Getting tomato seedlings in the ground at the right time is the difference between a plant that takes off and one that stalls for weeks. The key signal is soil temperature, not the calendar date - tomatoes need soil at least 60°F at the root zone before they will actively grow, and most planting failures come from rushing that window. This guide covers how to choose healthy seedlings, harden them off properly, amend the soil, and transplant them deep enough to build a strong root system.
In short:
• Plant when soil temperature hits 60°F minimum at 4 inches deep - not just when air temperatures feel warm
• Harden off all seedlings for 7–10 days before planting, even store-bought ones
• Bury the stem deep, up to the lowest leaves - to build a wider, more drought-resistant root system
• Add compost and slow-release fertilizer before planting; bone meal at the hole bottom gives roots an early phosphorus boost
When to Plant Tomatoes Outdoors
Tomato plants are essentially tropical plants, needing warm days and cool nights. They prefer soil temperatures between 70°F and 75°F. They (drop)blossoms if air temperature is cooler than 55°F or hotter than 90°F and are very sensitive to frost, which blackens and kills them.
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Tomatoes need at least 6 to 10 hours of sun daily plus some afternoon shade in really hot climates.
They accept almost any kind of soil, as long as it has lots of organic matter in it to help it hold moisture and drain well and is on the acid side (pH 6.0 to 7.0).
Add organic matter such as peat moss, chopped leaves or compost to lighten and loosen clay soil or plant tomatoes in raised beds to improve soil drainage.
Mix in some all-purpose slow-acting organic granular fertilizer or a fertilizer product for acid-loving plants when preparing the soil. Follow directions on the package label.
How to Choose Healthy Tomato Seedlings
When buying tomato plants at the garden center or farmer’s market, look for deep green leaves and for stocky, short plants that look healthy. Check the pot to see if the soil is moist and rich-looking. Pull one or two out of their container and see if the plant is root bound. Do not buy wilted plants or plants with yellow leaves. Especially be sure you know which of your tomato plants are determinant (grow 4 or 5 feet) or indeterminant (grow over 7 feet). And, don’t forget to protect your plants from the elements on the drive home. Translation: Don’t put them in the back of an open pickup and then fly home on I-5.
How to Harden Off Tomato Seedlings Before Planting
You should not even think of planting tomato seedlings until the soil temperature is at least 65 degrees; 70 degrees is better. When the soil temp is okay then you need to bring your seedlings outside during the day and then bring then back inside at night for at least two days. This helps the seedlings get used to real sun and the breezes.
Best Soil Amendments for Planting Tomatoes
Critical New Products To Make Planting Tomatoes A Huge Success
There are three new garden products and one old one that have become essential at planting time to our success in growing wonderful tomatoes.
Compost – Available forever, compost is incredibly valuable when included in the soil where you are planting tomato seedlings – we use a half a cup per plant.
Myke – Myke (www.usemyke.com) contains mycorrhizae which are beneficial fungi that attach themselves to the roots of a plant and literally double the size of the plant’s root system in a few weeks. This significantly increases a plant’s access to water and nutrients. It’s crazy not to use this product – a tablespoon per plant on the root ball and in the planting hole.
Actinovate – Actinovate (www.naturalindustries.com) is a biological organic fungicide that has the very powerful ability to prevent virtually every fungal disease that affects tomatoes including early blight, fusarium, verticillium, and late blight. You spray it on the root ball and on the plant at planting time and then spray the plant every two weeks throughout the season. Available on Amazon.com.
For a full list of tomato diseases and pests Actinovate protects against, see Problems of Tomatoes.
Vegetable Thrive – Thrive (http://www.usethrive.com/store/VeggieThrive/) contains Microbial Soil Bacteria and Mycorrhizal Fungi; two groups of beneficial microbes found only in very healthy soil with an organic content of at least 5%. Most of us do not have soil with 5% organic matter or higher so Thrive provides the services of critical beneficial soil microbes as we work to get our soil up to organic snuff. Available on Amazon.com. Spray on plants at planting and then spray plants every three or four weeks throughout the season.
How to Transplant Tomato Seedlings Step by Step
- Water seedlings well 1 to 2 hours prior to planting, to keep their rootballs intact during the transplanting process.
- Plant on a cloudy day or in late afternoon or evening to protect seedlings from the hot sun while they cope with transplant shock. The soil is cooler and the relative humidity higher then, reducing moisture stress on the new plants.
- Space transplants 1½ to 2½ feet apart. With a trowel dig individual holes a bit deeper than the seedling’s rootball or container. Deep planting encourages roots to form along the buried stem, increasing the plant’s capacity to take up water and nutrients, resulting in stronger plants.
- Gently coax each seedling from its container, set it in its hole and fill in with loose dirt. Press the soil gently around the main stem. Water transplants generously and then do not allow them to dry out.
- Drench the soil around each seedling with Actinovate and with Thrive.
- Apply an inch of organic mulch around the seedlings. After they get large enough add mulch until the depth is 3 to 4 inches.
Common Questions About Planting Tomatoes
What soil temperature do tomatoes need before planting?
Tomatoes need a soil temperature of at least 60°F at a 4-inch depth before planting - 65°F is better for faster establishment. Soil temperature lags behind air temperature by two to three weeks in spring, so a warm afternoon does not mean the ground is ready. Use an inexpensive soil thermometer, push it 4 inches deep in the morning for the most accurate reading.
How deep should you plant tomato seedlings?
Plant tomatoes deep - bury the stem up to the lowest set of leaves, pinching off any leaves that would be underground. Tomatoes are one of the few vegetables that grow roots directly from buried stem tissue, so a deeper hole means more root surface, better water uptake, and a plant that holds through dry spells. A planting hole 6 to 8 inches deep handles most transplants.
Do tomatoes need to be hardened off before planting?
Yes, hardening off is essential for any tomato seedling grown indoors or bought from a greenhouse. Without it, plants go into shock from direct sun, wind, and temperature swings they have never experienced. The process takes 7 to 10 days: start with one hour outdoors in a shaded spot, add an hour each day, and work up to full sun and longer exposure before planting in the ground.
When is the best time to plant tomatoes outside?
Plant tomatoes outdoors when two conditions are both true: nighttime air temperatures stay reliably above 50°F, and soil temperature is at least 60°F at 4 inches deep. For most US gardeners this window falls two to three weeks after the last frost date for their zone - which ranges from early March in zone 9 to late May in zone 5. Soil temperature is the more reliable signal; the calendar date is only a rough guide.
What should you add to the soil before planting tomatoes?
Work compost and a balanced slow-release fertilizer into the bed before planting. If the soil is heavy clay or sandy, the compost matters more than any specific amendment. Drop a tablespoon of bone meal at the bottom of each planting hole for an early phosphorus boost, and a handful of crushed eggshells adds calcium that helps prevent blossom end rot later in the season.
After Planting
Once the seedlings are in the ground - planted deep, amended well, and given a week to adjust, the foundation is set. Watch the first two weeks after transplanting: afternoon wilting is normal while roots establish, but if plants are still drooping in the evening, they need water. After that, move on to watering schedules, fertilizing through the season, and what to watch for as the plants grow - all covered in Caring For Tomatoes.
