Sturdy, healthy tomato starts are important for both hydroponic and outdoor gardens.
What’s the secret? Tough love.

WORDS: Heather Walker

Germination Basics

To go from seed to seedling, tomato plants need a moist growing medium, light, and warmth. I grow seedlings in my own organic potting mix of peat moss, vermiculite (some growers prefer perlite), green sand, bone meal, and organic soybean meal. You can add some aged compost too, but weeds may take over your seedlings if the compost wasn’t hot enough to kill the weed seeds. I soak the mix in a wheelbarrow with warm tap water (we’re on a well, so no chlorine worries here) then run the hose to add water until I can squeeze the soil mix and water runs out. I fill 4-inch pots with the wet mix, then plant one seed in the center of each pot and label it: name, date planted, open pollinated or hybrid.

I like to start my tomato seeds in 4-inch pots on the windowsill in my living room, directly above a baseboard heater: the additional bottom heat gives them that extra encouragement. This summer was the first year I tried heat mats, and I definitely noticed a shorter time to germination with those bad boys.

To know when to water, dig down an inch or so at the edge of the pot, and if it’s still moist then don’t bother watering. If it starts to look dry, soak the pot a few times with room-temperature water.

Eager tomato seedlings on a heat mat.

Seedling Mix

2 parts by volume sieved garden soil
1 part by volume sieved sphagnum moss
Add to each cubic foot (5 gallons) of mix:
1 cup agricultural lime or dolomite lime
1 cup cottonseed meal or soybean meal
1 pint soft rock phosphate or 1 cup steamed bone meal
1 cup kelp meal

From “Growing Vegetables West of the Cascades,” by Steve Solomon.

The Sprout

Once the sprout emerges with its first pair of leaves, it’s time for the tough love. If you spoil your tomatoes when they’re young, they will grow into leggy plants that will be ill-prepared for the real-world conditions in the outdoor garden. 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. The plant, receiving lots of warmth but not much light, reacts as if it’s being shaded by other competing plants: as a result, it grows fast and tall in an attempt to access the sun and out-compete the other plants that it thinks are crowding it.

These leggy tomato seedlings were exposed to too much warmth with not enough light, which encouraged them to stretch: the stems are thin and weak as a result. Weak plants are more susceptible to disease, drought, and pests. Photo: Greg Wagoner.

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 50°F (10°C), which can compromise a tomato plant’s development or kill it.

Ideal Tomato Growing Temperatures

Day: 65-70°F (18-21°C)
Night: 50-60°F (10-16°C)

It’s also important to expose your tomato seedlings to air movement from this point forward. Fans, ventilation, an open window, or even your hand brushing their tops a few times each day will encourage more stocky growth and prepare the plants for the realities of wind. Novice growers often leave the clear dome on their plant starts for far too long. Don’t be an overprotective tomato parent!

Transplanting

It’s very likely that your tomato plants will outgrow their starter pots before it’s safe to plant them outside. In fact, this is preferable: the more times you can transplant your tomatoes into larger pots, the better. Why? Because every time you re-pot a tomato plant, you bury it up to its “neck” (just below its top set of leaves), or as much of the plant as you can fit under the soil. The tomato will then send out roots from the newly-buried stem, creating a more well-developed root system. And a strong root system leads to a healthier, more productive plant! Transplanting in this way also helps control the ultimate size of your plant once it’s ready to go into the garden: it’s far easier to plant a foot of stem and foliage with 8-inches of well-formed roots than a 2 foot spindly monster that will snap in half if you look at it funny.

So once your tomato outgrows its four-inch pot, bury the plant up to its neck in a gallon pot of soil, with the top set of leaves above. If you start your tomatoes in 1-2″ cell trays, transplant them into 4-inch pots when they’re ready for more room, then eventually into the gallon pots. And once a tomato outgrows its gallon pot, it’s probably time to plant it outside.

Transplanting a tomato plant from a small pot to a larger pot: bury the plant up to its neck, leaving the top set of leaves above the soil.

Hardening Off

It’s a blue-skied, warm, sunny day: you’re ready to unpack those shorts and plant out your tomatoes! But hold on. It’s crucial that you gradually prepare your tomato plants for outdoor conditions, rather than abruptly moving them from their cozy, sheltered existence into the cold, hard 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 windowsill nursery, to the unheated greenhouse in the daytime, to the unheated greenhouse 24 hours/day. I’ll start leaving the greenhouse door open, then setting them outside for the daylight hours. It’s 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 sunburn too! Eventually there will be a warm night and I’ll leave them outdoors. If frost is in the forecast, or a storm, I’ll bring them under shelter until it’s clear again. Eventually the plants will become more hardy, and spring will really be here, and around late May to early June I’ll be able to risk planting them out.

Into the Garden

This transplanting technique from pot to outdoor garden minimizes transplant shock and encourages strong root development.

Rather than digging a hole and planting the rootball at the bottom, as when re-potting, lie each tomato in its place horizontally on the outdoor garden bed, then bury it in soil — again, up to the top set of leaves. Be careful to support the stem, to avoid snapping it. Carefully pat down the dirt to ensure plant/soil contact, then water the whole plant thoroughly. The top of the tomato plant will eventually turn up toward the sun and grow into a surprisingly strong stem, supported by the amazing root system you’ve helped it develop.

It might seem easier to dig a trench and lie the plant in it, to keep your garden bed nice and flat, but if you do this you risk exposing the plant to the chillier soil underneath that sun-warmed top layer. Tomato plants may turn blue/purple-ish as a result — a sign of transplant shock. They will take longer to recover, which may affect the time or quality of harvest.

Have your own tomato-starting secrets to share? Tell us about it below…