How to Start Tomato Plants Indoors

December 3, 2019

By Matt

Starting Tomatoes Indoors From Seed: A Complete Guide

Is there anything better in life than biting into a fresh, juicy, sweet tomato right out of the garden? We think not! 

In the cold and dreary winter, there’s nothing like the thought of a vegetable garden to get the mind racing about spring planting! Are you ready to get a kick-start on the spring season?

Advantages and Challenges of Growing Tomatoes From Seed

So why should we start tomatoes from seed, anyway?

Benefits:

  • By growing from seed you can benefit from a more diverse stock. You can choose from literally thousands of unusual varieties listed in seed catalogs and online instead of the few dozen common varieties typically available at your local nursery. 
  • One huge advantage is tomatoes indoors is they need a constant soil temperature of at least 60 degrees (preferably higher) in order to germinate. This can give you a significant jump start on the season so you can prime your tomatoes to be ready to go and hit the ground running when temperatures rise in late spring.

  • Starting early allows you to be flexible and ensures you have a bumper crop of tomatoes come summer since you can start your crops when it’s most convenient for you and your growing season. 
  • You can save some serious money if you’re growing dozens of plants. A packet of seeds costs a couple of dollars at most, while even just a six-pack of (already grown) plants can cost you anywhere from 3 to 5 bucks, depending on where you live.

  • It’s more fun! Watch your seedlings grow and enjoy the fruits of your labor this summer!

Challenges:

While seed starting certainly has its benefits, we also want to highlight some of the drawbacks to this strategy.

  • Starting seeds early does create a project in the form of energy and time along with purchasing supplies and possibly even grow lights depending on your setup.

  • Seedlings may not turn out healthy or as strong as store bought plants since they may be grown in less-than-ideal conditions.
  • Unfortunately, tomato seedlings don’t take care of themselves and usually require daily care (especially in the form of watering)
  • Late spring weather may not cooperate and you may need to delay moving your seedlings outside when they are desperately in need of transplanting into the ground.


We may be biased but, despite these challenges, growing tomatoes from seed can be both fun and rewarding. And hey, worst case scenario if our seedlings fail, backup tomato plants are always readily available from garden centers and home stores. Experience is the best teacher, so we recommend rolling up your sleeves and giving it a shot!

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Best Types of Tomato Seeds for Indoor Growing

In short, you can grow just about any type of tomato seed indoors, from cherry to beefsteak to grape, and everything in between! Experiment with a few types of seeds for best results, and you may be pleasantly surprised by some new tasty varieties you discover with which to cook and preserve. 

Consider growing heirloom varieties indoors as these are often challenging to come by as started seedlings. You will enjoy tastier, more unique fruits than you would if you purchased your tomatoes as seedlings or – heaven forbid! – had to purchase tomatoes at the grocery store. 

There are thousands of types of tomatoes, but they all fall into a few broad categories. Try to pick one or two plants/seed packs from each category to avoid thousands of tomatoes over the course of a season, keeping in mind that one seed packet can produce quite a few tomato plants, and one plant produces dozens of tomatoes. 

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Here is a list of the types of tomatoes you may want to consider starting with:

  • Cherry (varieties such as Sweet 100): These tomatoes typically have a sweet flavor and are ideal for salads.

  • Sauce (varieties such as Roma): These tomatoes are dense, rich-tasting fruits with low water content and are best for sauces and canning.

  • Beefsteak (varieties such as Big Boy, Cherokee Purple): These tomatoes are very large and are best served in sandwiches or even plain. 

Order your seeds a few weeks to a month before you intend to plant. In the dead of winter, it may be hard to purchase seed from your local farm and garden store if they haven’t stocked up yet for that growing season. Some stores do receive early shipments of vegetable seed, so just double check packets are dated for the correct growing season. Leftover stock from last year is often able to grow but sometimes germination percentages will go down over time in storage. 

Alternatively, purchase directly from a retailer who specializes in seeds, such as Burpee’s, Burgess, or Johnny’s Selected Seeds so that you can ensure you are getting the freshest and most viable seeds possible.

