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Growing Tomatoes From Seed: How, When and Ideal Temperatures

From his early days, Brandon helped his grandmother in her garden. He has always been passionate about tomatoes.

Before you begin reading this guide on how to grow tomatoes, it is important that you understand that the perfect time to plant tomato seeds varies, and it completely depends on your climate. Tomato plants are warm weather plants that originated somewhere in Central and South America and were traditionally cultivated by native Americans before spreading across the globe. Today, they are available in a multitude of varieties.

Ideal Tomato Plant Temperature

The first trick to knowing how to grow tomatoes is understanding the room or garden conditions they thrive in. Depending on the stage of growth, tomato plants fancy different temperatures:

Germination: Tomato seeds germinate best at constant soil temperatures of around 70–80 degrees Fahrenheit (21–27 degrees Celsius).

Seedlings: Young tomato seedlings, just after germination and until they have two to three new leaves, prefer temperatures of around 65 degrees Fahrenheit (18 degrees Celsius).

Grown Plants: Tomato plants once transplanted/planted outdoors, prefer warm temperatures. The night time temperature should not go below 55 degrees Fahrenheit (13 degrees Celsius). Lower temperatures make your plant suffer, thereby stunting growth a bit. This ultimately leads to bad fruit yields, or worse, the death of your plants.

Seeds Are Usually Germinated Indoors

Tomato seeds are almost always germinated indoors, either in greenhouses, under grow lights, or simply on a sunny window ledge. Some people choose to speed up germination by placing heat mats at the bottom of the container bearing the seeds. This certainly helps, but it is not necessary.

Most of your vegetable plants do not need to be started indoors, but tomato seeds are delicate and require a constant soil temperature between 70–80 degrees Fahrenheit to germinate. They therefore need to be started indoors in many parts of the globe. Germinating indoors also prevents the young seedlings from being eaten by slugs or other creatures in your garden.

If you live in a region with long, warm summers, you don't need to germinate your seeds indoors. Instead, you just need to make sure that the temperature of the soil outdoors is within the requirements listed above. However, for those of you living in temperate climates, the soil only reaches this temperature sometime mid-summer, and by then it is too late to plant seeds outdoors, as the plants won't grow, mature, and bear fruit before the first frost.

Growing tomatoes from seed on a windowsill.

Growing tomatoes from seed on a windowsill.

When to Plant Tomato Seeds

When growing tomato seeds indoors, it is typical to plant the seeds somewhere between six to eight weeks before the date of the last frost. This is also stated on most seed packets if you read the fine print.

But determining the date of the last frost and the date of the first winter frost later in the year is not easy if you haven't been gardening for years. Luckily, for the people in the United States, there are websites to help you out. Check out the Farmers Almanac or this helpful chart on hardiness and hardiness regions on the Modern Farmer. There are useful resources for other countries too. A simple Google search for frost dates in your country or city would suffice.

Do not worry too much about determining the exact frost dates. A few days here and there is not going to ruin your crop, provided you plant outside only once the soil is sufficiently warm.

When Is It Too Late to Plant Tomato Seeds?

Not everyone is fortunate enough to plan their tomato gardens well in advance. Sometimes life gets in the way, and we are delayed. If it's already spring, and you're wondering whether it is too late to plant your tomatoes, the following paragraph will help you decide.

It depends on the time your plant has to grow, right from the date you wish to plant, until the first expected frost date. All you need to do is determine the days to maturity for the variety of tomatoes you intend to plant. If the number of days from the day you plant to the first frost is greater than the days to maturity, go ahead and plant your tomatoes. If you've missed out don't worry too much, look for other seed varieties that require shorter days to maturity and plant those instead. But, if you're extremely late to the party, there's nothing that you can do about it other than to learn from your mistakes and plan better the following year.

How to Seed a Tomato

Starting tomato seeds successfully requires a few things, such as:

  1. The tomato seeds
  2. Containers to plant the seeds
  3. Soil for germination of the seeds
  4. Water
  5. Natural or artificial light

1. The Tomato Seeds

When choosing tomato seeds you have the option to use seeds from tomatoes bought from the grocery store, tomatoes from last year's harvest, or you could alternatively buy tomato seeds from a store or on the Internet. If you are buying seeds, you may want to look for seeds that are resistant to any diseases that are common in your area. Make sure you know exactly what you want to plant before you buy your seeds. Would you prefer heirloom or hybrid tomatoes? Determinate or indeterminate tomato plants?

