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Soil Temperature As Your Planting Guide |
By Ken Harr, Director of Horticulture, Green Bay Botanical Garden
As Wisconsin gardeners begin to plan and anticipate the upcoming season, the urge to rush Mother Nature and start planting early becomes greater with each warming trend that passes through. But even experienced gardeners with their trusted almanacs and latest weather data can get caught setting out transplants or sowing seeds too early, for it is soil temperature that's the real spark behind seed germination and early root growth. Soil temperatures regulate how water is imbibed into seeds and how much water and nutrients are absorbed through root hairs. The warmer the soil, the faster water absorption takes place. Soil warm-up can vary from year to year and is dependent on varying factors, such as sun exposure, type of soil, amount of current moisture and the level of the land.
In Wisconsin, most cool-grown crops germinate only when the soil reaches around 45 degrees F or above. And typically, warm-grown crops will wait until temperatures hit 55 degrees F, or warmer. The surest way to know when the ideal time is plant or sow seeds is to take the soil's temperature from 10:00 am to noon. This is the midway point between the coolest and warmest parts of the day.
Using a metal soil thermometer that can be purchased through most gardening catalogs or retail garden centers, and pushing it into the soil about 2" deep in various spots will give accurate readings that can be recorded over a period of time. This information will then give gardeners a good indication on when to plant. Cole crops, such as cabbage, cauliflower and broccoli, can be seeded once the temperature reaches 45 degrees, while tomato and pepper transplants need the soil to closer to 65 - 70 degrees F in order for extensive root growth to occur. Bean seeds need soil temperatures above 50 degrees with good drainage. Otherwise the seeds and many other types of vegetables, may simply wait until conditions are right to germinate, and if the wait is too long, they will rot and never germinate.
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Recording temperatures and waiting for the minimum or ideal soil temperature will further increase germination rates and percentages and help ensure a bountiful crop of vegetables or a full, beautiful bed of flowers. |
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Ken
Harr, Director of Horticulture at Green Bay Botanical Garden, uses a soil thermometer. |
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