How to Start Tomato Seedlings from Seed

Steps

1. 1

   Pick determinate or indeterminate

   Most tomatoes grow in one of two ways. Determinate, or bush, types grow to a fixed size. They then stop and ripen most of their fruit in one big flush. Indeterminate, or vining, types keep growing and making fruit right up until frost. Vining types need a stake or a trellis to hold them up. Bush types need far less, and compact ones may get by with a short cage or nothing at all.

2. 2

   Count back from your frost date

   Whichever type you choose, sow indoors about 6 to 8 weeks before the last expected spring frost in your area. Sources vary a little on the exact window. If you follow a certain seed supplier or grower, use their timing. Otherwise, use 6 to 8 weeks.

3. 3

   Start sowing on the first day of your window

   On the first day of your window, gather your seeds and work through the sowing steps below. Do not start before that. Seedlings started too early grow tall, thin and overgrown, and then struggle to settle in once they go in the ground.

4. 4

   Choose containers that drain

   When the timing is right, gather containers with drainage holes. Cell packs, plug trays and small pots all work. So do clean recycled items like yogurt cups and egg cartons.

5. 5

   Sterilize anything reused

   Scrub the old soil off any container that has held plants before. Wash it in hot soapy water, rinse it well, and stand it out to air dry before you fill it. This small step heads off damping off, a disease that rots young seedlings at the base.

6. 6

   Fill with sterile seed-starting mix

   Fill the clean containers with a light, sterile seed-starting mix. Do not use soil from the garden. Garden soil drains poorly in a small container. It can also carry disease, including the organisms behind damping off.

7. 7

   Aim for about a quarter inch deep

   Sow the seeds about 1/4 inch deep in that mix. Anywhere from 1/8 to 1/2 inch will do.

8. 8

   Sow a few seeds per cell

   Put 2 to 3 seeds in each cell or pot. That way you are covered if some seeds fail to sprout, and you can leave them all in place for now.

9. 9

   Label every variety now

   As you sow, mark each variety. Use a plant tag, a popsicle stick, or tape and a marker. Weeks from now you will not remember which cell holds which.

10. 10

    Cover the trays to hold moisture

    Put a clear humidity dome or a sheet of plastic wrap over the labelled trays. This holds in moisture and warmth while the seeds sprout. Take the cover off as soon as you see sprouts.

11. 11

    Keep them warm to germinate

    Under that cover, aim for a soil temperature of about 70 to 85°F. That's a commonly cited target range for sprouting tomato seed rather than a hard cutoff, so treat it as what you're steering toward, not a line the seed won't cross.

12. 12

    Keep the tray warm until the seedlings are up

    Keep the tray warm until the seedlings are up, setting it on a heat mat if you have one. Warmth from below speeds up sprouting a lot. Take the tray off the heat once they are up, since heat does the seedlings no good after that point.

13. 13

    Keep the seeds in their warm spot until sprouts appear

    Leave the seeds where you set them to stay warm. Do not move them to a bright spot yet. A tomato seed needs no light to sprout. Wait until the seedlings have come up before you start giving them light.

14. 14

    Water evenly, never soggy

    All through sprouting, keep the mix evenly moist but never soaked. Too much water is one of the leading causes of seedling failure and rot. Water from the bottom instead. That keeps the seeds where you put them and keeps the leaves dry.

15. 15

    Give them 14-16 hours of light

    Once the seedlings are up, light does matter. Give them about 14 to 16 hours of bright light a day. Grow lights are ideal. Hang them at the distance the fixture's manufacturer recommends for seedlings, since that varies a lot between weak fluorescents and high-output LEDs. Getting that distance right keeps the stems from stretching without scorching the tops.

16. 16

    Grow them cool and bright

    Pair that bright light with cool conditions. About 65 to 70°F makes the sturdiest seedlings. Warmer rooms push tall, spindly growth instead.

17. 17

    Set up a small fan

    Run a small fan near the seedlings to keep the air moving. The breeze makes the stems thicker and stronger. It also cuts down on fungal problems like damping off.

18. 18

    Feed after the first true leaves

    Do not use fertilizer until the seedlings have put out their first true leaves. True leaves come after the first pair, and they look like real tomato leaves. Then use a balanced water-soluble feed. Dilute it to about half, or even a quarter, of the strength on the label.

19. 19

    Pot up into bigger containers

    Move the seedlings into bigger pots of about 3 to 4 inches. Do this when they have about two sets of true leaves, or when they simply outgrow their cells. Set them deeper than they were. Bury part of the stem to encourage more roots.

20. Why the stem goes deeper: tomatoes grow roots along any buried part of the stem. The buried section turns into more root, so the plant ends up with a bigger root system. The same trick comes up again at transplant time.
21. 20

    Harden them off gradually

    Before the potted-up seedlings go in the garden, harden them off. This means getting them used to outdoor conditions bit by bit. Put them outdoors in shade for a few hours a day. Then steadily give them more sun and more time outside over about 7 to 10 days.

22. 21

    Wait for warm soil to transplant

    Plant them out only once all danger of frost has passed. The soil must have warmed to around 60°F. Night temperatures must also hold steadily above 50°F.

23. 22

    Bury the stem deep

    Set each plant deep enough that the soil comes up to its lowest set of leaves. Tomatoes grow roots along any buried part of the stem. So deeper planting builds a bigger root system. For what comes after, see how to plant tomatoes.

24. 23

    Save seed only from open-pollinated varieties

    Check that the variety you sowed was an heirloom or open-pollinated one before you save any seed from this crop. Open-pollinated means the seed grows into a plant like its parent. Seed saved from hybrid plants, also labelled F1, will not come true to type next year. Here is how to check whether the variety you sowed is open-pollinated.

26. 24

    Months later, save seed from a ripe fruit

    Once a fruit has fully ripened, months after you plant out, scoop its pulp and seeds into a container and leave it to ferment for a few days, until a layer of mold forms on top and the seeds settle to the bottom. Rinse the seeds, then dry them thoroughly before you store them.