When to Plant Tomatoes Outside: The Soil Temperature Guide

The Number to Wait For: 60°F Soil

Plant tomatoes outside when the soil reads at least 60°F at 3 to 4 inches deep, frost danger has passed, and air temperatures are staying above 50°F at night. Penn State Extension says tomatoes should not be transplanted until soil 3 inches beneath the surface reaches 60°F. Ohio State, Oregon State, Maryland and New Hampshire extension publications give the same 60°F minimum, and Oregon State adds that tomatoes ideally prefer 65 to 70°F.

Two sources set the floor lower. Colorado State University Extension says soil temperatures must be above 55°F before a transplant can be successful, and an Ohio State bulletin gives 55 to 60°F as the minimum for warm-season crops. Since the other guides here ask for 60°F, treat 55°F as the point below which planting fails and 60°F as the point where it starts to go well.

Why the Frost Date Alone Lets You Down

The average last frost date tells you when a freeze becomes unlikely. It says little about the ground, and Ohio State notes that a large difference between soil and air temperature is common early in the season. A plant set out on the right calendar day can still be sitting in cold soil.

University of Maryland Extension makes the point with a familiar rule of thumb. Tomatoes do not have to be in the ground by Mother’s Day, and a 40-degree Mother’s Day is bad for young seedlings. Its advice is to wait until the forecast shows at least a week of soil temperatures over 60°F and air temperatures over 50°F.

Air temperature matters on its own account too. Colorado State wants nights above 52°F and days above 60°F at transplant, and Penn State describes tomatoes as sensitive to night temperatures below 55°F.

How to Measure Soil Temperature

Use a soil thermometer, and push the probe to planting depth in the bed where the tomatoes will go. Ohio State recommends reading at about 4 inches for transplants, and Penn State uses 3 inches. Maryland notes that a compost thermometer is useful for checking soil temperatures, so the one from our compost thermometer guide can do both jobs.

One warm reading is not enough. Follow the Maryland approach and look for the reading to hold at 60°F or higher alongside a week-long forecast of mild nights. Expect different beds to be ready at different times: Oregon State points out that soil temperature varies with location and elevation, and that gardeners on the coast or in central and eastern Oregon often wait a few weeks longer than those in the western valleys.

What Cold Soil Does to a Tomato

Planting before the soil warms can lead to root decay, poor growth and disease, according to Ohio State. The most visible symptom is purple leaves. In cold soil the plant cannot take up phosphorus even if enough is present, and the shortage shows as purple discoloration, especially on the older leaves near the bottom.

University of Maryland Extension describes the same thing as purple leaf undersides and stems, calls it very common in new transplants, and says plants recover when the roots are fully established. Ohio State suggests a foliar feed with a water-soluble fertilizer that contains phosphorus as a stopgap until the soil warms.

The cost is lost time. Colorado State reports that a week of cool daytime temperatures below 55°F will stunt plants and reduce yields. Maryland describes tomatoes set out too early as suffering cold injury and nutrient deficiencies and taking a while to catch up, while those put into warm soil start growing with minimal shock. That is the case for waiting, above all when you have grown the transplants from seed yourself.

How to Warm the Soil Before Planting

Plastic mulch is the standard tool. Rutgers recommends applying plastic to moist soil 1 to 3 weeks before you plan to transplant. University of Connecticut gives the size of the effect: daytime soil under black plastic is generally 5°F warmer at a 2-inch depth and 3°F warmer at a 4-inch depth than bare soil. Under clear plastic the gain is 8 to 14°F at 2 inches and 6 to 9°F at 4 inches.

Clear plastic heats more, but Rutgers notes that black plastic prevents weed growth and clear plastic does not. For that reason black is the usual choice when the plastic will stay in place all season. Penn State’s production guide grows tomatoes on raised beds covered with red or black plastic mulch, and the same pairing works on a home raised bed.

Hold off on straw, leaves and other natural mulches. Rutgers explains that they keep soil cooler and should not be applied before the soil warms in spring, because mulching too early slows growth and delays harvest. Our guide to the best mulch for a vegetable garden compares the materials for later in the season.

Harden Off and Watch the Nights

Soil temperature is one of two gates. The transplants also have to be ready for outdoor conditions. University of New Hampshire Extension starts about ten days before transplanting, setting plants outside for a few hours each day and bringing them in at night and on days when temperatures fall below 60°F. The full routine is in our guide to hardening off seedlings.

After planting, keep a cover handy. Maryland’s advice is to throw a covering over the plants if frost is predicted, or really anything below about 45°F.

Typical Planting Times by Region

These dates come from the extension service for each area, and each one is a typical time for that state only. Your thermometer still decides.

Region	Typical time to transplant	Source
Maryland	Second or third week of May	University of Maryland Extension
New Hampshire	Memorial Day weekend; mid-June in northern frost pockets	University of New Hampshire Extension
Colorado Front Range	Late May	Colorado State University Extension

New Hampshire adds that gardeners in the south of the state can often plant a week or two before Memorial Day weekend. Maryland’s writer also reports a distinct chance of frost in May, which is why the forecast and the 60°F soil reading both have to agree before the plants go out.