Starting Tomato Seedlings

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There’s no one universal way to start tomato seeds but here is how I do it.

I use Park Seeds Bio-Dome, a heat mat, and lights.
(I usually use the 40 call planting block for tomatoes, but you could use a 60 cell if you wanted to – the jumbo size is really not needed, since you will be transplanting while the seedlings are young and tender.)

Keep the reward in mind!

Keep the reward in mind!

If you do not use the Bio-Dome, with it’s planting sponges, it is recommended that you use an artificial soilless mix for the initial seeding. Regular potting soil has lots of fungi in it and you increase the chances of your seedlings developing damping off which is characterized by the young seedlings developing a narrowed stem at the soil level, eventually falling down and dying. Some people recommend using Chamomile tea to deter damping off, I don’t know if that works or not.

Whether you use the bio dome sponges or a soilless mix, you MUST transplant the seedlings at least once in order to get a decent plant.

I start out by soaking my bio-sponges, then squeezing out the surplus water, so that they are nice and moist to start with.

Now I plant my seeds. I often plant only one seed per sponge. You might want to plant 2, but then you have to thin to one once it germinates. You don’t want two seedlings in the same sponge.

When you are done sowing, put the lid on the Bio-Dome, but leave it open to vent excess moisture to start out. You want to keep conditions moist, but at the same time allow for air circulation so as to not risk damping off. I often prop up one side of the Dome the first couple of days, until the Dome stops being steamy, otherwise you risk literally cooking your seedlings to death.

Once I am done, I set my whole Bio-Dome set up on a heat mat. Do not place under lights until the seedlings have emerged, or you’ll cook your seedlings. Tomatoes do not NEED bottom warmth for germination, as do most peppers, but they will germinate faster that on a heat mat. When you see the first seedlings emerge, immediately I remove the Dome, and set the tray under lights.

The lights must be kept about two inches above the growing seedlings. I think just one inch is even better. It means you’ll have to move the light fixtures often during early growth. It’s hard to grow good seedlings without strong light. If you use a windowsill be sure there are no drafts because cold drafts and wet mix spell doom and death to the seedlings. And you must also remember to turn the container each day so light reaches all sides of the plants. I much prefer growing under lights.

Mature Soldacki Tomato - YUM!

Mature Soldacki Tomato - YUM!

If you planted more than one seed per sponge thin out the young ones – you can do it just by pinching them or use a small pair of scissors. (don’t pull them out, you risk pulling the next one out with the one you intended to remove)

When using the Bio Dome, just add water from the bottom as needed.
With the lights, use a timer. Leave the lights on for 14-16 hours per day and NEVER at night. The plants need a dark period for proper metabolism.

The first little green things that emerge are NOT leaves, they are called cotyledons. They are followed by the first set of leaves and then the second set of leaves at which point you MUST transplant the seedlings to another container as described below.

Occasionally the seed coat doesn’t come off one of the germinated seeds and if you don’t remove it the plant will die. Moisten a cotton ball and hold it to the seed coat for a few minutes. Then gently grasp the seed coat with your fingers and it should slip off. If it doesn’t, you just lost a plant. Because these things do happen, it is always a good idea to plant a few more seeds than you need – that way you won’t be too grief stricken at the loss of one of your babies.  If you end up with extras, you will probably have some gardening friends who are happy to get a healthy seedling just in time for planting season.

By the way, once you start raising seedlings, you will realize why so many gardeners refer to their seedlings as “their babies”. When you raise them from seed, you really do form a bond with your plants.

I do not add any fertilizer at all up to this point. When I transplant for the first time, I use an organic potting mix and add organic fertilizer into the mix. I use red plastic cups from Costco. (tomatoes like red, and the cups from Costco I find to be a great size). Some people say no fertilizer at all, I find my seedlings do better with an organic fertilizer – it is mild and won’t burn the seedlings.

When you transplant, you bury the tomato seedling to leave only the top part sticking out over the soil. Why is it so important to transplant? Because it shocks the plant and retards foliage growth so that the plant will focus it’s energies towards root development instead.

If you don’t transplant you end up with huge leggy seedlings that flop over. Also, by burying the plant when you transplant, so just the very top of it sticks out over the soil surface, you stimulate root growth along the buried stem.

Newly transplanted from Bio-Sponges

Newly transplanted from Bio-Sponges

Make sure the seedling mix you are going to use is moist. Make a hole to be ready to receive your seedling, then push the bio-sponge out from the bottom and sink the seedling all the way down so that only the little leaves are above the soil line.
Again, I can’t stress enough how important this is. Tomatoes form roots wherever the stems make contact with soil so you want to sink those plants way down. When you finally transplant out into the garden you should do the same thing to make a strong root system.

