The talk among gardeners in the spring is when to start putting out vegetable plants. If you start too early, a late frost might get your tomato plants, but starting too late could mean a shorter growing season before summer heat slows down production.
There are a lot of interesting tips for planting vegetables by the signs of the moon or counting the weeks from Groundhog Day to predict the last day of winter. One of the best kept secrets of good gardening is knowing the value of checking soil temperature. Use a soil thermometer, or even a kitchen thermometer, to test
the temperature of your soil. Test the top one or two inches for planting seed, and test four to six inches for planting vegetable transplants.
Soil temperature is the most important factor to consider for good seed germination and seedling growth. Soil temperature for vegetable seed growth is classified into four categories: the minimum temperature for seed growth; the optimum temperature; a realistic temperature; and the maximum temperature for germination to occur.
The minimum soil temperature is the lowest temperature the soil can be for seed germination. Some cold season crops like beets, cabbage, chard, radish, and turnip have a minimum soil temperature of 40 degrees F. If we plant at the minimum soil temperature, we might have slow germination and weak seedlings. The maximum soil temperature is the highest temperature at which the seeds will germinate. Seeds planted at the maximum soil temperature or higher will not be likely to germinate.
The optimum soil temperature is the best temperature for seed germination. The optimum temperature for seed germination of cucumber, okra, and watermelon is 95 degrees F. At this temperature, we will get the best germination. Unfortunately, if we plant those seeds when the soil temperature is 95 degrees F, the summer heat will stress the plant and reduce plant vigor and yield.
Gardeners want to hit the sweet spot of the realistic soil temperature. This is somewhere between the minimum and optimum temperature so the seeds will germinate and have the best chance for growth. For example, the optimum soil temperature for seed germination of lettuce is 75 degrees F, and the minimum soil temperature is 35 degrees F. The realistic soil temperature to plant lettuce is 45 degrees F so the lettuce has a good chance to germinate, and the plant will have plenty of time to grow before the weather gets too warm.
Soil temperature affects more than just vegetable plants. Our warm season turf grasses like Bermuda, St. Augustine, and Zoysia need soil temperatures of 65 degrees Fahrenheit for significant growth of rhizomes, roots, and stolons. The optimum soil temperature is around 80 degrees F.
Soil temperature helps us to know when we might start to see grass start to actively grow in the spring. You can hold off on fertilizing your lawn until you get good, active grass growth. You can also wait to begin watering your lawn until temperatures get much warmer and the rain stops coming.
For more information about lawns and gardens, contact Kate Hajda at the Williamson County AgriLife Extension Office at 512-943-3300.