2. Containers to Plant the Seeds

You're going to want to use small containers, with a depth of at least 3 inches for the germination of the seeds. You could use larger containers too, but I suggest smaller containers as the soil used for germination is a bit different from what the plants would eventually grow in. Moreover, there are benefits to transplanting a tomato plant after germination—you bury the stem a bit to get more roots—which I would personally say is crucial for the future well-being of the plant. However, you should understand that this is just a suggestion and not a must. I would recommend that you read my guide on how deep tomato roots grow so you can make an informed decision.

Just make sure that the container you use has holes at the bottom, if not, make some holes yourself. This prevents the possibility of the soil getting too wet, thereby preventing root rot.

For the seed containers, you can use almost anything. For example:

  • Yogurt containers
  • Egg cartons
  • Cut off milk or juice cartons
  • Plastic containers that strawberries and other fruit come in
  • Plastic soda bottles
  • Plastic cups
  • Take away containers and just about anything else

In most of the above cases, you are going to need to make holes in the bottom of the container yourself. Don't forget to place the container on a tray, so that the water seeping out does not damage your furniture, floor, or windowsill. Check out these two resources for more ideas on home-made containers for seeds: Preparednessmama and Treehugger.

Tomato seedlings growing in plastic cups.

Tomato seedlings growing in plastic cups.

I do not like to unnecessarily buy stuff when you can recycle what you already have. But if you were looking to buy containers for the germination of your seeds, I would recommend Bio Pots by Window Garden, which were a perfect gift for a friend of mine who had just decided to try his hand at peppers. The roots of the tomato plants can penetrate through the container. Therefore, it could be placed directly into a larger container, and then later transplanted into the ground. This is very helpful if you are a new gardener and are scared that you may damage or kill the plant when transplanting the seedling.

Moreover, these bio pots use natural fiber soil and not peat and have no traces of plastic in them. I have personally tested this out by burning them to see if there was any plastic residue. Also, after using such pots and studying their structure, I make my own using newspaper and some plant fiber.

If you're sowing seeds for a home garden, you're probably not going to be sowing many. But for those of you with large gardens, greenhouses and farms you may want to plant your seedlings in trays. This helps you grow a lot in limited space during the initial germination stage. Check out the video below for guidance on how to plant in trays.

3. Soil for Germination of Tomato Seeds

Good germinating mixes are fine, uniform, allow for good aeration, are not compact but loose, and are free of pests and weed seeds. Also, germinating mixes usually do not need any fertilizer/compost if you intend to transplant as soon as the first true leaves appear. However, if you intend to leave your tomato plants in their initial container for longer than three to five weeks after sowing, you should seriously think about adding worm castings to the initial mix or alternatively supply liquid fertilizer after the appearance of the first true leaves. If you are not sure what true leaves are, check out the video below.

Best Fertilizer for Tomatoes

Tomato seeds do not require any fertilizer to germinate, as the seed contains all the necessary nutrients. It is only after the first true leaves that the plant must begin fending for itself. I've written a detailed guide on how to fertilize tomatoes that covers the nutritional requirements of the plant at different stages of growth.

Why You Should Never Use Garden Soil as It Is to Germinate Seeds

Backyard soil is typically too compact and is therefore not ideal for the germination of seeds. It tends to form a hard, crusty layer, and seedlings find this hard to penetrate and might ultimately die. Also, garden soil is not the best when it comes to aeration and drainage. Lastly, garden soil can harbor diseases and weed seeds. Weed seeds could easily be weeded out, but getting rid of the damping off disease and others is a different story.

Transforming Garden Soil Into a Seed Starting Mix

This is not recommended as the best practice, but if you want to be frugal, this is a good option. The rate of a successful germination may be lower, but it definitely is a lot cheaper to use garden soil and homemade compost as a starting mix. But it is vital that you first pasteurize the soil and compost. Check out this article on the best way to pasteurize soil.

Before you begin the pasteurization process, I would recommend that you use a sieve to only allow small particulates of soil and compost to pass through. I would also recommend you add some sand to this mix, if easily available. Now, pasteurize this mix and then add an equal proportion of moistened coconut coir to prevent the soil from clumping up and forming a brick. The coconut coir will help keep the soil loose and aerated. You can buy coconut coir bricks from your local gardening or pet store.

Which Commercial Seed Starter Mixes Are Best?

