Growing Tomatoes from Seed: Sowing times, compost, and instructions

Tips on growing tomatoes from seed. Covers where to get heirloom seeds, how to sow them, the benefits of growing from seed, watering and temperatures, and the best compost to use. Includes two instructional videos.

This is the first part of the Growing Tomatoes from Seed Series. It continues with pricking the seedlings out, planting tomatoes in the greenhouse, growing them, harvesting the fruit, saving seeds, and preserving tomatoes.

One of the most delicious and worthwhile crops to grow is the humble tomato. The scent of its sun-warmed leaves is synonymous with summer, and that’s aside from the real prize – the fruit. You’ll see them start as little yellow flowers. After being pollinated, they develop a tiny green bud that swells ever larger from hard and green to plump, glossy orbs. Small cherry types, medium-sized romas, and gigantic sculptural heirlooms. Their beautiful shapes and colors are only surpassed by their delicious sweetness. No supermarket tomato can compete in flavor.

Contrary to what it might seem, it’s not difficult to grow tomatoes, even from seed. This is the first in a series of pieces I’m sharing this year to help you get started. We’ll begin with growing tomatoes from seed. Later pieces will cover pricking out, planting out, tomato plant nutrition, staking, harvesting, and preserving tomatoes.

Grow tomatoes from plants or from seed

Most people grow their tomatoes from small plants that they purchase in spring. They’re common enough to find both at your local garden center or even online. It’s perfectly fine to start this way and can save a lot of time if you’re running late.

I’ve had plants from a friend who grows tomatoes commercially and has also been sent tiny plug plants from a seed company. I also start tomato plants from cuttings and overwinter them and grow tomatoes from seed. Each method has its time and place, but there are some real benefits to starting with seed. Seeds cost less than plants, are fun to grow since they can be sown so early in the year, and there’s an astonishing variety to grow.

Benefits of growing tomatoes from seed

There are currently over 10,000 tomato cultivars in the world. How many of those do you come across as plants for sale? Growing from seed means that you have so much more choice in the types of tomato you want to grow. From tiny berry-like currant tomatoes to beefsteak types that weigh up to three pounds at harvest like Gigantomo.

This year I’m growing five varieties, all in my standard-sized greenhouse. They include Ailsa Craig, Black Russian, Sungold, Red Pear, and Costoluto Fiorentino.
Some of the best places to find rare and unusual tomato seeds are heirloom seed catalogues. Here are some places you can browse online.

Tomato Seeds in the USA

• MiGardener (get 10% off all seeds with code LOVELYGREENS)
• Tomatofest
• Baker Creek Heirloom Seeds
• Seed Savers Exchange

Tomato Seeds in the UK

• Plants of Distinction
• Victoriana Nursery
• The Real Seed Company

Controlling Growth Cycle

Growing tomatoes from seed also enable you to control the plant’s growth cycle. Meaning you can choose when to sow, how to grow, and when to harvest. In the part of Britain where I live, the average minimum winter temperature is only ten degrees Celsius, different from the average maximum summer temperature. Basically, it rarely freezes here or gets really hot. Brassicas and leafy greens love it, but heat-loving plants like tomatoes struggle.

To grow tomatoes in most places with cold springs, you need to start seeds early, nurture the plants, and plant them in a greenhouse or polytunnel when the danger of frost is past. It’s difficult to grow outdoor tomatoes here thanks to cooler temperatures and Blight, a fungal disease.

This means tomato plants that grow from volunteers rarely have enough time to produce fruit here. Deliberately starting off tomato plants in difficult environments can mean the difference between a harvest or not.

When’s the best time to sow tomato seeds

The time you sow your tomato seeds is not necessarily what the back of the seed packet says. Everyone who orders those seeds, from Alaska to Texas, will have the same information given to them. There are actually two factors that dictate the earliest time you should sow your seeds. Planting location and last frost dates.

Planting Location

It’s difficult to grow tomatoes outdoors in Britain for two reasons – temperature and Blight. Tomatoes need shelter from wind and cold and lots of warm sunshine. Blight is an air-borne fungus that affects tomatoes and potatoes and decimates most outdoor toms. That’s why most of us choose to grow our tomato crops under cover in polytunnels, greenhouses, or in a warm conservatory in the house.

Those in warmer climes without tomato blight will be lucky enough to grow tomatoes outside. If you’re unsure about where you should grow tomatoes, ask a neighbor, garden center, or local gardening group. They’ll know the best way for your region.

When to Plant Tomato Seeds

Once you know the location, you can decide on when to sow tomato seeds. The general advice is to sow two weeks before your last frost date. If you’re planting under cover, your plants will have more protection. If you plant outdoors too early and there’s a late frost, then you can lose your precious plants.

Free-Draining Potting Mix

If you’re starting tomato seeds indoors, as most of us do so early in the year, you should not use garden compost or soil. It’s because both contain weed seeds, fungi, and pathogens that can kill seedlings when grown undercover. It’s a different case for direct sowing seeds in the ground when it starts warming up. Those factors are more controlled in their natural environment.

