When to plant tomatoes where you live

When is it safe to plant tomatoes outside?

Tomatoes are very tender, warm-season plants, and frost injures their top growth, so University of Maryland Extension advises transplanting them only after all danger of frost has passed and the soil has warmed. Utah State University Extension puts a number on that second condition and recommends setting out tomato transplants when the soil is warmer than 60°F or after all frost danger has passed. Illinois Extension places tomatoes with peppers, winter squash and cucumbers in the group of warm-season crops that should go in well after the average last frost date, because they grow poorly in cool weather.

Because both conditions depend on local weather, the right week changes from one region to the next and from one spring to the next. Clemson Cooperative Extension lists a spring planting window of March 1 to April 30 for tomatoes in the Coastal Plain of South Carolina, and Illinois Extension notes that gardeners in northern Illinois can plant about two weeks later than those in central Illinois. The dependable approach is to learn the frost data for your own county, check the soil in your beds, and plant once both line up. Your county Cooperative Extension office can tell you which planting window local gardeners use, and our garden section collects more seasonal guides.

How do you find your last frost date?

A last frost date is a statistical estimate drawn from decades of weather records, which is why it works as a planning guide and leaves some room for a late cold night. Illinois Extension explains that the frost maps published by the Illinois State Water Survey and State Climatologist use temperature data from 1991 to 2020 and show the median date of the last 32°F freeze, along with the earliest and latest last freezes and the dates for 1 and 9 years out of 10.

The example Illinois Extension gives for Macon County, in central Illinois, shows how far apart those dates can be. The median last 32°F frost there falls between April 8 and 15, the date passed in 9 out of 10 years falls between April 22 and 29, and the latest recorded last freeze falls between May 5 and 15. For that area, Illinois Extension says a cautious gardener, or one growing only warm-season plants, would plant outside in mid-May. When you look up your own dates, use the later, more conservative figure for tomatoes, since a single frost can injure the plants.

What soil temperature do tomatoes need?

Utah State University Extension uses 60°F as the soil temperature above which tomatoes can be transplanted, and a soil thermometer pushed into the planting area gives you a reading to compare with that threshold. Air temperature matters as well, and Clemson Cooperative Extension notes that tomatoes grow best at 70 to 80°F during the day and 60 to 70°F at night, and Utah State adds that flowers drop without setting fruit when nights fall below 50°F or days climb above 95°F. Checking both soil and air temperature before you plant gives the young plants conditions in which they can grow steadily and set fruit.

You can help the soil warm up ahead of planting. University of Maryland Extension suggests laying black plastic or landscape fabric over the bed two to three weeks before planting, and Utah State recommends waiting to spread organic mulches such as straw or grass clippings until the soil is warmer than 75°F, since those mulches keep the ground cool.

Does your USDA hardiness zone tell you when to plant tomatoes?

Your hardiness zone gives only a broad regional hint for tomatoes. The USDA Plant Hardiness Zone Map is based on the average annual extreme minimum winter temperature, shown as 10°F zones and 5°F half zones, and the USDA describes it as the standard for deciding which perennial plants are most likely to thrive at a location. Illinois Extension points out that most fruit and vegetable plants grown there are annuals, so the zones mostly apply to landscape plants. Tomatoes are grown as warm-season annuals that live for one season, which is why your local frost date and soil temperature are the numbers to plan around. If you want to understand the zone system in more depth, our guide to annuals vs. perennials explains how it applies to plants that come back each year.

When should you start tomato seeds indoors?

Clemson Cooperative Extension recommends sowing tomato seed indoors six to eight weeks before the last frost date in your area, and Utah State University Extension also allows six to eight weeks to grow transplants. Utah State suggests germinating seeds at 80°F until the seed root appears, then growing the seedlings at 65 to 70°F until they have five to seven leaves and a well-developed root system. Clemson advises a light soil mix and plenty of light, because low light produces tall, spindly plants, and it notes that supplemental light is needed unless the seedlings sit on a sunny, south-facing windowsill. If you buy transplants instead, Clemson and Maryland Extension both suggest choosing stocky plants about 6 to 10 inches tall.

How do you harden off tomato seedlings?

Seedlings raised indoors need a gradual introduction to sun, wind and cooler air before they go into the ground. Clemson Cooperative Extension describes a process that begins about a week before transplanting:

1. Start roughly one week before the date you plan to transplant.
2. Place the plants outdoors in bright, indirect light at first, such as under a tree or on a covered porch.
3. Increase the number of hours they spend outdoors each day.
4. Water less often than you did indoors, while making sure the plants never wilt.
5. Watch the leaves, which develop purplish veins as the plants harden off, according to Clemson.

How do you plant tomatoes once the timing is right?

Clemson Cooperative Extension recommends a well-drained site where tomatoes, peppers, eggplant or potatoes have not grown for at least three years, which helps you avoid soil-borne diseases and nematodes. It suggests transplanting in the evening or on an overcast day to reduce wilting, and spacing plants 24 inches apart in rows 3 feet apart. Set each transplant deep enough that only two or three sets of true leaves remain above the soil. If a plant has become leggy, Clemson and Maryland Extension both describe laying it horizontally in the planting hole, because roots form along the buried part of the stem. Press the soil firmly around each plant, leaving a slight depression that holds water.

Can you plant tomatoes before the last frost date?

Utah State University Extension explains that most gardeners in Utah transplant tomatoes through black plastic mulch for earlier harvests, and that with plastic mulches and row covers, plants can go out several weeks before the last frost date. Hot caps, plastic tunnels and fabric covers protect young plants from cool air, and Utah State advises ventilating them when air temperatures exceed 80°F. It also warns that the stem should not touch the plastic mulch, which can cause heat injury. If you plant early, keep the covers ready for any cold night in the forecast. Our home maintenance checklist can help you fit spring garden tasks into the rest of the season's chores.