How to Prick Out Tomato Seedlings and Pot Them Up

You’ve sowed the seeds. Now it’s time to plant tomato seedlings into their own pots. Tips on pricking out tomato seedlings, planting them into individual pots, and growing them on using grow lights. Includes an instructional video.

This is part two of the Growing Tomatoes from Seed Series. The first piece is on sowing tomato seeds and this piece will continue with planting tomatoes in the greenhouse, growing them on, harvesting the fruit, saving seed, and preserving tomatoes.

Growing tomatoes from seed is an annual ritual that gets me into the spirit of the growing year. In February to mid-March I sow them in pots and trays and grow them on in the house where it’s warm. After just a few weeks the seedlings begin to crowd one another out which brings me to the next step – pricking out the seedlings.

Pricking out tomato seedlings means gently separating the young plants and potting them up in their own containers. This piece shares how to do it, why it’s important, and gives tips on growing them on into mature plants.

Growing Tomatoes from Seed Series

This piece includes the third video in the Growing Tomatoes from Seed series, which you’ll find at the end. The first two steps in the process include an introduction to the varieties I’m growing this year, making a simple seedling compost, sowing, watering, and using a heated propagator.

We continue with pricking the seedlings out and planting them into individual pots. Throughout the coming year I’ll be sharing how to grow tomatoes from seed beginning with sowing, growing on, planting in the greenhouse, and finally harvesting.

Just a few weeks from sowing to pricking out

Tomato seeds germinate between 5-14 days after sowing. This time around, mine were up quickly which I attribute to their growing in a heated propagator.

The seedlings tend to come out of the compost stem first, looped back with their heads buried. After that they pop up and begin growing quickly. At first, they unfurl what are called seed leaves, which are more rounded than their true leaves will be. Soon after a second set of leaves emerge – the first set of their more jagged true tomato leaves. This takes about 1.5-2 weeks from germination.

When to prick out tomato seedlings

Though some gardeners will prick out seedlings before these true leaves are out, it’s more traditional to wait until they have them. I’ve tried it both ways and think it’s probably better to wait. That way you don’t have to worry about the plant being hurt if you accidentally damage a seed leaf.

If it’s damaged after the true leaves are out, no big deal. The little plants rely on seed leaves to power them until that point though.

Potting mix for tomato plants

In the first step of growing tomatoes from seed I showed how to make a simple seedling compost to sow seeds into. It was very free draining and lower in nutrients by being mixed with Perlite.

When planting up your seedlings you’ll need richer compost. I tend to use pure multi-purpose compost if potting on into new pots. If thinning seedlings out from pot-sown seeds, I’ll leave one plant growing in the original seedling compost but top the compost up in that pot with multi-purpose.

The tomato plants are likely to not need any additional feed until it’s much larger and growing in the greenhouse or outdoors.

What size pots to plant tomato seedlings into?

Tomato plants are about four inches tall when they’re planted out so they’ll eventually need to be planted in 6-inch diameter pots. You can plant them into this size immediately on pricking out or you can pot them up into smaller pots and then repot them on later.

The benefits of using smaller pots and then up-potting include saving space and compost, and reducing the chance of fungus gnats. Those are annoying fly-like insects that love breeding in moist compost. They’re a real nuisance with house plants and seedlings grown indoors. Their larvae can also damage plant roots.

In my video you’ll notice that I’ve planted the tomato seedlings into smaller peat pots and also 3” plastic pots. The ones in smaller pots are destined for this weekend’s Seed Swap and will need repotting by their new owners in a couple weeks’ time. The ones in 3” pots will be potted on in the next piece, after which they’ll be hardened off and planted in the greenhouse.

How to prick out tomato seedlings

To pull out an individual plant from a seed tray crowded with seedlings you should work from the outside in. Gently take hold of a seed leaf of a plant and gently lever it out of the compost using a pencil, plant label, skewer, or another implement.

Use the end of your implement to guide the seedling down into the hole of its new pot. You can bury the plant up to the bottom of the seed leaves if you wish. Roots will form from the stem that’s buried and if your seedlings are leggy, it’s a handy tip to promote a stronger stem.

Continue working through your seedlings until they’re all pricked out or discarded. I wouldn’t recommend pricking out tomato seedlings that look damaged, stunted, or less healthy than the others.

Growing tomato seedlings

Once planted, the seedlings will need regular watering to keep their compost moist. They’ll also need light and warmth, so keeping them in your house, conservatory, or a heated building is a good idea.

The multi-purpose compost will be enough to sustain them and they’ll grow from 1-2” in height to 4” in a matter of weeks. That’s why it’s important that you sow your tomato seeds not from when the seed packet says, but from using your regions specific climate to guide you. There’s a lot more information on that in the first piece in this series.

Lighting for tomato seedlings

Tomato seedlings need a lot of light to grow into healthy plants. The light from a window sill is often not enough, especially in late winter or early spring. If you don’t already have a plant light set-up, you can create a simple one using a grow-light that clips onto your window sill or by suspending a traditional light from above. I’m using both, and can recommend either.
The clip-on grow light is new to me this year and I’m pretty excited about it. Not everyone has the space for a dedicated grow-light shelf so it makes a window sill a bright enough place for the hobby gardener to start off tomato, pepper, and eggplant seeds early in the year. It’s also inexpensive.

If your plant’s stems are tall and spindly it probably means that they’re stretching trying to find more light. Give it to them by lowering your grow lights to a couple of inches of their leaves.

The next steps

This series on growing tomatoes from seed will continue with repotting, hardening off, and planting out in my greenhouse next month. From that point I’ll be training them to grow up a string, cover nutrition, watering, and pollination, and then show how to harvest and preserve tomatoes in the summer.