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A Step-By-Step Guide to Starting Tomato Seeds


Growing tomatoes from seeds

If you want to try your hand at starting seeds at home, tomatoes are a great place to start. Tomato plants at garden centers or greenhouses are limited to a few common varieties at a certain time of the year, and buying live plants adds up quickly.

If you want to start growing one of the 35,000 other varieties of tomatoes, then it’s time to learn how to start your own tomato seeds at home.

Tomatoes are one of the easiest garden vegetables to start at home. Put soil in a plastic container, plant a seed, keep it moist, and 8 weeks later you will have a tomato plant that’s ready to transplant.

One of the hardest things about starting seeds is resisting the urge to buy all the lights, trays, kits, or gadgets in the seed-starting section.

Tip: most are useless.

Tomatoes are almost foolproof if you have some basic gardening knowledge. And even if you’re a newbie, these plants are a great place to start.

Step 1: Choosing Seed

Before you can plant anything, you must first choose what kind of tomatoes you are going to grow. There are thousands of tomato varieties, but most hardware stores or garden centers only carry 4-5 different kinds.

If you want a standard commercial red slicing tomato, hardware store seeds will work fine.

You can also purchase tomato seeds online (link to Amazon).

However, there are many more flavorful and colorful options as you start looking into heirloom varieties. You can get pink, purple, red, orange, green, and even white tomatoes of all different sizes.

As you search for the perfect tomato variety, keep these points in mind:

Provenance

Provenance is a fancy name for “where the seeds came from”. Make sure you are getting your seeds from a reputable dealer or a registered heirloom provider. Some heirloom growers are backyard gardeners who supply larger registries with a rare variety.

This is pretty normal as you start looking at the more unique varieties, but you do want to make sure the supplier has good reviews and a process for reporting any problems.

One of the best places to find rare, heirloom seeds is by becoming a member with Seed Savers. They connect growers from across the globe who preserve rare lines of different fruits and vegetables.

You should also look for where the seeds are grown. Most hardware stores supply seeds that will grow well in your area, but if you start looking into more unique varieties, you want to make sure the seeds are coming from a climate that is not too different from yours.

If your tomato seeds are grown in zone 5, and you live in zone 11, the strain may be adapted to colder temperatures and not as resilient in the heat.

Try to find seeds from strains grown in a similar climate so you know the plants are able to handle your growing conditions.

Indeterminate/Determinate

This is one of the most confusing terms on the back of a tomato seed packet, but it’s also extremely important.

Determinate tomatoes are bush varieties that have a maximum height of 3’-4’.

Indeterminate tomatoes are vining varieties that can grow 20’ or more along the ground.

You can grow determinate tomatoes in cages, or you can tie heavy branches to wooden stakes and keep the plants pruned. These plants take up less space, but they also produce less fruit. Determinate tomatoes are better for shorter growing seasons and bulk harvests.

Indeterminate tomatoes are usually grown on a trellis. If you grow them in cages, they may spill over the top and trail along the ground (which doesn’t hurt anything). Large, sturdy cages will hold indeterminate varieties if you keep them pruned.

These tomato plants have a longer harvest season and are better for warmer climates.

Uses

Why are you growing tomatoes? Figure out what you will use your tomatoes for before you choose the varieties.

Sauce tomatoes are different than slicing tomatoes. Some tomatoes are easy to can, while others are better for salads. Find varieties that work well for your intended uses and then research which ones have the best flavor profile.

Step 2: Determine Growing Space

How much space do you have to grow tomatoes? Do you have one 4’ x 4’ raised bed? Do you have 5 acres? How much space are you willing to devote to tomatoes?

Each plant will need roughly 6sq’ if it is pruned and tied up to prevent overgrowth. Some indeterminate varieties will handle closer spacing if they are tied to a trellis.

If you love eating fresh tomatoes, but have no interest in preserving them, then avoid the urge to start too many seedlings. It’s easy to start 8 tomato plants with the intention of making a few jars of salsa, and then you’re 50 gallons deep in tomatoes in August.