Although many people will use standard multi-purpose compost, it’s better to use a free-draining seedling potting mix to start off many seeds, including tomatoes, aubergines (eggplant), and peppers. In their early days, they’ll have nutrients stored in the seed itself to get started.

Make Seedling Potting Mix

Seedling compost is fairly standard, and you can find it at most garden centers. They come in lots of different mixes that use loam, perlite, vermiculite, coco coir, and lots of other materials. The main objective of seedling compost is a fine yet light mixture with only a dash of nutrients. A common one you’ll find is John Innes seedling compost, and you could make that yourself with this recipe:

• 2 parts sterilized loam
• 1 part peat
• 1 part coarse sand
• For every 2-gallon (6 liter) bucket, add 10g (1/3oz) superphosphate
5g (1 teaspoon) ground chalk

If you don’t want to use peat, you can make other mixes, and there are loads of recipes you can find online. I don’t use peat in the garden, so tend to make a mix of 1-part vermiculite to 2-parts good quality multi-purpose compost. Sometimes I’ll use coco coir or perlite instead of vermiculite, though. This mix has enough nutrients to keep plants going for a while if necessary, and the vermiculite gives it good drainage and space for the roots to develop.

Tomato Sowing Instructions

You can sow tomato seeds in trays, modules, or in individual pots. Trays can take more seedlings, so if you’re planning on growing rows of tomatoes, then that’s your starting point. Prepare the potting mix by moistening it with a little tap water. Just enough to make it damp.

Fill a tray with about an inch or more of seedling compost, firm it down with your hand, scatter on seeds lightly, and then cover them very lightly with more compost. With pots and modules, fill them nearly full before setting two to three seeds on top. Cover with no more than a light sprinkling of potting mix, firm down, and water. If the temperature and moisture levels are sufficient, you’ll see seedlings emerge in 5-14 days. Grown this way, tomato seedlings will need pricking out after about three to four weeks.

Growing Tomato Seeds in Pots

If you’re growing a few plants in the greenhouse, it will save time if you use 3” pots. The plants will live for a longer time in those pots, so the compost they’re in will need more nutrients. In that case, use a more nutrient-rich potting mix to sow seeds into. In this case, I’d prepare a mix that’s about 1-part vermiculite to 3-parts sieved compost. Some gardeners will use plain multi-purpose potting mix, but it really pays to give your seedlings extra drainage.

Fill the pots to about a half inch from the top, firm down, and space two to three seeds on top. Cover lightly with compost. Allow to grow and pinch out the weaker seedlings after about a month.

Light & Temperature

To germinate, tomato seeds need a temperature of around 64-69°F (18-21°C). A little lower or a bit higher is fine, but below 50°F (10°C) and above 90°F (32°C) and they won’t do well. Because many of us start tomato seeds in late January to March, it can be difficult to give those kinds of consistent temperatures. If your home or growing space is cold, use a heated propagator.

Tomato seedlings need lots of bright light to help them grow. If you’re growing on a window sill, you’ll see your plants stretching for the window. You can try turning them if this happens to try to even them out. In the end, though, they’ll need a grow light if you’re starting them off really early in the year. Late winter sunlight isn’t usually strong enough to give tomato seedlings the energy they need. Put your seedlings under supplemental light as soon as you can see growth. I cover how I grow seedlings undercover in another piece.

Tap vs. Rainwater

I always use tap water to water tomato seedlings. I pour it into a bottle or jug overnight to let it come up to room temperature and to allow any chlorine to evaporate. That way, the temperature won’t shock the seedlings, and the chlorine won’t interfere with the growing process. Our tap water here is not fluoridated, but if yours is, you could consider filtering it.

Some years ago, I read somewhere that collected rainwater isn’t the best choice for seedlings. It’s not the water itself but the container and surfaces that water has come into contact with while it’s outdoors. Stronger and more established plants can easily deal with any microbes and fungi it can introduce. Seedlings are a bit more sensitive.
I’m not 100% sure this is a fact, but I tend to stick by this. If you’ve heard something similar, or have another opinion, let me know in a comment below.

Watering Tomato Seedlings

Getting the water to your seedlings without getting the leaves too wet or disturbing the compost or roots is the goal. A direct stream from a watering can or spray bottle can do a lot of damage. When watering, you can choose to do a few things:

• Set trays and pots temporarily in a shallow tray of water to allow it to soak up from the bottom
• Use a rose-headed watering can
• Use a misting spray bottle
• Protect the surface of the compost with grit

Growing Tomatoes from Seed

This year I’m taking you with me on the full journey of growing tomatoes from seed, to plant, to harvest. Have a watch of both videos in this piece that show you how I’ve sown my own seeds and set them up to grow. Just over a week from when those videos were filmed, nearly all of the tomato seedlings are up. Now two weeks on, the seedlings are growing uncovered under a grow light. Though they’re still in the propagator, I turned it off as soon as all the seeds had germinated — room temperature is fine for them.

The light I’m using is a red and blue spectrum flexible lamp that clips onto a window sill. It’s perfect for the home grower, especially if you only have a few seedlings to nurture indoors. I highly recommend it. I’ll cover grow lights and pricking out the seedlings in the next installment of this series.