The time to plant seeds in the garden or start seeds indoors depends on the variety of plants and the average date of the last frost in your region.
Cool-season crops grow best in the cool weather of spring and autumn. Warm-weather crops grow best in the summer months. Seed starting should be matched to the appropriate time that the variety you are planting will grow. The goal is to bring plants to harvest when the temperatures are optimal to complete their growth cycle or ripen fruit.
The average date of the last frost in spring and the number of days in the growing season are important starting points when sowing seed indoors or out.
Average Date of the Last Frost
The average date of the last frost in spring is a day around which you can plan to start seeds indoors, sow seeds in the garden, and set transplants into the garden.
The average date of the last frost in your garden or region is an approximation. It is a date based on an average of the last frost date for each of the last 30 or so years. It is an average or approximation – a date that will change from year to year depending on the weather.
The average date of the last frost is important because most warm-weather plants will not grow and many will die if they are hit by frost. Warm-weather plants most often succeed when they go into the garden two or three weeks after the last frost. By that time daytime temperatures have begun to significantly warm above freezing.
Most cool-weather plants can withstand a light frost. They can be started in the garden a week or two before the last frost in spring or on the date of the last frost in spring.
The average date of the last frost in spring for your regions is available from the nearest cooperative extension. You will also find the last frost date in many gardening books or online from The National Climatic Data Center of the United States government which produces a freeze-frost probability table for each state and the largest cities and towns in each state, and from Weather.com which produces a chart of high and low-temperature averages by month broken out by zip code and city name.
As you look at these charts, keep the following growing information in mind:
• Cool-weather vegetables require a minimum average soil temperature of 40° to 50°F for planting and an average air temperature range of 60° to 85°F (optimal is 70°F) for sustained growth. The maximum air temperature for cool-weather crop productivity is 86°F, above this temperature cool-weather crops will bolt or quit growing.
• Warm-weather vegetables require a minimum average soil temperature of 50°F for planting and an average minimum air temperature of 75°F for sustained growth. The maximum air temperature for warm-weather crop productivity is 110°F, above this temperature most warm-weather crops will die, just as they are likely to die at 32°F.
• 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 temperature.
Know Your Growing Season
The length of the growing season in a garden or region is most often expressed as the average number of frost-free days between the last frost in spring and the first frost or hard freeze in autumn. By the date of the last frost in spring, daytime temperatures have grown significantly warmer than freezing.
A more useful measure of the growing season might be soil temperature averages: the date in spring when the soil temperature averages 40°F or greater (when cool-weather plants begin growth) and the last date in autumn when the soil temperature drops below 40°F (when cool-weather plants stop growth).
Soil temperature in spring may lag by days and weeks behind the air temperature as the soil and collected soil moisture warm after the winter chill. (Soil temperature is affected by solar heat, air temperature, and 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 in spring).
The true growing season for a region or garden is the total number of consecutive days that the soil is warm enough to grow plants–including germination, maturation, and ripening before a killing frost or freeze. Minimum soil temperature is required for a plant to germinate and grow; minimum soil and air temperature is required for growth; the flowering temperature (required for fruiting vegetables) will be higher than the minimum growth temperature.
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 plan 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.
You can get more from your garden by protecting plants and giving them an artificially warm environment in which to grow. This is called growing season extension.
How to Extend the Growing Season
When soil and air temperatures fall below the average and minimum for growing, you can extend the growing season by protecting plants from cold and wind (the combination of wind and low temperatures can be lethal to plants) and by keeping soil and air temperatures at or near the temperatures required for growing. This is achieved with cloches, plastic tunnels, cold frames, hotbeds, and greenhouses. Conversely, when temperatures rise too high for growth, moderate the temperature and protect plants with shade structures, shade cloth, and additional water.
Season Extending Options
- Use inexpensive plant covers to thwart frost (these will raise the temperature by 2° to 3°F): paper bags with bottoms cut out; cardboard boxes turned upside down; milk cartons or plastic jugs with the bottoms cut out; buckets turned over; hats made of newspaper.
- Use more substantial covers can raise the covered temperature by 5° to 15°F:
- Vertical tomato wire cages wrapped in plastic.
- • Wooden A-frames covered with sheets of polyethylene.
- • Fencing or chicken wire arched across planting beds to form tunnels covered with plastic and held in place with staples or clothespins.
- Arched panels of ribbed translucent fiberglass.
- Hoop or wicket tunnels covering an entire bed: wickets or hoops of stiff wire arched and covered with clear polyethylene.
- Cold frames and hotbeds. A cold frame or hotbed can raise the temperature of protected beds by 20°F or more and extend the growing season by several months to all year. A cold frame or hotbed is a box with a plastic or glass top that captures the warming rays of the sun and holds the heat.
- Use 2 x 8 or 2x 10-inch board to form an open-bottomed box that will fit over your crops. (The top can be made less expensively with two layers of polyethylene.) Cut one end of the box to slant towards the sun, or place soil under the north end of the box so that it slants towards the sun and captures the solar heat.
- A hotbed is a heated cold frame. Dig a pit 18 inches deep and place fresh horse or cow manure in the pit or freshly cut grass or green manure. Place one inch of sand and 6 inches of soil over the organic matter to make a growing bed, or two inches of sand to form a floor on which to place pots or growing flats. The bacterial action of the organic matter decomposition will heat the frame. A hot frame also can be heated by placing light bulbs in the frame or soil heating cables.
- On warm sunny days, air temperature in cold frames, hotbeds, and under plastic tunnels can quickly become too hot. Ventilation–simply opening the top or lifting the cover–is required to keep plants from suffering from too much heat.
Nearly all forms of plant protection can increase the number of growing days by at least three or four weeks in both spring and autumn. That is a significant boost to the growing season. The combination of the extended growing season and higher temperatures will allow the growing of semi-tropical vegetables such as tomatoes, peppers, and eggplant out of season.
THE KITCHEN GARDEN GROWERS’ GUIDE will give you germination, planting temperatures, and growing temperatures for 80 vegetables and herbs. You’ll also find a list of frost dates and growing days in this complete growing guide.