Determine how you want to use your tomatoes before you start them.

If you have plenty of space, calculate your desired yield and work backwards to see how many tomatoes to start. Each determinate variety yields approximately 10lbs of fruit, while each indeterminate variety can yield as much as 20lbs if they are pruned and trellised.

Calculate about 3lbs per quart jar if you are canning. If you are dehydrating tomatoes, calculate 12lbs per quart jar.

If you want to can 12 quart jars of tomato sauce, you need 36lbs of tomatoes, or 2-4 plants. Each plant needs 6sq’, so you need a 12-24sq’ plot.

Step 3: Seed Starting Equipment

You don't need a lot of expensive equipment to get started growing tomatoes from seeds.

I love gadgets and tools that help streamline and improve gardening processes, but when it comes to seed starting, most of the products are completely unnecessary.

Seeds need a growing medium, moisture, light, and ventilation. Most seed-starting products interfere with these basic needs and cause many more problems than they solve.

Coconut Coir

Stay away from seed-starting products using coconut coir. This product has some awesome features for other stages of plant growth, but coconut coir is either always wet or always dry. There is no happy medium. This means your seeds will either mold or dry out, and if they do germinate they will struggle.

Avoid coconut coir pots, pellets, and growing media.

Peat

Peat moss behaves a lot like coconut coir. It is very difficult to regulate moisture, and most seeds either mold or dry out. Biodegradable pots pull moisture out of the soil and into the pot, so while the pot may look wet, the soil can be dry.

The more you water, the more the pot falls apart, and then the seedling suffers.

If you plant a seedling directly in the pot, the walls of the pot will not break down in time to allow the seedling to grow a strong root system before the end of the season.

Plus, if the pot sticks up above the ground, it acts as a wick that pulls moisture out of the soil and evaporates into the air.

These pots and pellets are expensive anyway, so don’t waste your money.

Humidity Domes

These little plastic domes create a miniature greenhouse environment so your seeds stay moist and have a higher germination rate. That’s the claim, anyway.

The problem with humidity domes is that not all seeds germinate at the same time, and if you leave the dome on after the seedlings emerge, they tend to damp off and die.

If you plant an entire tray of tomato seeds, they should all germinate within a few days of each other, and you can remove the dome and move on.

However, if you plant tomatoes, peppers, basil, marigolds, and kale in the same tray, you will have to remove it once the first few seedlings get their true leaves, which could be weeks before the peppers germinate.

This isn’t a huge problem, it just defeats the purpose of having a dome on some of the seeds.

The best humidity domes have a ventilation option to allow for air circulation. This cuts down on mold problems and lets you keep the dome on longer.

If you are starting multiple plants from seed, consider getting small trays with individual domes instead of starting everything in one large flat.

Heat Mats

If you have to choose between a humidity dome and a heat mat, get a heat mat. Domes usually last 1-2 seasons, while a mat can last for years. Seeds germinate much faster with bottom heat (unless they require cooler soil temperatures), and it improves germination rate more than a humidity dome.

If you want to start all your seeds in one tray, separate the seeds that need cool soil temperatures from the seeds that need warm soil temperatures. Put half the tray on the mat and leave the other half off.

This way, you can start all your seeds in one tray and avoid the problems of a dome and multiple seed trays.

Tomatoes thrive with a heat mat during germination. Seeds germinate faster and have a higher germination rate than room-temperature soils.

Growing Media

Compost

There are many different options for seed-starting mixes. Here’s a basic rundown:

Sterile Mixes

Most seed-specific mixes are sterile. Sterile seed-starting mixes have been heated to the point where all soil life, good and bad, has died. This can help prevent seeds and seedlings from molding, but it also means the mix has zero nutrients.

These mixes are usually very lightweight, hold water well, and allow plenty of space for healthy root growth. However, they are only truly necessary for very small seeds. Most seeds germinate just fine in regular potting soil or garden soil.

