Starting Tomatoes from Seed: The Secrets to Growing Great Tomato Seedlings

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Obviously, no garden is complete without homegrown tomatoes. Since they’re much-loved by many, you’ll find more varieties of tomatoes to choose from than just about anything else, when you’re plant shopping at nurseries and garden centers in the spring.  For that reason, a lot of gardeners choose to buy their tomato starts rather than fool with starting seeds.

Sadly, I’ve noticed a serious decline in the quality of tomato seedlings available in my area since the Covid-19 pandemic started, probably because of increased demand. Each year, a lot of the transplants for sale are already showing symptoms of Septoria leaf spot and early blight, diseases that spread rapidly in crowded greenhouses. These diseases will really slash your harvest if your plants catch them early in the season, so that’s definitely NOT what you want.

If you’re also having trouble finding healthy tomato seedlings where you live, or if you’d just like to experiment with growing some rare varieties, starting from seed is the way to go. This way, you have control over both the varieties and the condition of your seedlings.

I’ve had people tell me tomatoes are tricky to start indoors from seed, and that their tomato seedlings always tend to struggle. Tomato seedlings are picky about a few things, for sure. But once you know what they need, you can easily grow your own healthy tomato transplants at home.

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Choosing Which Tomato Varieties to Grow

You’d never know it from looking at grocery store options, but there are approximately ten thousand different varieties of tomatoes. I’m serious, you guys.

When it comes to tomatoes, picking the ideal varieties for your climate is paramount for getting a big harvest. There’s totally nothing wrong with growing a few wacky, novelty heirloom varieties just for fun. But if you want a real bumper crop, I suggest you also plant some varieties known to contend well with the weather conditions they’ll have to deal with in your garden. Tomato insurance, if you will.

Many hybrid tomatoes are specifically bred to resist certain common tomato diseases. If you’re new to growing tomatoes, you’ll get a good idea of the pests, diseases, and weather problems you’ll be up against after you’ve grown your first few tomato plants. You’ll also learn which kinds you enjoy the most, the more varieties you experiment with.

For starters, here are some basics to look into when choosing seeds:

Weather Tolerance– In general, tomato plants like warm (not blistering hot) weather and lots of sun. If you have hot summers or get lots of rain during the growing season, it’s a good idea to go with varieties that are known stand up to those conditions.

Determinate vs. Indeterminate- Tomatoes are divided into two groups based on their growth habits. You’ll want to look at this when choosing tomato varieties, since their care requirements are quite a bit different.

Indeterminate tomatoes produce rambunctious vines that need to be supported, trained, and pruned throughout the growing season. Determinate tomatoes grow into more compact bushes that set and ripen all their fruit within a few weeks or so. If you don’t have a ton of time on your hands, determinate tomatoes might be just for you, since they don’t require as much wrangling.

Read this post to learn more about finding the perfect tomato varieties to plant: Variety Matters: Choosing the Right Tomatoes for Your Garden.

Timing it Right: When to Start Tomato Seeds

Since tomatoes are a warm weather crop, they won’t survive any amount of frost. At the least, that means you’ll need to time your start date so that your transplants are ready to go out once the threat of frost is over.

While you can technically plant your tomatoes outside anytime after that last frost, they really don’t like cold weather. If nightly lows are still dipping into the 40s, tomato transplants will sit there and do absolutely nothing. They won’t grow, and they’ll probably start to look a little blue (literally). Plants dealing with temperature stress and transplant shock may also become a target of pests.  

For those reasons, it’s preferable to wait until nights are averaging warmer (50 F or higher) to transplant tomato seedlings. In many climates, that’s several weeks or so after the last spring frost. Tomato seedlings take about six to eight weeks to reach a good transplanting size, give or take a little. To hit that target planting time, sow your seeds about four to six weeks before your average last frost date.

Don’t be tempted to start too soon! One of the most common newbie mistakes with growing tomatoes from seed is starting them much too early, in areas where spring frosts and cold nights are a concern. This leads to a weeks-long headache of trying to keep your oversized seedlings happy until it’s warm enough to transplant them.

Once they get some momentum, tomato seedlings grow rapidly. They’ll outgrow their pots overnight, take over the whole room, and suck up a ton of water. Tomatoes are hungry, heavy-feeding plants when they start to get big, and their leaves will turn scary colors no matter how much fertilizer you offer them. You may start to worry your hangry tomato seedlings will eat the family cat before it’s over with.   

Unless you have a very short growing season, lean towards starting a little late over starting seeds too early. Tomato seedlings transplant well at a small size as long as the weather is warm enough. This will give you better results than rootbound, oversized plants.

When is it too late to start tomato seeds?

Depending on the variety and your weather, tomatoes take anywhere from 60 to 100 days to set and ripen the first fruits. Many varieties fall somewhere in the 70 to 90 day range.

