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Starting Tomatoes By Seed (4 Tips for Healthy Seedlings)

Tomatoes may have a reputation to be a little finicky when grown from seed, but with the right equipment and the proper care, tomatoes are one of the easiest vegetables to grow at home. Plus, there’s nothing quite like a juicy, homegrown tomato, and once you’ve tried your first, you won’t go back to store bought!

Healthy tomato seedlings require a quality seed-starting mix, clean seed trays, pots, or peat pellets, a heat mat and thermostat, as well as a grow light when getting an early start. With these supplies, you can ensure a good start for your seedlings.

A step-by-step guide to growing healthy tomato seedlings

Follow these tips to grow the best tomato plants from seed.

  1. Begin with the right supplies

    First things first. Before you start any tomato seeds, ensure you have all the materials you need. To give your tomato seeds the best possible start, you’ll want to gather the following supplies:

  • Seed-starting mix

    Not all potting soils are made the same. Potting soils contain different vitamins and minerals suitable for plants at different stages of growth.

    When you’re starting tomato seeds, you’ll want to use a lightweight and well-draining seed-starting mix. Seed-starting mixes have a lighter consistency than potting soil mixes, so the seeds don’t have to work as hard to break through the soil. Seed-starting mixes are also not as heavy on nutrients as potting soil mixes, since the youngest seedlings don’t need fertilizer—tomato seeds are already fortified with all the nutrients they need.

    As tomato seedlings grow and mature, you can ‘bump up’ the seedlings into bigger containers with heavier potting soil. When it’s time to transplant your tomato seedlings, look for a well-draining potting mix enriched with nutrients. Many garden supply stores carry potting soil mixes specifically made for tomato plants and other fruit-bearing vegetables.

  • Seed trays

    Obviously, you’ll need a container to start your seeds in. Most growers start tomato seeds in 72-cell flats, though you can start tomatoes in whichever containers you like.

    Seed trays are easiest to clean and store, since multiple trays nest together and can be stacked. You can also start tomato seeds in pots if space isn’t an issue for you.

    Or, you can invest in biodegradable Jiffy Peat Pellets—a great plastic-free option for the eco-conscious gardener. Since peat pellets can be directly planted in the ground, using these biodegradable ‘pots’ minimizes the risk of transplant shock for your delicate tomato seedlings.

  • Heat mat and thermostat

    Tomatoes are heat-loving plants and most varieties require temperatures between 65℉ and 85℉ to germinate properly, according to a study published by the University of California. Unless your house stays in this range, you’ll need a heat mat and thermostat to keep your tomato seeds warm.

    Many of our seed-starting kits at Seeds ‘N Such come with a heat mat, and you can purchase a thermostat separately.

    Heat mats are indispensable for successful tomato seed germination. Adding a thermostat allows you to control the temperature, which is difficult to achieve naturally in midwinter, when temperatures often fluctuate drastically.

    Though tomato seeds do need heat to germinate, it’s equally important to make sure that the seedlings don’t get too hot. Tomatoes don’t like temperatures above 80°F. Once your tomato seedlings begin popping up through the soil, you can usually remove the seed tray from the heat mat.

  • Grow light

    Though heat and moisture are all that’s required for tomato seeds to germinate, once the seedlings sprout they’ll be hungry for light. Most seedlings need between 14 and 16 hours of light each day to grow healthy roots and eventually produce tasty fruit.

    While you may be tempted to set your tomato seedlings near a sunny west-facing window, that often isn’t enough light—especially in late winter when the days are significantly shorter than in the summer. Tomato seedlings will stretch towards an indirect light source, resulting in spindly stems.

    Tomato seeds in a greenhouse or even a sunroom may get enough direct light already, but it never hurts to add a supplemental light source.

    If you’re starting tomato seeds on your kitchen table you’ll definitely need a grow light once the seedlings pop up. You’ll get the best results by using a LED full-spectrum bulb, like this one. Plus, this light is made to grow with your plants—as the plants grow, you can raise the bulb so that the light is always about three inches from the tops of your tomato seedlings.

  • Humidity dome

    Humidity domes are nothing more than clear plastic lids, but they work wonders in the seed-starting room. Humidity domes work in tandem with a heat mat to raise the soil temperature to an ideal range for tomato seeds to germinate. The simple lids also trap moisture in the seed tray, keeping the soil moist, so you have to water less. Once your tomato seeds sprout, you’ll want to vent the lids periodically to allow excess moisture to dissipate. Make sure you have an extra-tall humidity dome on hand for when your tomato seeds outgrow the smaller lids.

  • Optional: Dial Seed Sower

    Tomato seeds aren’t the smallest seeds, but they’re definitely not as large as other vegetable seeds like cucumbers or beans. We love to use the Dial Seed Sower for sowing tiny seeds like tomatoes, since you can load the seed sower with an entire packet or more, and gently tap out only as many seeds as you need for each cell or peat pellet. Sowing multiple trays of tomato seeds is a breeze with the Dial Seed Sower. You’ll be impressed at how quickly and efficiently you can seed multiple trays of tomato seeds.

    Be sure to read this previous post for more seed-sowing hacks.

