Tomatoes (Lycopersicon lycopersicum) are perennials in tropical climates, but in the United States they are typically grown as annuals in U.S. Department of Agriculture plant hardiness zones 4 through 11. The USDA growing zones only give you the average winter low temperatures, not the average first frost in autumn or last frost in spring. Soil and air temperature, not the calendar, are the best guides for planting tomato seedlings outdoors.
Best Soil and Air Temperatures
"After the last expected spring frost" is a phrase often listed as the best planting time for tomatoes. While tomatoes may survive if you plant them then, a more accurate way to gauge conditions for success is to pay attention to temperatures.
Soil temperatures should be from 50 to 55 degrees Fahrenheit for tomato seedlings to survive. To thrive, they need soil temperatures above 60 degrees F. Temperatures above 70 degrees F are even better.
Air temperatures below 57 degrees F will delay growth and make tomato plants susceptible to disease. Air temperatures from 75 to 95 degrees Fahrenheit are optimal.
You can buy special thermometers from most garden supply centers that measure soil temperature. Inexpensive models are just glass tubes with a metal tip. To get the soil temperature, poke a hole in the ground 5 to 6 inches deep with a screwdriver and insert the thermometer.
Zone 6 and Spring Frost
USDA zones say nothing about the severity of summer heat or humidity. Spring warming varies in different parts the same zone from year to year. Some locations stay colder longer than others. For example Akron, Ohio, is 230 miles northeast of Cincinnati, Ohio. They are both in USDA zone 6a. Akron’s expected last frost is April 18, five days later than Cincinnati’s April 13 date. Meanwhile, Spokane, Washington, also USDA zone 6a, has an expected last frost of May 2.
Overall, the last spring frost dates in USDA zone 6 range from March 30 to April 30.
There's not a lot you can do except pay attention to your local weather forecast. Increased accuracy of weather forecasting may give you a heads up about prolonged cold or early warming in your area.
Strategies for Early Planting
If you know the average last expected frost date in your area of USDA zone 6, there are ways to cheat a bit and get your plants out a little early.
- Lay clear plastic on the ground to warm
the soil, and cut slits in the plastic for your plants. As the soil starts to
warm, you can later cover the plastic with straw mulch to block the sun and
prevent the soil from getting too hot. You have to water with drip irrigation
to do this.
- Cut the bottoms out of plastic milk jugs and set them over the plants until the weather warms. This also requires drip irrigation.
- Buy small plastic tents
online or at a garden supply center to help keep your tomato plants warm.
Hardening Off Tomato Seedlings
Whether you grew tomato seedlings yourself indoors or bought them at a supermarket or nursery, the date of last expected frost does not mean you can immediately plant them in the ground and start calculating days to harvest. Seedlings have grown in a protected indoor environment. You need let them adjust to intense sun and to wind. This is called "hardening" or to "harden off" your seedlings.
Start by putting the seedlings outdoors in partial shade for a couple of hours, then move them back inside. Increase their exposure to sun and time outside gradually over six to eight days, and then plant them in their permanent location.