If you use a sterile mix, you must begin light fertilization a few weeks after the seeds have germinated if the mix does not include a fertilizer.

Potting Soil

Potting soil is not formulated for seed-starting, but many potting soil mixes are fine for starting seeds. The major difference between sterile mixes and potting mixes is that potting mixes are more coarse and may have real soil in them.

If you are starting tomatoes, most potting soils will work just fine. Just stay away from soils that have large chunks of bark.

Garden Soil

High-quality garden soil may work as a seed-starting mix, but most garden soils are too coarse or heavy for starting seeds. Garden soils hold on to lots of water and can be too dense for young roots to form a healthy root ball.

Low-quality garden soils may have large pieces of bark, wood chips, rocks, sand or waste soil. Most cheap garden soils are not suitable for amendments, let alone seed starting.

If you are going to use garden soil as a seed starter mix, buy a good one.

Homemade Seed-Starter Mix

These can be quite cost efficient, especially if you start lots of seeds. There are many options for homemade seed mixes, but try to keep an even balance of water holding materials, porous materials, and compost.

Homemade mixes work well for tomatoes.

Pellets

Expanding pellets and pods are expensive and they don’t work well. Save your money and use a real mix with a tray or cells.

Trays

This is the only truly necessary piece of equipment for starting seeds, other than soil. Trays can either be one large flat or divided into cells.

Large flats are good for starting flowers or large amounts of one type of seed. However, tomatoes don’t do well in flats unless you transplant them into individual containers as soon as they get their first true leaves.

Tomatoes work best in cells with 2 seeds per cell. Place the cell packs on a tray or something to collect water. One 6-pack of tomato plants will fill a 36sq’ space and yield 60-120 lbs. of tomatoes.

Recycled Seed Starter Trays

Pinterest and Instagram accounts would have you believe that toilet paper tubes and egg cartons are just as effective as plastic trays for starting seeds. I have tried these methods multiple times in the past and never had one viable seedling.

This is not to say it can’t work. But it takes a lot more time and attention to get healthy seedlings from recycled papery trays because they break down quickly and lose so much water.

If you treat them like plastic trays and only water them once per day, they quickly dry out and the seedlings die. Plus, there isn’t much room for root development.

Yogurt and other plastic containers are much better options for seed starting containers. Poke a hole in the bottom and fill them with soil. I have used these many times when I just want to start one or two plants and I don’t want to go to the hassle of buying anything.

Grow Lights

If you have a very well-lit area, you can skip the grow lights. But, even the most tempting windowsill generally doesn’t have enough natural light for seedlings to thrive.

Regardless of where you start your seeds, supplemental lighting is almost always necessary. Get a full-spectrum LED light that has an adjustable height (link to gift list) and put it on a timer.

Other light options, like halogen or florescent, can put off too much heat or not enough light for the amount of space your trays will use. LEDs are the most efficient and don’t risk burning your seedlings.

If you have soil or a seed mix, a plastic container, light, and seeds, you have all the equipment you need to get started.

Step 4: Planting the Seeds

Tomato seeds germinate well so long as they are provided with proper moisture and nutrient-rich soil.

Once you have your equipment, it’s time to plant some tomato seeds. Start seeds 8-12 weeks before the last frost of the spring.

  1. Fill each cell loosely with seed-starting mix. Do not pack the soil into the cells or else the roots will have a difficult time growing.
  2. Take 1-2 seeds per cell and plant them ¼” below the surface of the growing media. Cover and tamp down.

    Tomatoes have a very high germination rate, so if you want to conserve seed you can plant one seed in each cell or container and 9 out of 10 should sprout.

    If you want to plant only the strongest, healthiest seedlings, plant 2 seeds per container and pull the weaker one once they both germinate.

    Most tomato seed packets come with 25 seeds, unless you order large volumes or buy bulk seed packs.
  3. Use a spray bottle or plastic water bottle with holes poked in the lid to moisten the soil. Do not pour water from a container or use a high-pressure hose or watering can to moisten the seeds or else it will wash away the soil.

