my face, I'm ready to put seeds in the ground. I cannot control the urgency I feel to begin planting. This past spring, I learned a valuable lesson: air temp does not equal soil temp. Every type of seed has an optimum soil temperature for germination. It so happens that beans like 75-80 degrees F, and at this optimum temperature, they will germinate in seven days on average. Planting them in March when the mean soil temperature was 46 was destined for failure as most the beans rotted in the ground. The ones I planted in April took nearly two weeks to germinate, but by July, they were germinating in under a week.

Obviously, if we waited to plant everything until July, the growing season would be cut short, which is why many people use transplants rather than seeds for the home garden and plant after the danger of frost. I chose to grow as much as I could from seed indoors. I grew tomatoes, peppers, squash, various herbs and several other things which did not take off. Some did not work for unknown reasons and some simply did not handle transplanting well. I started broccoli from seed, but ended up using transplants, and I also picked out several varieties of pepper transplants which I had not purchased seeds for.

The current soil temperature in Raleigh is 67 degrees F. How do I know this? I used the State Climate Office website. The site allows users to look up daily, monthly, and yearly means for many different atmospheric, wind, moisture, soil, and solar radiation parameters. Raleigh's current soil temperature falls into the optimum ranges for the following fall crops: broccoli, cauliflower, lettuce, peas, radish, and rutabaga. Yesterday, I planted more spinach and various lettuces. While it is a few degrees cool for starting spinach, which can take up to two weeks to germinate, hopefully some will take. It looks like I missed the boat on carrots, which prefer 75 degrees F. I may still plant some and cross my fingers.

When you know your current location's average soil temperatures, it can make timing your garden planting more successful, especially when planting cool weather crops. In Raleigh, our average soil temperature never quite breaks 75 degrees F, but don't think that those heat loving crops like okra, peppers, squash, melons, etc won't ever germinate - they just take a little longer. You can always start some indoors in a mini greenhouse. I started squash outside in late spring without any trouble at all, so remember that optimum temperature doesn't imply that seeds will only germinate within the given range.

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