By ANNE LENOX BARLOW, Cornell Co-op Extension
If you grow vegetables, you are entering a very difficult time of the growing season.
The weather is warming up. We know that we have a short growing season. And in the Champlain Valley, we are officially past the average last frost date. Therefore, area vegetable gardeners are eager to get garden plants into the ground and have them produce as much as possible before the first frost hits at the end of September.
But for many plants, we must be patient just a bit longer for the soil to warm up.
Why is soil temperature important? Because our garden seeds will only germinate if the soil temperature is within a certain range. For example, a bean seed will only germinate if the soil temperature is above 60 degrees Fahrenheit but no warmer than 95 degrees. If you plant your bean seed in soil that is only 50 degrees, it will sit and wait for the soil to warm up.
As the seed is sitting, you are diligently watering the garden — creating a dark, moist environment for fungus to rot the seed. Then you have to go back and replant your bean crop because it didn't germinate well. By waiting, you would have avoided the situation.
To add to the process, all plants also have an optimum temperature for germination. This is the temperature at which that particular type of seed will germinate the best. In the case of beans, it's 80 degrees. If you were to wait until that temperature is reached to plant, you could expect nearly every seed to germinate.
But we don't live in a perfect world. We live in a cold climate with some summers when the soil temperature never reaches 80 degrees. And in the summers that is does, the bean crop might be killed by frost before reaching maturity. What's the use of waiting for near-100-percent germination if we can never eat the beans?
REALISTIC TEMPERATURES
Luckily, Dr. Jerry Parsons, extension horticulturist at the Texas Agricultural Extension Service, has identified what he calls a realistic soil temperature for germination. This is a temperature at which gardeners can expect good germination and strong growth but not 100-percent germination.
Cold-season crops have lower realistic temperatures than warm-season crops. Carrots, parsley, lettuce, radish, spinach and beets all have realistic soil temperatures of 45 degrees. Our soils are currently warmer that this, so it's a good time to consider getting these crops in the garden if you haven't already done so. Other plants that go into the ground as seed that require warmer temperatures include cabbage (54 degrees), corn (55), beans (72), cucumber (64), pumpkin (75) and squash (70).
For plants that we set out as transplants, it is also a good idea to wait until the soil warms up to the realistic temperatures. If you plant them too soon, their growth will stall and the plants will be stressed. The realistic soil temperature (in degrees Fahrenheit) of some popular transplants include 75 for eggplant, 64 for peppers and 55 for tomatoes.
There are a few tricks gardeners can do to warm their soils faster than Mother Nature would do if left to her own devices. Constructing a raised bed elevates and warms the soil faster than in the ground gardens. And covering the soil with plastic also traps the sun's warmth.
If you are curious about what your soil temperature currently is, simply purchase a soil thermometer from a garden supply store. These inexpensive devices cost as little as 6 dollars and, along with some patience, can really help get your garden off to a good start.
Anne Lenox Barlow is the horticulture educator with Cornell Cooperative Extension in Clinton County. Office phone numbers: Clinton County 561-7450, Essex County 962-4810, Franklin County 483-7403. E-mail questions to: askMG@cornell.edu.
