Ultimate Crash Course: Starting Tomato Seeds Indoors

Many people prefer to buy ready-to-plant tomato seedlings at big box stores, rather than growing their own plants from purchased or saved seed.

However, buying plants will cost a lot more than simply buying some seeds from your favorite varieties and trying to grow them yourself.

The process of sowing seeds and watching them sprout is incredibly satisfying for the soul, especially during the dark days of winter; It’s a great feeling to know that you nurtured those little plants all the way from seed to fruiting!

If you’ve never grown tomato plants from seed (or if you’ve just had trouble in the past) this guide will clear up any of your questions or concerns about growing these important kitchen garden staples.

If you’ve had issues with your seedlings in the past, pay close attention; we will address those symptoms along the way.

Selecting Your Tomato Seeds

In order to grow your own tomato plants from seeds, you’re going to have to buy (or trade for) some tomato seeds.

For the new tomato gardener, there are a lot of confusing terms thrown around on all of those online seed shops. Let’s clear some of that up.

Determinate Vs. Indeterminate

One of the most common (and important) details is whether a plant is indeterminate or not. This affects the plants growth habit, how long it fruits for, and when the fruit ripens.

  • Determinate: These plants are more bush-like rather than vine like. They’re smaller, overall. The plant will flower in successful flushes, but will eventually stop flowering; the fruit will begin to ripen all at once, allowing you to process most of the fruits at the same time. Once the flush of blooms is over, the plant’s maximum potential yield is reached. The plant dies after fruiting.
  • Indeterminate: These are the vining varieties of tomatoes that can reach over 12′ in height, or length. They produce suckers that can be rooted. They flower and fruit until frost.

Organic VS Conventional Seed

When shopping, you’ll find both organic and conventionally grown seeds. Typically, organic seeds will cost you a bit more than conventionally grown seed, but not always. This depends on the source of the seed, how easy it is to grow and harvest that particular type of seed, and how common the plant itself is. For newly released items, you’ll generally pay even more during their first year of being cataloged by the distributor.

Most people prefer to opt for organic seed, but personally, I believe that status doesn’t affect the crop that is grown from those seeds. Rather, I believe buying organic seed is simply a way to support cleaner, environmentally friendly agricultural processes; if you don’t have much to spend, you shouldn’t feel worried about conventional seed.

When a plant is grown with traditional pesticides, negligible (if any) amounts of pesticide would make their way into the seeds. By the time the seed has germinated and grown into a plant, there would be no traces of the pesticide left behind. Some may say it could hinder germination rate; it my experience, this is false. I’ve had excellent germination rates from both conventional “cheap” seeds, and seeds from companies like Baker Creek.

Purity and Quality of Seed

Your biggest concerns when purchasing seeds are the purity of seed, and the quality of seed.

If a seed variety isn’t pure, you might wind up with some plants that are not producing the vegetables that you ordered; a result of cross pollination. This is a very concerning issue among squash.

The quality of seed is important, as well. It should be fresh, properly processed for storage (fermentation for seeds from fleshy fruits), and stored properly prior to sale (not in a hot building, for example.)

Seed Companies VS Trading Seeds

There are two major ways that you can obtain seeds: through seed companies, and through trading.

With seed companies, you’re going to pay store price for your seeds. You will also have seeds that are pure (aside from the rare mix up, unlikely but it does happen!) and hybrids that are exactly what they are supposed to be. The seed should be fresh, unless it is on clearance. There are several companies that stand behind their seeds, offering a free replacement if the seeds do not germinate or produce true to type.

With seed trading, you’ll get your seeds free aside from the cost of postage. In return, you’ll send the other trader seeds as well. Seed trading is incredibly popular, as you can expand your tomato collection quickly for the price of a $0.70 stamp!

However, you’re not guaranteed that seeds are fresh, as they may be a few years old. For most of us, this isn’t an issue at all. In addition, you must be diligent about questioning the purity of seed; for example, most home gardeners are not practicing hand pollination to prevent cross pollination; thus, some crops could cross easily. With tomatoes, some varieties are unlikely to cross while others stand more of a chance. It’s a risk you take, but tomatoes are much more likely to remain pure than other crop types.

Preparing for Sowing Tomato Seedlings

Once you have your seeds in hand, it’s time to prepare for sowing season.

Based on your available space and the number of seedlings you start, you should choose the method that’s right for you.

Choosing a Method

Are you simply starting 3 or 4 seeds of 4 or 5 varieties?

Or are you starting 20 or 30 seeds of 10 or 20 different varieties?

For those who are only starting a few tomato seeds, sowing in small nursery pots would be ideal; for those who are growing hundreds of seeds, this could be largely wasted time and space during the first four weeks of growth.

The two options below are the most common methods for starting tomato seeds, with the first being ideal for those who are growing more than a handful of plants.

Sowing Tomato Seeds in Large Seedling Flats (Or Takeout Boxes)

Trying to fill hundreds of nursery pots with seed starting mix is going to take a while. That seed starting mix is quite expensive when you’re starting all of those seeds, as well.

