Tomatoes: Indoors to Outdoors

A grow light really comes into its own when starting tomatoes from seed. There's no better way to banish the winter blues and encourage the onset of Spring by starting your seeds as early as February or March.

Germination Basics

Growing from seed is by far the cheapest option too and you get access to a mind-bogglingly huge range of varieties. Seed will keep for around three to four years if stored well. If you choose to buy young tomato plants, you’ll invariably be limited to a relatively tiny range of commercially popular varieties, although they are probably tried and tested in your region.

Tomato seedlings in a propagator indoors under a T5 fluorescent grow light.

Tomato seedlings enjoy a rich, moist growing medium, together with light, and warmth. I grow seedlings in my own organic potting mix of peat moss, vermiculite (some growers prefer perlite), bone meal, and organic soybean meal. I moisten up the mix in a bucket with tepid, de-chlorinated water with Orca (liquid mycorrhizae) 0.5mls per gallon, General Hydroponics Diamond Nectar (3mls / gallon) and General Hydroponics Bioweed (cold pressed seaweed) (3mls per gallon). A T5 high output or very high output fluorescent grow light is an ideal way to get them started, but be sure to consider your timing if your tomato plants are destined for outdoors or your greenhouse.

Tiny tomato sprouts in a cell tray.

Nighttime temperatures are your cue for when its okay to put tomatoes out into the real world (or greenhouse.) You don’t want to be starting your tomato seeds more than six weeks before nighttime temperatures (either outdoors or in your greenhouse) are reliably above 57 °F (14 °C) although most varieties will tolerate temperatures a little cooler—note emphasis on 'tolerate!'

I like to start my tomato seeds in cell trays in propagators under a multi-array T5 fluorescent grow light. I start my first batches in late March—if it’s still chilly I sometimes sit the propagators on thermostatically controlled heat mats. I find that the additional bottom heat gives them that extra encouragement. Heat mats certainly reduce germination times but you may wish to discontinue their use after sprouting as excess heat can cause them to leg up.

The Seedlings Emerge!

Tomato sprouts in a cell tray filled with rich organic compost.

Once the sprout emerges with its first pair of cotyledon leaves, it's time for some tough love. Don’t spoil your tomatoes when they're young—no matter how tempting—or they will grow into leggy plants that will be ill-prepared for the real-world conditions of the outdoor garden. As already mentioned, if you give your tomatoes lots of warmth when they aren't getting a lot of sun or supplemental light, they can get "leggy," growing tall with a spindly, weak stem. This is particularly important for growers in the Northern half of our continent, where the sun's strength and height in the sky in March/April may not offer enough light in the greenhouse. Your best bet is to keep them under your T5 fluorescent grow light for 18 hours light a day. Remember, too much heat and not enough light tells the plant that it’s being shaded by other competing taller plants so it starts to accelerate its vertical growth.

These tomato seedlings received too much heat and not enough light—hence the tall, leggy growth pattern.

To prevent leggy tomatoes and encourage stocky, strong growth, narrow the gap between the light and heat the plant is receiving. To do this, steel your heart and move every tomato with leaves into an unheated greenhouse during the day, unless it's unusually cold. The greenhouse protects the young plants from the wind, cold, and rain/snow, but exposes them to cooler temperatures than in the house, and more sunlight through the poly-plastic roof and walls: they will receive more light and less heat than on the windowsill or heat mat. Bring them in at night until you're confident that the temperature won't drop below 57°F (14°C), which can compromise a tomato plant's development or kill it.

Ideal Tomato Growing Temperatures

Day: 65-77°F (18-25°C)
Night: 57-64°F (14-18°C)

Use a small oscillating desk fan to create a little air movement to help strengthen those stems. Beginners often leave their propagator lids on over their starts for far too long, inviting damp issues and generally creating weaker starts. 


Which tomato varieties are you growing this season?

Transplant your tomatoes from their cell trays when they are no more than two weeks old with at least one set of true leaves. Try and wait until the medium in the cell tray is quite dry as this will ease the transplanting process. Take the opportunity to bury your tomato seedling deeper in its new home (e.g. a 3” or 4” pot). The tomato will then send out roots from the newly-buried stem, creating a better-developed root system.

These tomato starts will need transplanting to a larger pot soon.

So once your tomato outgrows its intermediate pot, you can either plant out or, if it’s still too early in the season, transplant it on to a gallon pot. And once a tomato outgrows its gallon pot, it's probably time to plant it outside!