  • Optional: Growing Tomatoes From Seed Guide

    If you’re one of those people who prefers to read real books over virtual, you’ll appreciate Growing Tomatoes From Seed, a 36-page guide all about growing tomatoes at home. You’ll learn about tomato seed sowing, transplanting, grafting, harvesting, and more! All in one easy-to-read guide.

  1. Use quality tomato seeds

    Not all tomato seeds are created equally. At Seeds ‘N Such, we make sure that all of our tomato seeds are of the highest quality on the market, and are always non-GMO. And just because you’re a beginner gardener doesn’t mean that you can’t experiment with unique hybrid and heirloom varieties!

    We specialize in some of the most stunning heirloom tomatoes, including Red Brandywine, Cherokee Purple, and Pineapple. Our hybrid varieties are just as stunning in terms of flavor and disease resistance, but even easier to grow. We love varieties like the All-America Selections winners Midnight Snack and Pink Delicious Hybrid VFFTSt. Ready to explore the many tomato varieties we have to offer? Check out our selection of over 500 tomato varieties here.

  1. Start tomato seeds at the right time

    Tomatoes take a little more time than some other vegetables to grow from seed, and it’s not generally recommended to direct sow tomato seeds. Most tomato varieties take between 60 and 100 days to mature, so you’ll want to get a jumpstart on the growing season by starting tomato seeds indoors. Start tomato seeds too soon and you’ll end up with leggy plants—but start tomato seeds too late and you won’t get as prolific a harvest as you would if you’d started your seeds at the right time.

    Tomatoes need to be started indoors about two months before the last frost of the year. This allows time for the seedlings to grow big enough to survive the fluctuating temperatures of early summer. Tomatoes are not frost tolerant, so make sure that your last frost has passed before transplanting tomatoes outside, and don’t skip the crucial step of hardening off. Depending on your local climate, this could be anywhere from March through May.

    Most varieties of tomatoes can be started between six and four weeks before the last spring frost. Go here to find the magical date for your area.

  1. Ongoing care of tomato seedlings

    Getting your tomato seeds to sprout is only half the battle! Pay attention to watering, fertilizing, hardening off, and transplanting to ensure your tomato seedlings blossom into healthy, mature plants.

  • Consistent watering

    Tomato seedlings are vulnerable to damping off, which is why it’s important to not over-water your seed trays. Check your tomato seedlings at least once a day, during the hottest part of the day, by lifting the bottom of the tray to check for moisture. Let the surface soil dry out between waterings to discourage mold and the growth of fungal diseases.

    Keep the soil moist, but not waterlogged. For the best results, water from the base of the seed trays or pots by nesting the seed trays in a bottom tray that holds water. Be sure to drain the bottom tray after 30 minutes.

    If you do choose to overhead water tomato seedlings, water lightly and evenly by holding the watering wand a foot or so above the plants. To avoid excess moisture, invest in an oscillating fan—it will improve air circulation and protect against damping off.

  • Fertilize seedlings

    Once tomato seedlings have been transplanted into a bigger pot, it’s important to monitor the fertility of the soil. Tomatoes are nutrient-hungry plants and need fertilizer to grow, flower, and produce fruit.

    When your tomato seedlings have reached between four and six inches tall, you can begin fertilizing them with the right balance of nutrients. Use a balanced liquid fertilizer, like a 10-10-10 NPK mixture, and dilute it to half the strength recommended on the label.

    Start with a mild solution of water and fertilizer to give your tomato seedlings a nutrient boost. Feed your tomato seedlings with this diluted fertilizer solution once a week for the duration of their time indoors.

    It’s best to use a fertilizer specifically made for tomatoes since they contain all the nutrients tomatoes need in the right concentrations. If you’re not sure what fertilizer to use, ask your local garden store for advice.

  • Hardening off

    Once your seedlings are big and strong, it’s time to transplant them outside. This process is known as “hardening off” and it’s essential for healthy tomatoes. Hardening off involves gradually exposing the plants to cold temperatures and wind, so the roots are better prepared for the outdoors. Start with 30 minutes of exposure and increase the time by 30 minutes, until your tomatoes can be transplanted.

  • Transplanting

    As tomato seedlings grow, they will inevitably become rootbound. Once your seedlings have four or five true leaves, they’re ready to be transplanted outdoors. Choose a cloudy day to help the seedlings transition—bright sunlight can actually be detrimental to newly translated seedlings.

    Choose a well-draining site that receives full sunlight—at least eight hours per day. Bear in mind that most tomato varieties grow between three and six feet tall, so staking is essential. Plant tomato seedlings about two feet apart to allow room for mature growth.

    When transplanting, make sure the roots are barely disturbed and the soil ball is intact. Unlike other plants, you can bury tomato seedlings much deeper than they were growing in the seed tray—in fact, the deeper the better, because the tomato stems will just produce more roots. Just make sure that the leaves are all above ground.

    Water tomato seedlings before planting and give your seedlings consistent moisture after transplanting, about an inch a week.

Conclusion

With these tips, you’ll be on your way to growing your own tomatoes from seed in no time. Enjoy the fruits of your labor–there’s nothing quite like juicy homegrown tomatoes!

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