To calculate the latest date you can start your seeds, find the days to harvest for each variety you plan to grow. This info will be somewhere on the back of the seed packet. Then, add 30 days or so to that number so you’ll have a nice window for harvesting. You’ll need to get your seedlings transplanted with at least that many frost-free days left in the season.

Here’s a handy calculator to find your average frost dates by zip code, if you’re not sure.

If you live in a warmer zone where frost isn’t a concern, you may need to plan around intense summer heat instead of cold. Tomatoes don’t produce well with daily temperatures well over 90. For best results, plan to get your transplants in the ground with enough time left for them to start ripening fruit before major heat sets in.

Soil and Containers for Starting Tomato Seeds

Soil- For tomatoes, it’s especially important you don’t use garden soil, homemade compost, or regular potting mix for sowing seeds, since they’re highly vulnerable to soil borne disease as young seedlings. It’s best to use a soilless medium specifically meant for seed starting.

Containers- Sowing tomato seeds in plug trays or small cell packs can help you avoid a few potential issues with your seedlings, like cold soil and waterlogging. Since tomato seedlings start out tiny and grow fairly slowly for the first few weeks, large pots can hold water for too long and potentially drown your seedlings’ roots, especially if you accidentally water them a bit too much. Smaller containers are also much easier to keep at the proper temperature with a seedling heat mat, if you’re using one.

For more about choosing the best seed starting mix and seedling containers, read this post: No-Fuss Supply List for Indoor Seed Starting.

Germinating Tomato Seeds

Sow tomato seeds around ¼ inch deep in pre-dampened seed starting mix, making sure the seeds are completely covered. After sowing, gently press the soil surface to close any air pockets. If you want, you can sow two or three seeds per cell or plug and thin down to the strongest seedling later. It’s not really necessary, though, since tomato seeds rarely fail to sprout.

Tomato seeds germinate quickly as long as the soil is warm and moist enough. You’ll usually see sprouts within 7 days or less. While you’re waiting for your tomato seedlings to pop up, keep the soil evenly moist on top with a fine mist spray bottle. Be sure to check on them at least once or twice a day, since allowing the soil to dry out completely might kill the germinating seedlings.

Do you need a heat mat to germinate tomato seeds?

Bottom heat isn’t a requirement, as long as the room where you’re starting seeds stays at 65 F or warmer. However, using a seedling heat mat to keep the temperature consistently at or above 70 F will give you faster and more even sprouting. I use this seedling heat mat and soil probe thermostat to keep the soil temperature at 75 while waiting for seeds to germinate.

Keep in mind that seedling heat mats are bad to dry the soil out from the bottom up. Watch the soil moisture and set the seedling containers in a pan of shallow water for a minute to rehydrate them as needed. When the seedlings sprout, either remove the heat mat or turn it down to the 70 F range using a soil probe thermostat. Too-warm soil can cause your seedlings to get leggy, once they’re growing.  

Placing your seedling trays in a high place such as on top of your fridge is another way to boost the soil temperature for germination. Just make sure to get them underneath grow lights as soon as they emerge, since tomatoes will quickly get tall and leggy without enough light.

Watering Tomato Seedlings

Like most seedlings, tomato seedlings prefer evenly moist soil that isn’t overly saturated or frequently allowed to dry out. Let the soil dry a bit between waterings, but don’t let it get too dry, to the point it becomes hydrophobic (meaning water will not sink in) or begins to pull away from the edges of the pots. Once you begin feeding your tomato seedlings, they’ll have trouble absorbing nutrients if the soil isn’t damp enough.

As tomato seedlings get a little older, they’ll start to grow faster, and they get thirsty! Once they have two or three sets of leaves, they really start to hog the water. I find I need to check on them a little more often than other seedlings.

Tip: Don’t water tomato seeds or young tomato seedlings with ice cold tap water, as this can slow germination and growth. Room temperature water is best. I like to keep a couple designated “seedling water” jugs sitting out so it’ll be just right. 

Read this post for more about the best practices for watering seedlings: How to Properly Water Your Indoor Seedlings.

Lighting: Grow Lights are a Must!

While you might can squeeze by with growing certain herbs and flowers in a brightly lit window, tomato seedlings won’t develop well with windowsill light. You’ll want to make sure they’re under a grow light that stays on 14-16 hours per day from the time they sprout.

Also, make sure the grow light is close enough, especially if you’re using a lower-cost LED light. If your tomato sprouts have grown over an inch tall and aren’t showing any leaf development, that’s a sign you need to move the light down a little closer.  