If you're a first-time gardener or if you do not intend to plant many seedlings, it is easier, cheaper and quicker to use a commercial seed starting mix such as the Black Gold seedling mix. It has no added fertilizer to it—it is not a potting mix—but just the perfect blend of ingredients to create a germinating media for your seeds. This was the mix my friend used to plant his peppers, and the pH of the mix was neutral, which is a good thing for most seeds. I stole some soil (with permission) to plant a few tomato seeds, and I realized that the mix I made at home and this mix produced similar seedlings. When you use this, you will have to transplant or add fertilizer after the growth of the first true leaves.

Some people tend to use the Epsoma SS16 seed starter premium potting mix, but this leads to lower germination rates, as it's more of a potting mix than a seed starter mix. But, as stated earlier, if you intend to leave your seedlings in their original containers even after the appearance of their first true leaves, the Epsoma mix is a better option. Just plant three to four seeds in each container instead of one or two as you would usually do.

How to Make Your Own Seed Starter Mix

To make your own seed starter mix, you only need three or four ingredients, depending on whether the mix is solely a seed starting mix or a dual-purpose seed starter and potting mix:

  1. Coconut coir (2 parts if dry, 8 parts if pre-moistened)
  2. Vermiculite (1 part)
  3. Perlite (1 part)
  4. Screened worm castings to also serve as a potting mix (4 parts)

Coconut coir is 100% organic and is a by-product of the coconut processing industry. It serves as the base ingredient of your germinating mix. It helps create a light, well-draining media for the seedling roots to grow.

Vermiculite helps retain water and is ideal for germinating seeds and young seedlings that require the soil to be evenly moist but not wet. Adding this ensures that the plant has sufficient water, without you having to water very often. Also, it helps with nutrient retention, if you add any fertilizers in the future.

Perlite is the white pieces that you see in commercial seed start mixes. On its own, it does not hold a lot of moisture. It prevents the compaction of soil and is, therefore, a helpful ingredient to a seed starting mix. It also helps the soil drain quickly (you do not want wet, but just damp soil for your seeds and seedlings) and helps with soil aeration.

To create a seed start mix and also a potting mix, you could either add screened (through a sieve to remove large pieces) compost that is created using the Berkeley hot composting method, or you can add screened worm castings.

Sowing Your Tomato Seeds

  1. Once you've got your containers, seeds, and germinating soil ready, you're going to want to fill up your containers with the soil (to the brim).
  2. Now make holes into the top of the soil, around a quarter of an inch deep using your finger, a stick, or a pencil.
  3. Plant two to three seeds in every section/container by dropping seeds into the holes you've just made. When planting two or more seeds, make a hole for each seed, do not place two seeds at the exact same spot.
  4. Finally, cover up the seeds with a pinch of new soil filling up the hole.

The video below shows you another way to do it, by placing the seeds on the soil and then pushing them in.

4. Watering Your Tomato Seeds

It is best to dampen the soil before filling it into the containers, because the loose germinating soil settles down and gets compacted a bit on the addition of water. After you've placed your seeds you should water again, to moisten the seed. You could sprinkle a few drops of water on the top, ensuring the seed (below the soil) is dampened. Alternatively, you could water from bottom up, as demonstrated in the video above.

Until germination and beyond, you're going to want to make sure that the soil is always moist, not wet. The holes in your container and the coconut coir would make sure that your seeds are never overwatered, so don't worry too much about overwatering at this stage of growth. For the later stages, watering is one of the most crucial aspects and you could use my guide on watering tomatoes to help get you started.

Creating a Greenhouse Effect

As you've learned right at the beginning of the article, tomato seeds prefer warm conditions for germination. In case the room with the tomato seeds gets too cold, you could try covering the containers with plastic wrap, thereby creating a greenhouse effect. The heat of the sun during the day is entrapped (to some extent) within the wrap. Many people report that this has been beneficial, and I can see why. However, if you do this, make sure that the moment the shoots get off the ground—even before the first dicotyledon leaves open—you remove the plastic wrap. Failure to do so would kill your plants due to the lack of oxygen.

5. Providing Adequate Light to Your Seedlings

Tomato plants require sufficient light to grow well. I have personally never had to use grow lights because I always lived in regions with at least 12 hours of warm sunlight on a south-facing windowsill. However, if you do not have the same luxury, it is beneficial if you provide your tomato seedlings with grow lights. My guide on tomato plant light requirements would help you decide if you need grow lights.

Note: Grow lights are not essential as long as you get at least a few hours of sunlight, but anything less than 12 hours of warm sunlight is not sufficient for your tomato seedlings. If you do not provide artificial light, your plants will not die, but they will not grow well. And a bad growth stage at the beginning of the plants' life may not lead to the best possible crop later on.

Why Use Grow Lights?

