Learn What Tomato Seedlings Need to Grow After They Germinate

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When tomato seedlings pop through the soil, the whole game changes. 

It’s a lot like what happens when you bring a new baby home from the hospital. Your new sprouts require lots of TLC.

There are 4 secrets to grow tomatoes successfully:

  1. Light
  2. Water
  3. Air
  4. Nutrients
What tomato seedlings need after they germinate to flourish with Tomato Dirt #GrowingTomatoes #HomeGardening

1. Light: let there be lots of it!

Bright light is super-important for healthy tomato seedlings! As soon as they sprout, they’ll reach up for sunbeams (or fluorescent beams). Seedlings need 12-18 hours of light each day. Do yourself a favor and give it to them. Otherwise they will become leggy and weak very quickly, which will set back production in your garden.

4 best ways to let there be light

1. Heated greenhouse. If your greenhouse is situated where it gets maximum exposure to sunlight, use it! Caution: out of sight is not out of mind. Your greenhouse may not be as accessible as your living room or kitchen, but make sure you check on your tomato seedlings every day. They can’t navigate this stage of development alone. (Find greenhouse plans here.)

2. Cold frame. The best ones face south. Make sure yours has a tight-fitting lid so that heat stored up during the day from the sun will keep seedlings warm during cool nights. When the forecast is for below 20°F overnight, place an outdoor electric light inside the cold frame (one 60-watt bulb for every 12 cubic feet should be enough). During days when temperatures rise to 50° F and higher, prop open the cold frame lid. You’ll prevent baked tomato plants.

3. Fluorescent lights are among the most affordable and popular light source for seedlings. Mount lights with cords or link-type chains so you can adjust light height as seedlings grow. Keep lights 3-6” from tomato tops. Yep, that means you’ll have to check your seedlings daily and move lights up and down occasionally. Lights should remain on 12-16 hours a day. Turn lights off at night. (Seedlings need sleep, too.)

4. Sunny window. This is the most challenging lighting scenario for young tomatoes. Most home windows simply don’t have enough light for seedlings, especially in late winter and early spring. Choose a window facing south. Rotate tomato seedlings daily to prevent stem bending.

2. Water

Hydration in the tomato nursery can be tricky.

Water do’s and don’ts

  • Do water your tomato seedlings enough so that you don’t let soil dry out or seedlings wilt.
  • Do water from the bottom. (Remember … you poked holes in your containers, right?)
  • Don’t overwater. Too much of a liquid diet fills in the air pockets in soil. When the roots can’t absorb all that water fast enough, they suffocate. Overwatering = drowning!

3. Air

Tomato Dirt best advice: steady temperatures!

  • Take off plastic or clear covers from seedling containers to let seedlings breathe. Circulation will help prevent nasty diseases and fungi from spreading.
  • Keep room temperature consistent, about 70°F. If your tomato seedlings are near a window, watch for drafts. Cold air mixed with over-wet soil is a deadly combo for seedlings. They’ll dampen off or develop nasty fungi faster than you can say “transplant time.”

4. Nutrients

The jury is still out among tomato gardeners about how much to fertilize seedlings at this point in their young lives. One side says plants are still feasting on the nutrients provided by their seeds. The other side says, “Give them some extra food! They need a strong start.”

On the other hand, mature tomato plants are like teenagers: you just can’t feed them enough. (Read more about fertilizing tomato plants.) Your seedlings aren’t quite at that point yet. It’s up to you during their tomato seedling babyhood to decide whether or not to feed them, and if so, how much.

A large number of successful home gardeners recommend waiting to fertilize until you transplant your baby tomato seedlings into their next-size-up containers. Others say to feed them right away.

Tomato Dirt best advice: keep an eye out for yellowing leaves, which indicate the need for food. If possible, wait until your seedlings have at least 2-3 sets of leaves before feeding.

If you decide to give your plants some additional nutrients now, then feed seedlings with a weak (1/4 strength) water-soluble tomato fertilizerKeep an eagle eye out for crystallized salts on soil surface. That will tell you you’re feeding your seedlings too much.

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