The Importance of Light and Fertilizer for Tomato Seedlings

Lots of folks report a similar problem when attempting to start tomatoes indoors. Their seedlings always grow to a certain size and then sort of…stall out. The leaves turn purple, yellow, or brown at the tips, and they suffer from leaf drop and stunted, slow growth. Meanwhile, all the other seedlings look fine. What gives?

As a new gardener, I was actually advised by several people not to bother starting tomatoes indoors, when I asked around about how to help my sickly seedlings. I was told you can’t raise healthy tomato seedlings without a greenhouse.

Since then, I’ve found there are two easy steps you can take to avoid the tomato seedling slump. Tomatoes are energetic, high-strung plants, and that means they have a bigger demand for light and nutrients. The keys to keeping your seedlings green and healthy are early feeding and early sun exposure.

Fertilize your tomato seedlings early.

Tomato seedlings tend to drop their cotyledons, or seedling leaves, a lot sooner than other seedlings do. The cotyledons supply nutrients for sprouting seedlings, and tomatoes use up this nutrients in a hurry. It’s largely normal for plants to shed their seedling leaves once true leaves develop, but yellowing or dropping cotyledons in tomato seedlings means it’s time to fertilize, stat. Since many seed-starting mixes don’t contain any nutrients, heavy-feeding seedlings like tomatoes will start to suffer at this point, if you don’t fill in the gap.

In my experience, fertilizing your tomato seedlings before the cotyledons wither and drop results in healthier, more vigorous plants. While most of my seedlings don’t get fertilizer until they have at least one or two fully developed sets of true leaves, I feed tomatoes as soon as those first little leaves start filling out.

What kind of fertilizer should you use for tomato seedlings?

Some plant or seaweed-based liquid fertilizers typically recommended for seedlings have very low NPK (Nitrogen, Phosphorous, Potassium) values of 1-1-1 or less. These “gentle” fertilizers may be enough for certain kinds of plants, but they won’t phase hungry tomato seedlings. Tomato seedlings want the plant equivalent of a steak dinner.

To keep tomato seedlings green and happy, you ideally need a balanced, water-soluble fertilizer with NPK values in at least the 4 to 5 range. Nitrogen and phosphorous are especially important, since young tomato seedlings need plenty of both to develop healthy leaves and roots.

It’s difficult to find organic water-soluble fertilizers within this NPK range. If you do like to keep things all organic, liquid fish emulsion is a decent (but very stinky) option. I prefer to use Fox Farm’s Grow Big for my tomato seedlings. It’s not 100% organic, but it’s high-quality, gentle, and specially formulated for young plants.

How and when should you fertilize tomato seedlings?

Tomato seedlings do great with weekly fertilizing. For tiny tomato seedlings that are just starting to develop true leaves, dilute the fertilizer to a quarter (1/4) strength the recommended amount. Once the seedlings are bigger and have two or three sets of true leaves, you can bump it up to half strength. As much as they like their nutrients, young tomato seedlings are still vulnerable to fertilizer burn if the concentration is too strong.

For more on the best practices for fertilizing seedlings, plus the reasons why I’m not against using synthesized fertilizer for seedlings I’m raising indoors, see this post: How to Fertilize Seedlings (and Why You Should Be).

Give your tomato seedlings real sunshine ASAP.

Throwback to the first time I tried starting tomatoes indoors. Once my seedlings were almost a month old, they’d started to look worse and worse. A dose of fertilizer didn’t seem to turn them around like I’d hoped. They were turning pale and yellow, their leaves were falling off, and they’d stopped growing. I finally just stuck the whole tray out on the deck to free up space inside so I could start over.

We were having an early spring warm spell at the time, and the spot where I put them happened to have some nicely filtered sun. Three days later, I went out intending to dispose of dead plants. And instead…they were greener, growing, and looked a lot happier.

Turns out, fertilizer isn’t quite enough to support tomato seedlings’ high energy demands. While most seedlings can easily get by on artificial grow lights until they can be planted outside, tomatoes need more light early on. Now, I always introduce my tomato seedlings to real sun ASAP once they’ve developed their first set of true leaves. This is the best way to give them the light they need, if you don’t have a heated greenhouse to raise them in. Do this, and you’ll notice a huge difference in the health and color of your seedlings.

When can you start taking tomato seedlings outside?

You can set your tomato seedlings outside to sunbathe on days when the temperature is around 60 F or higher. When you first take your tomato seedlings out in the sunlight, introduce them gradually to avoid sunburn. Start by setting them out for an hour or so in filtered shade, then add a little time each day as you slowly move them into brighter light.  

Tip: Placing your tomato seedling trays in a clear tote while outside will hold in some warmth, and it’ll shield them from any blustery wind that might stress them out before they’re fully hardened off.

