10 Must-Know Tips for Growing Tomato Seedlings to Plant in Your Garden

Growing tomato plants from seeds is relatively easy. The challenge comes in growing those tiny, tender seedlings into strong, healthy plants that will produce lots of perfectly ripe tomatoes for you. These essential tips will help you avoid spindly, weak seedlings, prevent fungal diseases, and guide you through transitioning seedlings from indoors to your garden. Your sturdy tomato transplants will then be primed to deliver a hearty harvest.

1. Don’t Start Too Early

Aim to plant tomato seeds 5 to 6 weeks before the last anticipated spring frost in your area. It’s easy to get swept up in the joy of impending spring and start tomato seeds too early. Seeds started more than 6 weeks before the last spring frost will be ready to transition outside long before the outdoor conditions are hospitable. Wait to plant for the best summertime harvest.

2. Keep Seeds Warm

Tomato seeds benefit from gentle heat as they are germinating. Warming the soil to 75 to 90°F spurs the tiny seeds to put out their first roots and send up a stem. Use a waterproof electric seed mat as a heat source, or place planted seed trays or pots on top of a refrigerator or radiator. Remove the seed trays or pots from the heat source as soon as the seeds sprout. Seedlings grow best when the soil temperature is between 65 and 75°F. 

3. Light Them Up

A bright, sunny window is not enough light to grow sturdy tomato seedlings. Sunlight streaming in a window produces seedlings with lanky, floppy stems. A simple set of fluorescent bulbs or grow lights is needed to grow seedlings that are compact and have strong stems and branches. Seedlings grow best when the light source is positioned 2-4 inches above the foliage. Use a timer to illuminate seedlings for 14 to 16 hours per day. 

4. Water Smart

Seedlings grow best when the soil is consistently moist but not wet. When you touch the soil surface, you should feel moisture, but water should not emerge when the soil is lightly pressed. Maintain moist soil by watering lightly every day. Don’t allow the soil to completely dry out. A watering can fitted with a sprinkling head is handy. 

5. Thin Out Seedlings

Use a pair of clean pruners or your thumb and forefinger to pinch away seedlings growing too close to each other. Aim to space tiny seedlings about 2 inches apart. Well-spaced seedlings will develop a strong, supportive root system and ample foliage to support future growth. 

6. Provide Fertilizer

After seedlings develop their second set of true leaves—true leaves are smaller versions of the mature leaves that cover a full-grown plant—begin to fertilize with an all-purpose liquid fertilizer diluted to half-strength. Aim to fertilize plants once a week.

7. Add Movement

When seedlings are 3-4 inches tall, place a fan nearby to gently move air around your seedlings. The idea is to mimic wind, which helps the plants develop strong stems. The daily air movement also works to keep soil fungal problems at bay.

8. Pot Up Seedlings as Needed

If you started seedlings in a seed tray or a small pot, move young plants to a container that is about 4 inches wide when the plants are 3 inches tall. The increased soil volume and above-ground growing space will allow the plants to develop a larger root system and strong branching.

9. Take a Trip Outdoors

After the last chance of frost is past in spring and daytime temperatures are regularly in the 60s, it’s time to help seedlings prepare for transplanting. Acclimate seedlings to the rigors of outdoor living by placing them outside in a protected location for several hours a day for a week or more. Gradually increase the seedlings' exposure to sunlight and wind, bringing them inside each night. This process is called hardening-off. Moving plants in and out daily might feel tedious but putting in this extra effort helps minimize transplant shock so your seedlings have a higher chance of thriving when planted outdoors.

10. Transplant Well

Wait to transplant seedlings into the garden until nighttime temperatures are regularly above 50°F. Warm nighttime temperatures leads to warm soil, which is essential for good tomato seedling growth. Tomatoes transplanted into cold soil struggle for weeks before good growth begins. After planting, spread a 2- to 3-inch-layer of mulch around plants to prevent weeds and reduce soil moisture evaporation.