    The first time watering seeds takes the longest. Most seed-starting mixes are very dry out of the bag, but once they are fully moistened they rehydrate easily.

    You may also put the soil into a large bowl or bucket and add water to moisten the soil before you put it in the trays. This makes it easier to thoroughly moisten the soil mixture and it is less likely to disrupt the seeds after you plant. However, it does make a mess. Just make sure the soil is damp and not wet.
  4. Place the seed tray in a well-lit area where the temperature stays above 60 degrees. The top of a fridge or dryer works well as long as you have a grow light. Windowsills can also work as long as they are sealed and do not leak cold air at night.

    If you don’t have a well-lit area, set up a grow light a few inches off the top of the soil. Put the light on a timer to shut off for at least 8 hours each night.
  5. Place tray on heat mat (optional) and put the dome (optional) on top of the tray.

    If you use a dome, remove it twice per day to let the air circulate and prevent mold. Do not water as often as open-air seed trays. The soil inside a dome should never be wet; only damp.

    If you use a heat mat, set the mat on a surface that can handle constant heat for 3-4 weeks. Wood tables or painted surfaces may peel or discolor.
  6. Keep soil moist, but do not overwater. It is always better for soil to be a little too dry rather than a little too wet (source). Use a spray bottle or plastic water bottle with holes poked in the lid to water seeds and seedlings.

When seeds emerge, start watering less frequently but make sure the water runs out the bottom each time.

Step 5: Taking Care of Seedlings

When your seedlings emerge, they will have a set of smooth, oval leaves. These are the seedling leaves. Once the first set of larger lobed leaves appear, your seedling can be moved into a larger container (if necessary) and must have fertilizer if your media was sterile.

Seeds usually germinate in 2-3 weeks. Once the seedlings have come up, you can remove the heat mat, but the plants will grow faster with the extra soil warmth.

If you planted the tomatoes in traditional 6-celled trays, they can remain in the trays until they are 12 weeks old and ready to transplant. Make sure you fertilize them regularly, according to whatever growing media you bought, and that they remain under grow lights until you can harden them off.

Step 6: Hardening Off

Seedlings can be transplanted outside once they have 3-4 sets of true leaves. Seedlings are grown in a protected, controlled environment, so it’s important to gradually expose them to the outdoors to prevent transplant shock.

A few weeks before the last frost, start taking the seedlings outside and set them in the sun for a few hours each day. The plants will begin to acclimate to sunlight, wind, temperature changes, and other natural stressors.

Each day, extend the time they spend outdoors until the nights are warm enough to transplant them into the garden.

Step 7: Transplanting

Transplanting sprouted tomato plants is best done in late spring to early summer.

Once the seedlings are hardened off, it’s time to transplant. The time to transplant varies depending on the climate, but mid-May to early June is generally the time to plant tomatoes outside.

Tomato plants can grow roots all along the stem, so dig a hole almost as deep as the entire tomato plant, and remove all but the top few sets of leaves. Bury the entire plant underground with only the top few inches above the soil.

Each node where you removed a leaf will turn into roots, and the plant will have a strong root system that tolerates drought better during the summer.

If you are planting on or before the last frost date, you can use glass or plastic cloches, or miniature greenhouses, to cover the transplants until the nighttime temperatures are about 55 degrees consistently. These covers also help the tomato grow faster and may speed up harvest dates.

Conclusion

Tomatoes are one of the easiest plants to start from seed, and once you have the equipment it’s much more economical to start your own seeds each year instead of buying plants from a greenhouse. Especially if you want to start growing rare or unique varieties.

Once you harvest your tomatoes, you can harvest and save the seeds (although it’s a messy process) and save them for 5-10 years in a dry, airtight container.

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Sydney Bosque

Sydney has over 15 years of experience in lawn maintenance, landscape design, and organic gardening. She has an A.A.S. in Landscape Design/Organic Produce Production from the Nebraska College of Technical Agriculture.

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