The most useful measure of the vegetable garden growing season is soil and air temperature averages.
Warming soil temperatures activate root growth and the uptake of moisture and nutrients. Soil temperature is very closely affected by the air temperature.
As days lengthen in spring and the air temperature warms toward summer, the soil temperature follows. Soil temperature usually lags behind air temperature; the same sunlight that warms the air warms the soil.
With rising air and soil temperatures, vegetable plant growth happens.
Soil and air temperatures
Here are temperatures needed for growing vegetables:
• Cool-weather vegetables require a minimum average soil temperature of 40° to 50°F (4-10°C) for planting, and an average air temperature range of 60° to 85°F/15-29°C (optimal is 70°F/21°C) for sustained growth. The maximum air temperature for cool-weather crop productivity is 86°F (30°C), above this temperature cool-weather crops will bolt—meaning flower and set seed–or quit growing.
• Warm-weather vegetables require a minimum average soil temperature of 50°F (10°C) for planting and an average minimum air temperature of 75°F (24°C) for sustained growth. The maximum air temperature for warm-weather crop productivity is 110°F (43°C), above this temperature most warm-weather crops will die, just as they are likely to die at 32°F (0°C).
Temperature affects a plant’s rate of growth. The higher the soil and air temperature above the minimum, the faster a plant will grow. Two-thirds of the growth time necessary for a plant to reach harvest and maturity should be at or above the optimal air temperature; the remaining one-third should be between the minimum and optimal air temperature.
Soil temperature in spring may lag by days and weeks behind the air temperature as the soil and collected soil moisture are slowly warmed by solar heat after the winter chill. (Soil temperature is affected by solar heat, air temperature, and amount of soil moisture.) The soil generally cools more slowly in autumn for the same reasons (residual solar heat remains in the soil from the summer season and the soil is generally drier in autumn than spring).
Temperature and growing season
The true growing season for a garden is the total number of consecutive days that the soil and air temperature are warm enough to grow plants–including germination, maturation, and ripening before a killing frost or freeze.
Use the length of the growing season in your garden to set planting dates for early, midseason, and late-season crops. Use the length of the growing season to plant succession crops; cool-weather spring crops, followed by warm-weather summer crops, followed by cool-weather autumn crops. If you grow crops without protecting them from killing temperatures, your growing season will be determined by nature.
Cool-season hardy vegetables
Here is a list of hardy vegetables. (“Hardy” is a gardening term often used to describe a plant’s ability to withstand cold temperatures.) These crops can be planted—seeds sown or transplants set in the garden—two to four weeks before the average last frost date.
- Brussels sprouts
- Chinese cabbage
Cool-season half-hardy vegetables
Half-hardy vegetables can tolerate only light freezes–that is short-term exposure to subfreezing temperatures. Half-hardy crops should be planted around the date of the last spring frost.
Here is a list of half-hardy vegetables. Sow the seeds of these crops or set out transplants near the average date of the last frost in your area. Half-hardy crops include:
- Chinese cabbage
- Swiss chard
Warm-season tender vegetables
Tender vegetables are best planted one to two weeks after the last frost. (“Tender” is a gardening term for plants that can not withstand cold temperatures.) The fruit and leaves of these crops can be injured by a light frost if planted too early. Tender vegetables include:
- Asparagus peas
- Shell beans
- Snap beans
- Malabar spinach
- New Zealand spinach
Warm-season very tender vegetables
Very tender vegetables should not be planted until at least three weeks after the last frost in spring. These crops demand warm temperatures to grow, usually above 70°F (21°C). They can be stunted by temperatures below 50°F (10°C).
Very tender vegetables include:
- Lima beans
- Sweet potatoes
Season extension is a term used by gardeners to describe their efforts to grow crops outside of the natural growing season. When temperatures are too cold for vegetable crop growth, crops can be protected under a plastic tunnel, frost blanket, row cover, or greenhouse. Those season-extending devices warm the air and soil and allow crops to grow past their natural growing season. See articles on Season Extension.
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