How to Start Tomato Seeds Indoors: It's Easier Than You Think!

Any kind of tomato can be started from seed: beefsteaks for slicing, romas for sauces and salsas, and cherry or grape tomatoes for salads and appetizers. The cherry tomatoes are always a favorite with beginners; Texas Wild (a pea-size red heirloom), Aunt Ruby’s German Green cherry, Super Snow White cherry, Tumbling Tom cherry, and Chocolate cherry are all prolific.

It’s always fun to grow a few heirlooms. Try Pineapple, Schimmeig Stoo, Amish Paste, and Japanese Black Trifele. How about a blue or black tomato?

When to Start Tomato Seeds

Tomatoes are a warm-season crop and can’t be outside until the soil is quite warm (65°). This is when nighttime temperatures are consistently above 50°F. 

The challenge is that tomatoes take a long time to harvest (60 to 100 days, depending on variety), so some gardeners don’t have a long enough growing season to get much yield. 

What to do? Many gardeners start the seeds early inside so they can set the small plants (“transplants”) outside as soon as the soil is warm enough!

It’s best to start seeds about 4 to 6 weeks before the last spring frost date. Don’t start too early, or you’ll just end up with leggy seedlings and later stunted growth and wilting. See the last average frost date in your area. 

How to Start Tomato Seeds 

Required materials: Seeds, seed-starting containers, seed-starting mix, plastic wrap, and water.

• Any type of shallow container will do—from a traditional seed-starter tray to recycled salad containers or yogurt cups. Just make sure the container has drainage holes, or you poke in your own holes.
• A seed-starting mix is not dirt or soil. It’s a soil-less medium that is very fine, light, and fluffy so that tiny seeding roots can grow easily. You can buy this mix at the store, or here’s how to make your own potting mix. 

How to Start Tomato Seeds

1. Use a fresh seed-starting mix. Moisten the mix and fill your seed-starting containers. 

2. Press in your seeds about 1/2-inch deep, and water very gently (so you don’t dislodge the seeds).

Tip: Some gardeners use mycorrhizal fungi inoculants when pressing seeds into the planting mix. Simply dampen the seeds and roll them in the powder. Fungal hyphae or threads form on plant roots almost immediately and gather extra nutrients and water to fuel development.

3. If you are growing multiple varieties, label everything so you can keep track of your plants. Popsicle sticks are a handy way to label varieties, but you can also use laundry pegs, wine corks, pot fragments, pebbles, or whatever is handy.
4. Cover the containers with plastic wrap or place them in a tray that has a dome lid to keep them moist. They shouldn’t need watering again until they germinate, but check them daily just to make sure they haven’t dried out. Water gently again if necessary. 
5. Place in a warm, dark location. Tomatoes germinate best at 65 to 85 degrees. Tomatoes are a warm-season crop, so setting seed flats or pots on a heat mat promotes rapid germination. But don’t let it get too hot. Don’t go over 95 degrees. 

6. Once the seeds are up, remove the heat source. Move them to a place with strong direct sun or place under grow lights where they will get bright light for most of the day. An LED grow light or fluorescent shop light kept on for 16 hours a day works best. Temperatures between 50 to 70 degrees are perfect for growing rugged plants. You want them to be short and stocky, not long and leggy. Keep the grow lights two inches from the plant tops. I have to adjust the distance every other day.

7. When your plants have two sets of true leaves, it is time to pot them up into larger individual containers, such as 16-ounce plastic cups with holes poked in the bottom. If they are getting leggy, bury the lower part of the stem to bring the leaves closer to the surface of the soil. They will grow more roots along the buried stem.  

8. As the plants grow, set up a fan to blow gently across them or brush the tops with your hand to encourage strong stems. I also run a small oscillating fan directed at the plants. The constant movement triggers the release of hormones that build thicker stems. The resulting tomato transplants end up stockier than normal with sturdy stems, which are more durable when set into the garden bed. You won’t have leggy plants, and you’ll avoid diseases and rot by having to bury long, thin stems.

9. Don’t overwater! It is the leading cause of seedling failure.

10. If your plants outgrow their containers before it is time to transplant them outside safely, move them up into a bigger pot. Each time you repot them, bury the lower part of their stem to stimulate more root growth for a stronger plant. 

11. When you’re ready to move them outside, you need to harden off the seedlings by gradually exposing them to direct sunlight, wind, and night temperatures outside before planting them out. For about 10 days, you will need to move them outdoors for a few hours of the day in a dappled shade area and then bring them back inside. Do not leave them outside in temperatures lower than 50°F or on windy days. If your climate is very windy, you may need to consider a cold frame or plastic sheeting for protection. 

Check out these videos I recorded for my local library during the early days of COVID-19. At that time, many homebound people were starting their first vegetable garden, and we wanted to give them some tips.

How-To Start Tomato Seeds Video

How-To Transplant Tomatoes Video

12. When you’re ready to put the tomato seeds in the ground, always choose a sunny spot and ensure your soil is well-prepared with rich organic soil. Transplant your small plants into the ground outdoors after all danger of frost has passed and the soil is at least 60°F. See our Planting Calendar for suggested transplanting dates

Other warm-season vegetables such as melons, peppers, and eggplant should be started from seed in the same manner. Add a bit of Epsom salts to the potting mix you use for peppers to give emerging plants an immediate dose of boron, which they crave. Plants will bloom faster in the garden and produce more peppers.

Save your eggshells, too, for when you transplant tomatoes into the garden. Crush them, scatter them over the soil, and then set them in plants. The shells gradually release calcium into the dirt, preventing blossom-end rot. Calcium also helps all types of transplants produce vigorous root systems, spurring plant growth.

Here is the Almanac’s complete Growing Guide to Tomatoes for more information on planting, growing, and harvesting tomatoes.

Have you ever tried growing tomatoes from seed? We would love to hear about your experience!