Now water in the newly transplanted seedlings.

Put your transplants back under the lights keeping the lights no more than two inches, or so, from the leaves – 1 inch is even better. Put your transplants back on the windowsill if not using lights and keep rotating the containers each day so they get even light.

Tomato plants develop best when grown at cool temps. Commercial growers will
usually have one greenhouse set at about 55-60 F degrees. If you can duplicate that you’re going to get a better plant. Warmth is needed for germination and early seedling growth but once you transplant you want cool conditions for optimum plant development. If you can’t, you can’t. Don’t worry too much about it. I keep mine in the garage after the seedlings have emerged.

You keep growing your plants until they get to be maybe about 10 – 12 inches high and it should be close to when you want to transplant them outside.If you want to run a fan near your growing plants that’s fine also; good for air circulation. And if you want to run your hands or a ruler over the foliage a couple times a day that’s fine too. The plants respond well to touch, that is sometimes reflected in even better growth.

Before planting out, you must harden off the seedlings. That means putting them outside for a few hours each day , initially in a shaded spot, and then increasing exposure to the sun as the days pass. Protect from harsh winds and bring the plants inside if the weather gets too cool. The seedlings must be hardened. During this period, don’t water the seedlings too much either. Don’t water them until they are on the verge of wilting.

If your plants start forming blossoms prior to transplanting out in the garden,  remove every single blossom on the plant. The earliest growth of a tomato plant must be devoted to vegetative grwoth of leaves, stems and roots, not a sexual cycle of reproduction and setting fruit, etc. So get all those blossoms off the plants. Blossoms that develop once the plants are out in the garden are fine to leave on the plant.

Tomato Seedlings ready for the garden

Tomato Seedlings ready for the garden

Choose a spot for your tomato plants that get the maximum amount of sun, and where AM sun will burn off the early morning dew, this is important for disease prevention.

Transplant out to your garden when the risk of frost has passed. Remove any leaves of the plant that have turned yellow. It’s natural that the leaves towards the bottom of the plant would have turned yellow and most of the time those yellow leaves will fall off naturally. If not, you take them off. Now dig a horizontal hole, long enough to hold all but the tip of the plant. Lay the tomato plant down sideways in the hole, and allow just the very tip of the plant to peek out above the soil. Laying them sideways allows you to bury the whole stem while still keeping at a warmer layer of soil.  Don’t be discouraged at how very tiny your seedlings look right after transplant, it’s supposed to be that way, and in no time at all they will grow up to be strong and sturdy plants.

After transplanting to the garden water them in well.

At transplant time, I again add organic fertilizer, right into the planting hole. (be aware that I garden organically, if you use chemical fertilizers, don’t follow my advice on fertilizing, you will probably end up burning the roots.)

Just transplanted to garden

Just transplanted to garden

Note in the photo above, that very little of the seedlings are above ground, compared to the photo of the seedlings above, just prior to transplanting into the garden.

Thanks to Dr. Carolyn Male for providing the guidance for me as I learned to start my tomato seedlings – though I have modified her method, especially as regards use of fertilizer, and she would NOT approve the above. Nevertheless, her instructions were invaluable to me as I got started with growing tomatoes from seed, and she should get credit for teaching many new gardeners the joy of raising tomatoes from seed. The method described here works for me. Probably every person raises their seedlings a little differently – you have to find out what works for you.

Seeds should be started 6-8 weeks before the last average frost date. I tend to start mine about 2 months before the last frost. Perhaps these instructions can serve as a guide for you to get started by.

Grown up!

Grown up!

The Reward!

The Reward!

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  1. Hi, i have a very large garden(about 1200 sq. ft.) and was very succussful last season. I use the Parks BioDome and this season things are not going well at all. Im losing so many seedlings.
    the tomatoes start off nicely but they start to die off. I do remove the dome when the seedling emerge but thats when they start to die. if i put the dome back the seem to perk back up. Some of my other veg just never recover. Is the dome keeping in too much moisture? Do i need more heat? I dont have a heat lamp but the window sill they are in gets planty of sun. When transplanting them what do you recomend? Are peat pots a good container? I see that you are using plastic cups. Do you put any holes in the cups for drainage? Im so frustrated. I have a green thumb but right now i feel like i have 2 left feet. Please drop me a line. Thanks, Joe

  2. HOw do I find or read the answer to the above problem and inquiry? Thank you for your assistance.

    Richard Hill

  3. Hi Joe & Richard,

    Do you have photos of your failing plants by any chance? It sounds to me like they suffer damping off, and there can be many causes of that. Did you sterilize the planting block with bleach before reusing it after last season?

