How To Grow The Best Tomato Seedlings

Tomato sungold F1 seedsOne of the things first time vegetable gardeners ask about when they call or email us is tomatoes – everyone wants to grow their own tomatoes. This is a bit unfortunate as for the most part they are not really suitable for growing outdoors in Ireland and are much better suited to a glasshouse, polytunnel or the Mediterranean, all of which require significant investment.

Having said that homegrown tomatoes are top of the list for difference in flavour from their bland shop bought cousins. You simply can’t compare the taste of a homegrown tomato with the thick skinned supermarket varieties that have been bred for long shelf life and handling qualities rather than what should be most important – their taste.Growing tomatoes may not be for someone who’s having their first go at vegetable growing but if you’re decided it’s for you or are considering a greenhouse or tunnel they will be one of the most rewarding crops you will grow.

We’re covering sowing tomatoes in this article, for planting and tomato crop care please see our ‘How to plant tomato seedlings’ article.

When to sow tomato seeds
Sow in late February to mid March. This may seem a little early (and cold) for heat loving plants but tomatoes need a long growing season for the fruit to form and ripen before light and heat levels fall in Autumn.

If you have a warm and sheltered South facing garden and wish to plant outdoors sow in mid March as plants can’t be placed outdoors till May and earlier sown seedlings will have to spend too long in their pots.

Seed compostWhat compost to use?
Use a specially formulated seed compost as it’s lower in nutrients (higher nutrient levels can inhibit germination and damage young roots) and a much finer consistency than potting compost. We use Klassman organic seed compost for our seedlings and reckon you’ll struggle to find better. Available in peat and peat free versions.

You’ll need heat for your tomatoes to germinate with 21˚C being the optimum. Use a heat mat, propagator or soil warming cable to achieve this. You’re ready to go with the first two straight out of the box but if you’d prefer to make your own you can see how to make a soil warming cable heat box here.

Sowing method
I’m afraid I’m a bit lazy when it comes to sowing tomatoes and don’t bother to prick out seedlings and pot them on. I need to feed my tomatoes a little after 4 weeks or so to keep them going before I plant them but I still get very happy plants which produce good yields of tomatoes. It’s a time problem with me, you might want to do it properly so I include both methods below:

Seed propagation trayOpen seed tray method
Sow seeds thinly in an open seed tray (not split into modules, like a small plastic roasting tin) or in a large 9cm pot. Barely cover the seeds with a very light layer of low nutrient fine seed compost (a soils sieve is handy for this), and firm the compost down gently with a wooden tamper.

The seed tray should be kept moist but take care not to over water at this stage. You must also make sure the compost never dries out so a bit of a balancing act here! I find inexpensive bottle top waterers ideal for this job as they deliver a fine accurate spray which won’t soak your compost.

After about 10 days the seedlings should be pricked out and re-planted in a larger 10cm pot filled with a higher nutrient multipurpose potting compost. ‘Pricking out’ means gently removing a seedling by loosening the roots with a dibblet or small stick. Hold the seedling by the seed leaves (cotyledons) and gently lever the root ball out with your dibblet. Re-plant in the new compost with the seed leaves just above the soil level.

The plants should be left on the heat bench at this stage but turn the heat down from 21˚C to about 18˚C  (15˚C is the minimum temp for tomatoes at this stage). If plants are grown in individual pots space them out as soon as their leaves touch to prevent them growing tall and spindly.

6 cell modular seedling tray6 Cell tray method
Most plants you buy from nurseries are grown in 6 cell modular trays. We direct sow 1 seed per call with the plant remaining in the seedling cell until it’s ready to be planted. To be honest you probably will produce a show winning plant with the first method but as long as you feed the seedlings in the lazy method I’d be surprised if it makes much difference in the long run.

For best results sow 2 seeds per cell in a 6 cell modular tray, nip out the weaker seedling with a nail scissors once they have germinated. Tomato seeds can be pricey, especially F1 varieties so if you’re being thrifty you can prick out the spare seedling and re plant in a cell that’s failed to germinate. The same method applies as above, adding a fine layer of compost, firming down and keeping moist.

The difference with sowing in 6 cell trays is you will need to feed your seedlings before you plant them out. As soon as the first true leaves appear (the second set of leaves after the seed leaves) feed with a seaweed tomato feed diluted to half strength. IMPORTANT: Make sure you stick to a half strength mix as overfertilized seedling plants can become stressed and die. Feed again 4 weeks later.

Seedling plants protected by horticultural fleeceSeedling Care
Make sure your propagator is turned down as we’ve said but also make sure your seedlings get plenty of daylight. Clean the glass or plastic on your greenhouse or polytunnel to allow as much light in as possible.

Remember the heat from the propagator comes from the bottom so the foliage of your plants will still be sensitive to freezing night time temperatures. Place a layer of horticultural fleece over the plants on cold nights to keep the heat in and avoid frost damage.

If any flower buds form while in the pot stage nip them off as you want to keep all the energy for growth rather than reproduction a this stage.

Growing tomato seedlingsPretend to be the wind
In their natural environment stems will get stronger and thicken up by being moved by the wind, of course this won’t happen in a protected environment indoors. Lightly brush your hands over the tops of the seedlings to jostle them around a bit which will encourage to grow thicker stems and become the sturdy little plants you want them to be. Do this for a minute or so rather than a quick rub on the way past, it’s not a chore, I bet you’ll get benefit just as much as they do!


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3 Responses to How To Grow The Best Tomato Seedlings

  1. Daniel Charbonneau says:

    I am growing seedlings in a cell box with water below. The cotyledon leaves have pushed but the stems are long…some are from 8″ to 10″ longs. Is this normal, and will the true leaves show up later?
    The cotyledons appears ten day after planting.
    I have removed the heating pad and moved the trays toward the sunlight; is this enough?

    Thanks for your help.

    • Andrew says:

      Hi Daniel

      8″ to 10″ is very tall without secondary seed leaves. It sounds like the seedlings got too much heat and not enough light. I don’t expect the plants will do well and would advise re-sowing your tomatoes. Are you growing in a greenhouse or indoors? If indoors, next time remove from the heat pad as soon as the seeds germinate and place in sunlight, preferably a warm south facing windowsill.

      I hope this helps


  2. Tomatoes love heat. Cover the planting area with black or red plastic a couple of weeks before you intend to plant.

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