Grow lights are perfect for planting seedlings indoors until the plants are mature and the temperature warm enough to move them into the garden. While natural sunlight offers a full spectrum of color, allowing plants to absorb the frequencies they require, grow lights mainly provide light in the red and blue regions which are essential for plant growth.

What Kind of Lights Work Well as Grow Lights?

Home gardeners love using fluorescent lights because they provide a high intensity of light while producing little heat. Also, they are not expensive. You should not use incandescent lights because they produce too much heat which could kill the young tomato seedlings. More importantly, they lack light in the blue spectrum. Light in the blue spectrum helps plants grow green leaves and become stocky. An insufficient amount of blue light results in leggy seedlings, and this is not what you want.

People often discuss full-spectrum lights. These are definitely a better option because they release light in all the seven colors of the rainbow and all frequencies are important for the growth of tomato plants (red and blue are just the most important among them). But these are a bit expensive and in my opinion, fluorescent lights are good enough for the early days, that is, before transplanting into the garden or placing the container outdoors.

To know more about the wattage required (number of bulbs) per square foot if you use fluorescent light bulbs, check out this SFGate article on CFLs and tomato seedlings.

The Distance of the Light From the Plants

Plants are designed to grow towards the light. Seedlings, tend to get leggy if the light source is placed high above them. Therefore, it is suggested that you never place the light source more than 3–4 inches above the seedings (or foliage). You are going to have to keep adjusting this as the plants grow.

How Long Do I Leave the Grow Lights On?

There are different kinds of plants, and tomato plants are long-day plants, which means that they need long hours of natural sunlight (14–18 hours). However, it is also very important that the plants get periods of darkness (night) for them to be in their natural rhythm. I would suggest you turn off the light in the night when you head to bed and switch them back on when you wake up. Alternatively, you could use an automated system to switch on and off the lights.

Tomato Plant Germination Time

You probably want to know how long it is going to be until your tomato seeds germinate. There are many factors at play here, right from the variety of the tomato, the soil temperature, the seeds themselves and of course the soil. But in general, expect your tomatoes to rise up from the ground anywhere between 5 to 10 days. Some people have reported that it has even taken them up to two weeks. Make sure you try and maintain the ideal soil temperature for germination, which is 70–80 degrees Fahrenheit.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.


Brandon Lobo (author) on October 31, 2018:

Cool. I hope you have a fun time doing that, Larry. You should definitely look up articles by others here on Dengarden, I've found a few gems that have helped me grow some pretty good crops this year.

Larry Slawson from North Carolina on October 31, 2018:

Really interesting! Thank you for sharing! My wife and I have been talking about growing some tomatoes and other vegetables next year. Some really good ideas here.

Brandon Lobo (author) on October 31, 2018:

That would be perfect Poppy, thanks for taking the time to comment.

Poppy from Enoshima, Japan on October 30, 2018:

It's so important to know how to grow your own food. I'll be bookmarking this and perhaps try to grow tomatoes on my balcony, which gets a lot of sunlight in the morning, next summer. Thank you for this wonderful information.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on September 25, 2018:

Hi, lobobradon, I got the correct time like summer. But now in Nigeria, winter is approaching so I can not plant a tomato that is all I said. By now, there become more expensive and a luxury item. Good that you will look into the weight issue. Glad you did. Thanks.

Brandon Lobo (author) on September 25, 2018:

Yup, winter is not the best time. But in Nigeria, it may be different. I mentioned the ideal temperatures required to grow tomatoes on this hub, you may want to check that out.

I'd have to look into the weight and cooked tomatoes thing, I find it very interesting.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on September 24, 2018:

@Brandon, and @Liz, thanks for the updates. I will take all these into account when I began tomatoes garden next year. Now winter is approaching, and the cool frost will destroy the plant, right?

Let me add a little word here. Tomatoes, when cooked, can help a skinny person gain some weight! This is hardly noticed. Have a nice time on HubPages.

Brandon Lobo (author) on September 24, 2018:

Thanks for the comments Miebakagh and Liz. Here in central Europe, I planted just before the beginning of spring so that they were pretty big by the time I planted them outdoors. Turned out very well. But, next year I intend to plant a bit later.

Liz Westwood from UK on September 24, 2018:

In the UK most people plant tomatoes in spring and then move them outside when the weather (hopefully) gets warmer. Some keep them in greenhouses all the time.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on September 24, 2018:

Hey, Brandon, thanks for all the informative and education features. I have bookmarked the article for reference. I am a lover of tomatoes fruits, and I hope to plant same.Thanks again