Did I mention how much space all of those pots would take up, and how difficult it would be to keep lights on them for 16 hours a day?

With seedling flats, you can plant a hundred seeds in something the size of a saucer! A flat is a shallow rectangular tray, similar to a baking dish. You simply toss the seeds in, and allow the seedlings to grow in slightly crowded conditions; it’s a baby plant nursery, if you will. Once the seedlings are a few weeks old, they are transplanted into their own pots, fill with potting soil; since potting soil is far cheaper than seed starting mix, you’ll save a ton of money here.

Flats also fit much more easily beneath grow lights, requiring fewer lights and less dedicated space for seedlings.

Sowing Tomato Seeds Individually in Seedling Pots

If you want a simpler method for fewer tomato plants, you can start the seeds in 3″ seedling pots. With this method, you start the seeds and only transplant when it’s time for the tomato seedlings to go outside.

This is going to take up more space and more time to set up initially, but that’s usually not an issue for those who are only growing a handful of plants for their gardens. You will save a step (transplanting midway to setting them out in the garden), and the plants will not have to adjust to that transplant, either.

Selecting Your Seed Starting Mix

Your seed starting mix needs to be of the highest quality that you can afford, ensuring that your seedlings have the best possible start in life. A good seed starting mix will:

  • Hold water well, and drain well.
  • Be fertile, feeding the seedlings in their early stages of growth.
  • Be light and fluffy, unlike clay.
  • Not contain large pieces of organic matter, such as wood chips or leaves.
  • Be sterile, without potential contamination from soil-borne disease or illness.
  • Not contain insects, especially pests that may damage your plants.

Should You Use a Grow Light or a Greenhouse for Your Seedlings?

No matter how you approach the situation, seedlings need light to thrive and grow. Not only that, they need adequate light; otherwise, you might wind up with leggy or weak seedlings.

A seedling grow light (or several) is the best option when starting seeds during the middle of winter. You can control how much light they receive, as well as how close the plants are to the lights. This is the optimal way to start seedlings during the shorter days of winter. It is also the most affordable solution if you are not opting for a DIY project.

A greenhouse is better for late winter or early spring seedlings, when daylight hours have increased and the sun’s rays have grown stronger. Under the cover of plastic, plants remain warmer, untouched by frosty mornings. You can also build your own DIY cold frame to keep seedlings safe on freezing nights, which is budget friendly. However, a prefabricated greenhouse will cost you $50 or more, with the cheaper ones being quite small.

When to Plant Your Tomato Seeds

You should plan to start your seeds roughly 6 to 8 weeks before the estimated last frost date, if you have a way to provide light for them. You can find your frost date by entering your location here.

You don’t want to start your seedlings too early, as you want to avoid flowering and fruiting before the plants are transplanted into the garden. They may become root bound, as well.

If you start your seedlings too late, it’ll take longer for the plants to reach maturity and begin fruiting; they may also be quite small at the time of transplanting, which means they won’t have the same head start.

What If You Don’t Have Access to a Grow Light or Greenhouse? Growing Seedlings without Light

If there is no way to provide your seedlings with supplement light and weather protection, you could still start your seedlings 4 weeks before the last frost date. Keep the seedlings in the sunniest, south facing window in the home, pulling them away from the glass on cold nights.

When the weather is warm, introduce the seedlings outdoors, to the sunlight. You must harden off the seedlings slowly, and you must protect them from temperatures that dip into the low 40’s or colder. It is possible to grow seedlings without supplemental lighting or greenhouses, but it’s time and labor intensive; you must pay close attention to the weather, and move the seedlings frequently.

Planting the Tomato Seeds: Starting Your Tomato Garden

Once you have your seedling zone setup and a plan in place, it’s time to plant your seeds.

Gather your flats (or pots), fill them will your chosen soil mix, grab your seeds, and get ready for the fun!

Wet Your Soil Mix

Some people sow their seeds before wetting the soil mix; this isn’t my preferred method, though.

I recommend moistening your seed starting mix roughly thirty minutes before sowing seeds. Be sure that the soil has adequate drainage after wetting it, too.

Once the mix is wet, you don’t have to worry about water displacing the seeds you’ve just planted- unlike watering from above.

Depth to Sow

After your seed starting mix has had time to absorb water and drain off the excess, make small impressions in the mix for your seeds.

Tomato seeds only need to be planted 0.25″ deep, so there is no need for planting holes.

When planting in flats, it’s easier and faster to make trenches along the width of the flat, sowing varieties directly into the trenches.

Keeping Germinating Tomato Seedlings Warm

Once planted, be sure that the seeds are kept in a warm place.

Your seedlings could germinate within 5 days if they are kept warm; but it could take as long as two weeks if they are too cool.

The ideal temperature for germination is between 68* and 78* degrees Fahrenheit.

Variations In Germination Time

As previously stated, the warmth of the flat or pots could greatly improve or delay germination time, so be sure to keep them warm.

Another factor is the age of seed. Sometimes, older seed can take a bit longer to germinate.