When growing tomatoes in containers, remember they need regular water—but be sure to allow the pot to dry out and become fairly light before watering—and always water so that there is at least 20% run-off. This technique encourages the tomato to fully exploit all the growing media in the container. 

Hardening Off

These tomato plants are outgrowing their 5-inch pots and our desperate to get to know terra-firma!

So—you’ve waited patiently for a beautiful warm, sunny day and you can’t wait to plant your little darlings out, but it’s also important that you gradually prepare your tomato plants for outdoor conditions, rather than abruptly moving them from their cozy, sheltered existence into the real outdoor world.

Plants must be "hardened off" for a week or so by gradually exposing them to less-hospitable conditions for increasingly longer lengths of time each day. My plants progress from their T5 fluorescent grow light and grow tent to a sunny, sheltered terrace during daytime and back indoors at night—it can be a bit of a chore but my little tomatoes are worth it!

When it’s time to plant out it's actually best to put them out on a cloudy or partly cloudy day, as a full day of direct, hot sun can be hard on a plant. Plants can suffer from sunburn too—especially if they’ve been raised under grow lights

Into the Garden

Prepare each plant site by digging a deep hole and partially back-filling with rich compost—this helps to counter the sandiness of my soil—(great for drainage but not so good for nutrient and water retention.) I use a mixture of green compost from my garden pile and worm castings from my wormery. I also add some Mykos (mycorrhizae inoculant), two or three pouches of “Once and Done” organic, slow-release plant food, and Azos (nitrogen fixing bacteria) – all from Xtreme Gardening. Insert your tomato plant into the hole and carefully pat down the dirt to ensure plant/soil contact, then water the whole plant thoroughly. You may find that they look a bit sorry for themselves for a few days—some leaves may pale off, but after a few days to a week your tomato plants will eventually turn up toward the sun and grow a surprisingly strong stem, supported by the amazing root system you've helped it develop.

 Tomato cane supports are essential for non-determinate (cordon) varieties.

If you’re growing cordon (vine) non-determinate varieties it’s absolutely essential that you create some sort of support structure—bamboo canes are the most popular option. As your tomatoes grow, gently secure it to the poles and be sure to remove side shoots to concentrate all growth on the lead growth tip. I use Velcro garden tape to loosely secure my toms to my supports every 10 - 12 inches or so.

Tomatoes and cane structure - early in the growing season.

Depending on your climate, water once or twice a week—very well. This will encourage your toms to send down deep roots rather than shallow, surface roots that need to be mollycoddled with regular waterings. Many growers cover the ground with plastic to decrease moisture-loss through evaporation, planting their tomato starts through little 'X' shaped slits. The best time to water is first thing in the morning—i.e. at first light. Try to avoid getting water on the foliage. Remove yellowing leaves throughout the growing process but be careful not to over-do it. 

Tomatoes, early in the season, not yet big enough to be attached to the supports!

Finally, remember that tomatoes are heavy feeders—they will reward well-prepared soil that is rich in organic composted matter. As the season progresses you can supplement nutrient levels by using a liquid organic feed such as General Organics BioThrive or Botanicare PurBlend.  

These young tomato plants are well established now in this rather sandy soil thanks to 'Once and Done' pouches by Xtreme Gardening. 

We threw in two to three 'Once and Done' organic plant food packs into the planting holes. These ingenious little pouches, manufactured by Xtreme Gardening, have really helped our young plants get a leg up and become well-established with thick, study stems and lush, deep green foliage.

Remove side shoots on indeterminate varieties to encourage more vertical growth, less foliage, and more fruit!

Don't forget to remove side shoots on indeterminate varieties of tomatoes. "Indeterminate" means that the tomatoes will grow to practically any size (it's indetermined!) depending on environmental and soil conditions whereas "determinate" varieties (less common) will grow into a bush shape without any need for pinching out side shoots.

Side shoots can easily be identified and removed. Try to pinch them out before they grow more than an inch long. 

Pinching out side shoots (try to do it before they grow over an inch long) will encourage vertical growth with less foliage and more fruits! You need to constantly be on the look out for these side shoots. You'll have to do it multiple times at the same internodes as they will reappear at the same points—determined little things they are!

Tomatoes in early June starting to ripen

By early June we have lots of trusses of green tomatoes—some of the earlier varieties (Latah, shown above) are well ahead of the pack!

Tell us what varieties of tomato you are growing this year!