When to Plant Tomatoes

The two signals: frost and soil temperature

Tomatoes are killed by frost, so no planting date works until the frost risk has passed. Clearing that bar is not enough on its own, though. Tomatoes also need warm soil. Utah State sets the transplanting threshold at a soil temperature of 60°F, and Oregon State says the plants do best with soil at 65°F or warmer and air above 60°F. Use the dates for your zone above as your window, then confirm with a probe-type soil thermometer at planting depth before you plant.

Waiting pays off. Oregon State notes that plants set out into cool, wet weather grow slowly, and cold weather at flowering causes catfaced, misshapen fruit on the first clusters. Utah State puts the night limit for fruit set at 50°F; Clemson and UC put it at 55°F.

When to start seeds indoors

Count backward from your planting-out date. The sources differ slightly:

• Clemson: 6 to 8 weeks before the last frost.
• Utah State: allow 6 to 8 weeks to grow a transplant.
• Oregon State: 5 to 8 weeks, depending on how warm you keep the seedlings.
• NC State’s plant profile: 5 to 6 weeks before setting plants out.

Choose the shorter end if you grow under strong lights in a warm room, and the longer end for a cool basement or windowsill. The target is a stocky plant 6 to 10 inches tall with 5 to 7 leaves. Plants that have started flowering in the pot establish slowly, so starting too early is a real mistake, not a head start. Allow one more week to harden plants off outdoors before planting.

Planting earlier with protection

In short-season areas, gardeners gain time by warming the soil and the air around the plant. Utah State says that with black plastic mulch plus row covers or hot caps, plants or seeds can go out several weeks before the last frost date. University of Maine notes that row covers can raise daytime air temperature by up to 10°F, and that water-filled plastic rings keep young plants a few degrees warmer on cold nights. Open or vent covers once the air inside goes above 80°F, and remove them when the plants flower.

Direct seeding is possible but slower. Utah State allows sowing about 10 to 14 days before the last frost, and Oregon State notes that direct-seeded plants mature about 25 days later than transplants. In cold areas such as central Oregon, Oregon State recommends transplants only.

Regional timing

Cold north and high elevations. Your season is short at both ends, so plant on time and choose early varieties. Oregon State recommends adding 10 to 14 days to the days-to-maturity on the packet where nights are cold. The University of Maine advises growing only early varieties in northern Maine.

The Southeast. Clemson recommends a spring crop everywhere in South Carolina but a fall crop only on the coastal plain, not in the Piedmont. NC State advises planting as early as frost allows, because hot summers make blossoms drop before fruit sets.

Texas. Texas A&M sets out fall transplants about 100 days before the first expected frost, giving the plants time to ripen a crop before frost.

California. UC Cooperative Extension recommends spring planting for most of the state, with a second planting in early to midsummer for a fall crop. On the cool central and northern coast and at high elevations, it advises planting later in spring and skipping the midsummer planting.

Low desert of Arizona. University of Arizona’s Maricopa County calendar has a late-winter transplanting window and a second one in midsummer. It suggests delaying fall planting until whitefly numbers fall in cooler weather, and waiting in spring until the soil is warm and dry, to reduce fungal and bacterial disease.

Frost-free Florida and zones 9 to 11. Tomatoes become a fall-to-spring crop. UF/IFAS plants in late summer and again in late winter in central Florida, and from late summer through winter in south Florida. The aim is to set fruit before the summer heat, when flowers drop.

Working back from fall frost

For a late crop anywhere, start with your first fall frost date and subtract the variety’s days to maturity, then leave extra time to pick before frost arrives. Remember that fruit needs 25 to 35 days from flower to ripe. A determinate or early variety is the safest choice for a late planting, because it sets its crop quickly.