Potting Up Tomato Seedlings

Tomato seedlings sown in small cells or plugs will need to be potted up at least once before transplanting outside, usually when they have a couple sets of true leaves. Don’t wait until they get too cramped. Lots of roots poking out of the drain holes or visible roots at the soil surface means it’s high time for more space.

Since tomato seedlings can grow strong roots from any point on their stems, it’s good practice to bury them deep (up to the seedling leaves) when you re-pot. This will help them develop a larger, stronger root system.

How big should the new pots be?

Container size for up-potting depends on how much longer you’ll need to keep your tomato seedlings in pots. If you suspect you started too early and you’re looking at another month or more before you can plant your seedlings outside, consider moving them into good-sized containers like 5-inch pots. That way, you can possibly avoid having to do the whole process over again.

I really like these tall 5-inch pots for up-potting tomato seedlings. Their shape makes it easy to plant deep and provides plenty of space for root development.

What kind of soil is best for potting up tomato seedlings?

At this point in the game, it’s safe to mix some compost or regular potting soil into your seed starting mix for re-potting tomatoes. This will give them an extra nutrient boost and help with water retention when they start to demand more water. I like to blend seed starting mix and all-natural potting mix at a 50/50 ratio for potting up tomato seedlings.

Hardening Off Tomato Seedlings

Like any other indoor-raised seedlings, tomatoes need to be properly hardened off before moving outside to their permanent container or the garden. Despite being vigorous little monsters, tomato seedlings are quite sensitive to environmental stress and sudden changes. So, don’t rush the process!

Even if you’ve already started placing your seedlings out in the sun on warm days, you’ll need to acclimate them in small increments to conditions they haven’t experienced before, like gusty winds and cooler evening temperatures.

Some folks assume it’s safe to leave seedlings out on the deck overnight as long as no frost is expected, but that’s not the case! Young tomato seedlings that are used to living indoors can easily die of shock and exposure if left outside for a long time in temperatures below 50. I like to start acclimating my tomato seedlings to chillier temps in the mornings, when they can get a little sun at the same time.

Tip: Running an oscillating fan on your seedlings for a few hours a day indoors will help get them ready to handle real wind. 

Common Tomato Seedling Problems

Here are a few common issues you might see when starting your own tomato seedlings, plus what you can do about them:

Leggy Tomato Seedlings

Tomato seedlings are really, really prone to getting tall when started under artificial light. Your plants might look a bit awkward and lanky compared to the nursery seedlings you’re used to seeing, but this doesn’t mean they aren’t doing well. It’s essentially just impossible to get short, compact tomato seedlings unless you start them in a greenhouse from the very beginning.

Fortunately, leggy-ness is an easy problem to remedy with tomatoes, since they readily root from the stems. Of course, you’ll want to avoid extreme cases of leggy-ness, which cause poor stem, leaf, and root development. If your plants are just tall, though, it’s no big deal. Just bury the stems when you pot up and again when you transplant them to the garden. At transplanting time, you can remove some of the lower leaves and bury up to two-thirds of the stem.

Tomato Seedlings Turning…Purple?

Purpling on the stems, leaf veins, and underside of the leaves is a common concern with tomato seedlings. Last time you checked, tomato plants are not purple, right?

The cause of purple tomato seedlings is usually somewhat of a mystery. In some cases, it may be from temperature stress or low phosphorous uptake. The seedlings may need feeding or may not be getting enough nutrients from the fertilizer you’re using, if there are other signs of nutrient deficiency, like stalled growth.

If you notice your seedlings turning purple, make sure the temperature where they stay isn’t dropping below the 60-65 range overnight, and make sure your fertilizer contains enough nutrients. Otherwise, if your seedlings look to be growing normally, the purple tone isn’t anything to worry about. It will fade away once the seedlings are planted outside.  

Nutrient Deficiency

As we’ve established, tomato seedlings are highly prone to nutrient deficiency if they don’t get fed early on. They’ll also display some impressively dramatic symptoms. Seedlings that were growing steadily will suddenly stall out and languish. The lower leaves may turn yellow, brown, or purple, often all three at once, and they may quickly start to wither and drop from the plant.

These symptoms can show up pretty much overnight, and you might think your seedlings are done for. But tomato plants are known for being drama queens! They’ll quickly recover once they get some proper nutrients. While the affected leaves won’t turn green again, new growth will be healthy, and the plants will be fine. Just don’t delay feeding, once you notice unsightly color changes in your seedlings’ leaves.

Ever since I started growing tomatoes from seed instead of getting starter plants, I’ve noticed the seedlings I raise myself usually make healthier, more vigorous plants once they go out in the garden. Not to mention all those variety options! Growing from seed allows you to always grow the varieties that produce bumper crops in your area, instead of just waiting to see what’s in stock on the shelves. You’ll never go back to buying plants, after you’ve raised your first batch of seedlings.