When to Plant Tomato Seeds 

The ideal time to plant tomatoes indoors depends on your individual growing needs, as well as the climate and weather patterns where you live. Typically, tomatoes are started about six to eight weeks before the average date of last frost. Count backwards from your expected date of frost to figure out this date. 

Because tomatoes are native to the tropics, temperatures below freezing will kill them. They take about three months to produce fruit, so you really want to give yourself as much time as possible. 


Quick tip: **With tomatoes, it is always better to be a little late than a little early. Tomatoes that are exposed to cold air in early spring (even if above freezing) can be stunted and will be surpassed in growth by plants that were planted later but at proper warm temperatures.**

To figure out your average date of last frost, estimate based on the last few years’ worth of data (you can find this information online or through your local cooperative extension). For the Mid Atlantic region, universities such as Maryland, Rutgers, Virginia Tech, and Penn State are great places to start.

If you aren’t willing to play the guessing-estimation game, you can also consult the Old Farmer’s Almanac to give you an idea of a range. 

In general, you should start your plants sometime between January and March, depending on where you live and how early or late you want to transplant tomatoes. You can even start seeds indoors as late as April or early May, when the weather warms and your plants will have access to more sunlight. 

By beginning seeds in April or May, your harvest will be later, but in many cases in the Mid Atlantic region, the growing season is long enough to accomodate for this delay even into September and October. Plus, since the soil will be its warmest when you transplant in late spring, you won’t have to worry about stunted growth or as much transplant shock. 

How to Plant Tomato Seeds

Step 1: Location
Before planting, you first need to select the ideal indoor location. They prefer lots of sunlight–if you have access to a heated greenhouse, this is your best choice. Otherwise, opt for a grow light, a sunny windowsill or similar area.

Step 2: Proper Starter Soil
Prepare your planting containers with a potting mix. Make sure not to plant in topsoil or compost as they are too thick and hold moisture too well for seedlings. 

One of the best products you can use is specially formulated organic seed starter mix, such as this mixture manufactured by Espoma, that is ideal for root growth and development. Seed starter mix tends to be less dense and is formulated with more nutrients for plants in their early stages of growth.

Whatever you choose, you want to utilize a base that is light and airy, and will allow air and water to filter easily. A heavy, dense soil will make it hard for tomatoes to grow, as their roots will have a difficult time penetrating through the clogged soil. They may not even germinate, as waterlogged soils could cause the seeds to rot.

Step 3: Picking a Container

The containers you use to plant your tomato seeds can vary. You can use plastic cell packs, which are tiny pots grouped together in packs for flower and vegetable seedlings, or you can even use small pots. In a pinch, you can even use recycled items like old, clean yogurt containers punched with a hole in the bottom to allow for proper drainage. 

Some of the best containers to use are seed starter peat pots that are specifically designed to cultivate seeds. These biodegradable seed starter peat pots by Jolly Grow are ideal, allowing plenty of air flow and root penetration as your plants develop. 

Keep in mind that a small pot that is at least three or four inches tall and wide will be the easiest, as this will allow your seedlings to fully develop without their roots becoming constricted. Be aware that once tomato plants begin to grow, they can grow very quickly and rapidly. 

To make sure you have plenty of space, consider using a cell seed starter tray, such as this product by Bootstrap Farmer. With a variety of sizing options, these trays are designed for starting plants so that you don’t have to worry about overhandling or suffocating your plants by mistake. 

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Step 4: Fill the Container & Plant the Seeds
To start, fill your containers with a seed starting potting mix to about half an inch from the top. Place a pair (or even three) seeds on top of the soil near the center of the pot. Many people skip this step when they plant their tomato seeds, believing that they only need one tomato per container in order to avoid overcrowding.

It is indeed very true that you want one plant per pot. However, it is best if you start with at least two seeds as added insurance in case one doesn’t grow. If both sprout, simply choose the best looking seedling and pinch off the rest. (Don’t dig them out- this will disturb the roots of the remaining plant.) 

Once you have set your seeds on top of the soil, cover them with no more than a quarter of an inch of soil and then compress it very lightly with your fingers. You want to make sure the seeds aren’t squished, but that they have good seed-to-soil contact for germination. Sprinkle water on top of the seeds so that the soil is thoroughly moist and you’re good to go!

