How to grow tomatoes from seed

• Is it better to start with seeds or plants?
• What is the best type of tomato to grow?
• What is the difference between determinate and indeterminate tomatoes?
• How to tell if the seeds are still good?
• How to grow tomatoes from seed indoors?
• When and how to start tomato seeds indoors
• How to care for seedlings
• When to transplant tomato seedlings into pots?
• How to prevent leggy seedlings
• When should I bring my tomato plants outside? 
  
   

Can you imagine a hamburger without ketchup or spaghetti without tomato sauce? It should be of no surprise that the tomato is the most consumed of all North American vegetables.

Once you have bitten into a home-grown tomato, you may wonder what happened to the taste of the grocery store one. Commercial tomatoes are grown for uniformity and for toughness that allows for farm machinery, warehouse and store handling without damage, as well as for longevity in storage. 

These are all important qualities for best efficiency and less waste. However, somehow, the excellent taste part got left out.

Tomatoes are by far the most popular vegetable grown by home gardeners, even for those that have no "real" inground garden to play with, because tomatoes are easily grown in containers. For the home gardener, starting tomatoes from seed gives you unique opportunities for stretching the harvest season and for experimenting with the most tasteful varieties.

 

Is it better to start with seeds or plants?

Yes, you can buy “ready-to-transplant” tomato seedlings each spring at garden centres, but by growing them from seed instead, you get a much greater choice in flavours, colours and characteristics. Commercial transplant seedling growers select the most popular varieties to grow each winter, of which garden centres might carry up to 10-12 different tomato types. But a quick glance at Canadian seed companies’ catalogues shows that they offer between 24 and 36 different varieties.  

Discover exceptional tomato varieties here: Best tomato varieties to grow in your garden

 

What is the best type of tomato to grow?

Are you looking for season-long snacking or an all-at-once crop for canning? What is your preference? The selection is abundant: Beefsteaks for slicing, Romas for sauces and salsas, and cherry or grape tomatoes for salads and appetizers. 

There are varieties that are disease-resistant, if that might be a problem in your garden. Others are what they call “heirloom” (plants that have been grown without crossbreeding for at least 50 years). Some are early-maturing, while others will provide tomatoes later in the season. The seeds come in both regular and certified-organic formats.

 

What difference between determinate and indeterminate tomatoes?

You’ll see that some tomatoes are termed as “indeterminate”, which for you means that plants need staking. These are tall, lanky plants, often considered almost a vine and they will provide fruit continually over a long period. 

Others are termed “determinate”, which means they tend to be more compact and do not always need staking. Determinate tomatoes are the type that are likely to mature all at once, bursting out with a predefined crop all at once, which is handy if canning or preserving.

 

How to tell if the seeds are still good?

So much to pick from… Remember that seed packets usually have a 2-year expiration date stamped on them. But if you keep them dry, your leftover tomato seeds should last at least five years. If they are dry and cool (in airtight jars in the basement, garage, etc.), they can keep for 10 to 12 years. 

So, even if there are more seeds in the packet than you need this year, the leftovers can be germinated next year or later, or can be shared with family and friends.

 

How to grow tomatoes from seed indoors

Why use Seed Starting mix for tomato seeds?

Do not use garden soil or bagged compost, which drain poorly and may harbour pathogens. Your best bet is a soilless potting mix, or even better, a seed starting mix. These are sterile and designed for optimal germination. We recommend: PRO-MIX Organic Seed Starting Mix

Which containers for tomato seedlings?

Pots, cell packs, peat pots, or used food containers that you drilled drainage holes into. The bigger the container, the longer it holds moisture, compared to a small cell pack, which dries out more often.

The importance of labels

If using a few different varieties, you will need to identify them. This is for your benefit at harvest time, because you can monitor which plant is a keeper for next year. Labelling will also remind you about how much real estate within the garden is needed at planting time - was it a dwarf or tall indeterminate plant?

How to choose tomato seeds

Perusing through seed catalogues might be overwhelming. But focus on what your needs are. Want a dwarf or bush type that doesn’t get too big and can stay in a patio pot or in a small garden all summer? Do you like cherry or grape tomatoes?

They are available in both determinate and indeterminate formats. Keep in mind that in healthy garden soil, some indeterminates can grow 1.5-2 metres (6’) tall. For sure, the tall indeterminate tomatoes will produce more fruits over a longer period. Check the “dates to harvest”. 

By having both early and slow-maturing plants in your garden, you can enjoy a continual harvest. Got a bit of disease issues in your soil or compost? Look for disease-resistant cultivars. 

Do tomato seeds need light?

Although a sunny window might supply good light, you need at least 12 hours of light a day for tomato seedlings. Winter and early spring sunlight isn’t nearly as intense as summer sunlight and there are fewer hours of daylight. It is worth looking at bringing in grow lights. 

Good light is needed for growing compact plants. Inadequate light causes the plants to stretch to reach the light and they become “leggy”. Keep the lights 5 cm (2”) from the plant tops. Adjust the height every few days. If your grow lights cannot be lowered, simply raise the containers to the desired height.

How much water should you give tomato seedlings? 

Pouring a strong jet of water from a watering can will unfortunately move around the seeds and potting mix. Instead, use a mister to gently water your seeds and seedlings until they are firmly rooted. Once sprouted, don’t spray the seedlings directly, just the potting mix. 

You can also “bottom water” by having your containers in a solid tray and pouring water in it so it can wick upwards.