    What environment are the seedlings in? Temperature etc?

    It’s strange that the tomato plants perk up when put back in the dome. Other than being covered, how is the environment different than where you have them when you uncover the dome? Are they warmer? More light?

    I do find a heat mat is of great help while germinating, but I generally take them off the heat mat once they have germinated. Peppers would benefit from staying on a heat mat even after germination, but that may not always be possible. I don’t have two heat-mats, so my peppers are relegated to being under lights but without heat once the dome cover is off.

    A lamp (does not have to be a heat lamp) helps a great deal – the tomatoes will do much better under light 16 hours a day, than they do in a window. I use a florescent grow light, with fixture it cost me just around $17 at BiMart.

    When I transplant I use MiracleGro Organic Potting Mix, with EB Stone organic fertilizer 5-5-5. As for the red cups, I got those from Costco, and yes I do cut holes for drainage.

    Sorry I can’t be of more help. Let me know if you can provide a few more details, and maybe I can be of more help? I know how frustrating it is when seed starting goes wrong. The fall onions I started last summer all failed to thrive, and I usually have great success with onions. I ended up having to buy onion starts instead, much to my chagrin. Fortunately, my spring planting went well!

    What part of the country are you in, by the way?


  4. Colin Lane says:

    Hi Boel great to see your vegie gardening i just home grown veggies & fruit as the products one gets from the super market are just chemical (one might just as well eat plastic ) I guess I am lucky as where i live in the northern suburbs of Adelaide of South Australia we don’t have frosts, with mild winters and very hot dry summers temperatures of 100ºF are usual plus i live in a aged pensioners home unit with a small back yard in which I planted drought proof native plants but i grow my vegies in pots so i can keep the water up. We often go 4-6 months with out rain & have water restrictions so I use my shower water on my plants. It works well keeps pests off too. Regards Colin.

  5. Hi Joe,
    I recently started a terrarium in my closet, in hopes that i could start all of my wonderful vegetables early, and then transplant them to the garden… I was very happy to see that my 24 hour light system worked great, and my veggies were sprouting and growing nicely.. But then came the shocker.. As soon as I took the plants out of the fluorescent light, and put them in normal sunlight, they all died overnight… After a little research, I decided that the best thing to start indoors are tomato plants.. plant them in your artificial light, and let them get a really good height and look to them.. It is important that you don’t try to transplant weakly, less sturdy plants.. Once your plants have gotten to 3 or 4 inches, place them in another container, (its okay to bury the first few leaves in soil) and move them to a dark location, one that gets a very small amount of sunlight, but not a pitch black area either.. you want a warm environment for your plants over the next few days, water them, and keep the soil moist, so that they can root and keep growing… after about two or three days, move your plants to a window sill, or another area that gets light.. Be sure not to put them somewhere that gets too much light, or move them if the sun gets too intense.. You want to gradually introduce them to sunlight.. Then, when you are ready to put them out into the garden, and all chances of frost have past, move your potted tomato plants outside under a shelter for a week, so that they really get used to our wonderful sun. After completing all of these steps, plant your tomato plants into your garden, water them, care for them, and watch them in turn care for you.

  6. Hi Jacob,

    My experience sure does differ from yours!

    I have raised seedlings under lights in the garage for years – and they do great. The only gradual hardening I do is to bring the plants out a few hours during the day at first, then longer and longer, until they are out all day and just back in at night.

    I always bring the tomato seedlings straight from under the lights to full sun however – and I have never had any problems with that, in fact they really start thriving when they get exposed to the full sunlight — as long as the transition is gradual.


  7. Last year we planted a few different kinds of tomatoes in urns on a back porch due of the poor soil quality in our neighborhood. When doing this you need to be sure the pots are big enough for respective plants potentiall root growth and you will probably need to water them almost double as much as the dirt tends to dry out. Also right before they turn red either cover them or harvest them as birds will destroy your plants as they cant stay away from the bright colors.

  8. Yes, having a large enough container is very important. I also find that determinate tomatoes make better container plants – or small cherry tomatoes, because they are not subject to blossom end rot, which tends to plague container grown full size tomatoes.

    I have never had the problem with birds eating my tomatoes – but that’s probably because there is so many other things for birds to eat on my property. I personally can’t imagine harvesting before they turn red (or yellow or orange depending on the variety) – vine ripened tomatoes are to me the whole point of home-grown tomatoes.

  9. Malisa Rothbart says:

    Excellent info over again.! I am looking forward for more updates;)

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