Some Post Planting Tips: Cultivating Your Indoor Crops

The easiest way to ensure your tomatoes germinate is to provide them with plenty of sunlight, airflow, and water.

Light

Sunlight can sometimes be a challenge, particularly if you live in a cloudier climate or have a house that is shrouded in shade. A professional grow light is likely your best bet here–a good one might set you back $20 to $50, depending on the size. They come in various shapes and styles so you can pick one that fits your budget and shelf space.

Alternatively, try to set them in the sunniest window possible that receives afternoon sun. If your seedlings become tall and leggy, they’re telling you that they are not receiving enough sunlight. You may also find the seedlings will lean towards the light and you’ll have to spin them around once or twice a week to keep them straight.

Airflow

In addition to warmth and sunlight, you also need plenty of airflow after the seedlings have developed. If airflow is a problem in general, consider installing a fan or ventilator in your room. This will simulate an outdoor environment and keep the air moving and fresh so that your tomatoes don’t become limp or succumb to disease.

Warmth

Remember that the warmer it is, the easier it will be for your tomato seeds to germinate and grow. Keep the room’s temperature above 60 degrees Fahrenheit and try to avoid wide swings in temperature. Placing your tomatoes near heating vents can be a good way to provide plenty of warmth and sunlight, but make sure they aren’t being exposed to drafts or variations in temperature. If they are near vents, make sure plants aren’t drying out too often.

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Water & Starter Fertilizer
Water your tomatoes regularly. When you water, do so deeply, so that the water penetrates to the bottom of the container and for the soil to be thoroughly moist.

Water when the soil is dry to the touch. Be careful though, because sometimes the soil will be dry at the top but the bottom is still soaked. If you aren’t sure, take a pencil and poke it into the soil to determine the moisture level. Seedlings can (and will) die from being constantly soaked. This will cause fungus problems and damping off disease.

Organic starter fertilizer can be applied at this stage, but don’t overdo it and make sure to follow all instructions and directions on the label for timing and frequency. 

Transplanting Your Tomatoes 

You may think that once the weather has warmed adequately outside (remember, soil temperatures need to be at least sixty degrees Fahrenheit, and warmer is better–totally past your last frost) you are ready to get your seedlings into the ground. 

Not so fast! You need to harden them off first. 

The hardening off process, which involves gradually introducing your plants to the great outdoors, should happen over a 7 to 10 day period. You need to allow your tomatoes time to adjust to outdoor conditions. 

To do this, find a sheltered place where the seedlings can sit in filtered sunlight, but out of the wind. Bring them outside during the day, starting with just a few hours at a time before bringing them back inside. Each day, increase the amount of time you leave the plants outside until day ten, when you can plant. This helps to reduce transplant shock and ensures adequate growth.

Try to plant on a cloudy, windless, and warm day. These conditions provide an optimal growing climate, although you can make do with less ideal conditions, too. Your seedlings should be at least three inches tall and nighttime temperatures should be above fifty degrees. 

Dig a hole a few inches deeper than the depth of the pot your seedlings are in, spacing your plants between three and five feet apart. Leave enough space because tomato plants grow large and need air space in between plants! Remove each seedling and loosen its roots before placing it in the hole and packing it in with soil. 

Tomatoes should be planted deep with some of the stem buried beneath the surface. Roots will grow off of the stem tissue and help make your tomato a stronger plant. The general rule of thumb is to plant ⅓- ⅔ of the stem underground when planting. 

The last step? Grab a glass of your favorite beverage, sit back, and enjoy the fruits of your labor–your beautiful homegrown seedlings! 


Ready to Begin?

Starting your tomato seeds indoors is a great way to enjoy the benefits of a homegrown garden, as well as to save a little bit of money. Plus, there is no better cure for winter fever than to start your planting indoors. Growing your tomato plants from absolutely start to finish–seed to harvest–is one of the most enjoyable endeavors there is, and should be pursued by every gardener at least once in his or her lifetime. 


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