If you sow lots of seeds, you’re going to have seedlings on each end of the timeline, with some extremely early and others much later. Roughly 75% of your seedlings should come up within a 3 to 5 day range, within the same variety. Other varieties may take more or less time.

Seedling Care During the First Two Weeks

Hooray! At this point, your seedlings have started to sprout, and are ready to take on this year’s gardening adventures….

That is, as long as they make it through their most delicate stage.

Most problems that occur with tomato seedlings are easily prevented.

Adequate Lighting for Growth: Preventing Leggy Seedlings

Adequate lighting is vital for your tomato seedlings. If they don’t receive enough light, they will become spindly and leggy. This ultimately results in weaker plants, which may not thrive very well, and they may not be able to withstand storms or heavy winds very well.

Plants should be stout and bushy, and they should not fall over. Tall, sparsely leaved, poorly supported plants aren’t receiving enough light. If the plants are too far from the light source, they won’t grow well either. Keep plants under lights for a minimum of 8 hours a day, and keep the lights within 2″ of the top of the plants.

Watering from Below

Tomato seedlings are prone to bacterial, fungal, and viral infections. Even with a sterile seed starting mix though, dampening off can still occur. This is the cause of a majority of losses, as a seemingly healthy seedling suddenly begins to wilt and die.

With dampening off, the base of the stem will appear to be pinched. This fungal condition is usually due to wet soil- particularly surface soil. Splashing the soil can quickly spread infections between seedlings and up into the foliage. Avoid splashing water and soil at all times.

Watering from below concentrates water in the lower sections of soil, allowing the top to dry out a bit. It encourages downward root growth rather than upward, as well.

Improving Air Circulation

I cannot stress enough the importance of circulation among your seedlings. By keeping a fan on your seedlings, you’ll help them in many ways. Fans help to promote:

  • Even temperatures among plants
  • Distribution of oxygen and carbon dioxide
  • Drier surface soils, which is not ideal for fungal growth
  • Even humidity, reducing wet or dry pockets of air
  • Clean soil, as fungi prefer still, moist, warm air
  • Plant strength, as the breeze forces the plants to stiffen up to protect themselves from high winds

Preventing Drafts: Tomato Plants Enjoy Heat

Cold air can cause your tomato plants to stall out, as the plants do not like cool temperatures. If you have a drafty door or window, keep the plants in a different area on cold nights. You could also insulate the source of the draft, reducing temperature swings.

If your otherwise healthy plants are getting plenty of light and nutrition, yet seem to be slow growing, drafts may be to blame.

Transplanting Seedlings from Flats

Once the seedlings have outgrown their nursery trays, they need to be transplanted into bigger pots- whether they’re traditional 3″ pots or even solo cups (these are also reusable!). The tomato plants need to have room to grow, otherwise they will become root bound and growth will be stunted.

When to Move Seedlings

Ideally, you want to give your seedlings enough time to strengthen themselves and grow their first true leaves. This may take two weeks or so. Once all of the seedlings have developed their first set of true leaves, get your pots filled with soil, and prepare yourself with a pencil.

How to Transplant Quickly and Easily

Tomatoes are one of the easiest plants to transplant. They handle this process incredibly well- I’ve actually never lost a seedling from transplant shock. The quickest, easiest way to transplant tomato seedlings is to:

  • Carefully lift seedlings with a spoon from beneath.
  • Gently tease the roots out of the soil, giving you a bare root plant.
  • Grab the transplant pot, and create a very deep hole with a pencil.
  • Gently drop seedling into the hole, guiding the root in with the pencil if you need to.
  • Once the plant is in, pinch the soil around it to close the hole.
  • Water in well, allow to drain, and you’re done!

Fertilizing Tomato Seedlings Between 2 and 8 Weeks of Age

Once the transplants are in, give them a week to adjust to their new pots. There should be plenty of nutrients in the soil to keep them happy for the first week (probably longer!)

Thereafter, give them a weekly watering with half-strength liquid fertilizer to promote healthy growth, until the seedlings can go in the ground.

If the seedlings become abnormally bushy or leggy, you may need to reduce fertilizing to once every 2 weeks or so. If tomato plants receive too much nitrogen, they will undergo excessive growth which can weaken the plant as a whole. As the plant stretches tall and wide, the resulting branches may be thin and weak, resembling a plant that has suffered from inadequate lighting.

Transplanting Tomato Plants into the Ground

When the plants are ready to go into the garden (around a week after last expected frost date), start by digging holes that will bury 2/3 of the tomato plant. This sounds like an odd process, but it allows the plant to root along the main stem and keeps the plant more stout as it continues to grow. This results in a strong, healthy plant.

When back filling the hole, be sure that you don’t compress the dirt. Tomatoes love to have rich, loose soil around their roots. Some also add a variety of amendments to the hole to help the tomato plant grow and fruit prolifically, such as compost (we enjoy Black Kow when not using our own), dolomite lime/Tums/eggshells (for the calcium), or epsom salts. There are other less common amendments that others use as well (fish, for example, which is a practice that was started by the Native Americans; those who actively fish may choose to use their waste.)

Don’t forget to water in plants well, and never water from above to prevent fungal infections.