Optional material to grow tomatoes

Heat mat

For starting tomato seeds, you don’t need light the first week, but you need warmth. If you don’t have a heat mat, find a warm spot to get them started, best if between 20 ˚C - 39 ˚C. Once the seedlings pop up, you should discontinue the heat mat (if using) and move the seedlings into light. Seedlings prefer slightly cooler temperatures after germination. 

Temperatures between 13 ˚C – 20 ˚C are perfect for growing sturdy plants. Remember: first 7-10 days: heat, light is not important. After sprouting: lower heat and bright light.

Small fan

A fan can be on for 1/2 hour once or twice a day, blowing at the lowest setting in several directions. This causes the plant stems to react by toughening up… all part of producing strong stems. The fan also provides good air circulation and reduces the chance of disease problems. 

If you don’t use a fan, you can simply run your arm across the seedling tops, pushing them gently to one side and then doing it again in the opposite direction. Do this a few times a day. It makes the stems tougher.

 

When and how to start tomato seeds indoors

1. To know when to start, you must count backwards the weeks from the last frost in your region. Tomato seed starting should begin 6 weeks or so before the last frost. If May 24 is the date in your region, for example, start seeds indoors mid-April. Click here to download your seeding and planting calendar.
2. Start by moistening the potting mix before filling the containers. Not too wet, just moist. Fill them to within 1.3 cm (½”) of the top. Press down to push out any air but don’t push so hard as to compact the medium. 
3. Place two seeds in each container. You can eliminate the weaker seedling later, leaving you with just one strong one taking over the container. Now cover the seeds with a thin layer of potting mix. And water with the mister, just moist, not soaking wet.
4. At this point, just find a warm spot, and if you can, place a clear dome or plastic film over the container to keep things moist. Light is not important at this point, until you see sprouts in about a week to 10 days.
5. At this point, remove the covering and place the seedlings in a sunny spot or under grow lights. Keep an eye for watering. Remember that smaller containers dry out more quickly.
   
    

 How to care for seedlings

• If you are starting your seedlings on a windowsill, rotate the pots every day or so, so the plants grow upright instead of leaning toward the light.
• If under grow lights, adjust the height regularly to stay 5 cm (2”) over the tops.
• Don’t let the medium dry out, but don’t overdo it as to waterlog the potting mix.
• Thin out the weaker seedling in the container. Best to use scissors to simply cut the stem. Tearing out the runt will disrupt the roots of the one you are saving.
• Once the first two true leaves appear, you can begin to fertilize the plants. When a seed germinates, it puts out a set of seed leaves at first, followed by the “true” leaves, the familiar ones that you would see on maturing tomato plants. 
• Once this second set appears, you can start using a diluted fertilizer dose (½ the dosage recommended on the label). At this point, the seedlings need phosphorus and potassium. The best product here would be PRO-MIX Organic-Based Garden Fertilizer 3-6-12 for Tomatoes, Vegetables and Fruits once a week.
• Learn all about seedling fertilizing here : Fertilization guide
• Before it’s time to set your seedlings outside, they may need to be transplanted into larger containers as they may outgrow their original pots. 
  
   

When to transplant tomato seedlings into pots?

With most annual seedlings, you always need to replant them at the same depth that they were growing initially. 

But tomatoes are different: they can be planted deeply because tomatoes are able to generate roots along their stems. In fact, whenever you are transplanting a tomato seedling to a bigger pot, it is recommended that you go deep. Put a thin layer of potting mix at the bottom of the larger pot, place your rooted seedling plug on top and fill the new pot to 1.3 cm (½”) from the top, leaving a short stem and leaves exposed. 

 

​​​​​​How to prevent leggy seedlings

A common problem for new indoor gardeners is finding that their tomato plants are tall and spindly, or “leggy”. Here are the main reasons for that:

• Insufficient light - increase light exposure, seedlings need at least 12 hours daily. If using grow lights, keep them 5 cm (2”) on top of seedlings.  
• Planting too early – hold off starting seeds until 6 weeks before last frost.
• Overcrowding – space the seedlings out.
• Excessive heat - temperatures need to be between 13 C˚- 20 ˚C.
• Overwatering – allow the soil surface to dry between waterings.
• Lack of air circulation – using a small fan helps.
  
   

When should I bring my tomato plants outside? 

When you are ready to move the seedlings outdoors after the last frost, you need to acclimate them to the new conditions. Remember that they have been pampered in moderate conditions. Outdoors, there are day and night temperature swings, brighter sunlight, and there’s also wind. This “hardening off” process is necessary for all annual transplants.

All you have to do is put the pots outdoors in a shady spot for a few hours each day. Bring them in at night. Gradually give them a bit longer time each day and start giving them exposure to some sun and wind. Keep this up for a week or so. That will get them ready for the shock of full sun, wind, rain and night temperatures.

For the big planting day, always choose a sunny location in the garden. It’s nice to have the garden soil well cultivated and mixed with compost and organic fertilizer, especially with bonemeal or gypsum for the extra calcium that tomatoes appreciate.

Pick a cool or overcast day to minimize the transplant shock. Once again, plant them deeper than they were in their pots, so new roots can form along the buried stem. If the seedlings are “leggy”, you can bury the stem in a trench horizontally.  They can be planted all the way up to the top couple sets of leaves. The tomato leaves will find the sun and grow upright in a few days.

It's good to stake the seedlings just to keep strong winds from pushing them over. Some gardeners cut the bottom out of old pots and place them over the transplants for the first week or so. Eventually, you should consider putting tomato cages or a similar type of support for the indeterminate tomatoes.

This is all worth the effort. You’ll see once you have bitten into that first home-grown tomato!