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Getting a jump on tomato season: how to time indoor planting just right

03/31/2015 09:51 EDT | Updated 05/31/2015 05:59 EDT
Occasional warm, spring-like breezes and longer hours of sunlight kindle an urge in me to plant tomatoes, starting them indoors, of course.

After all, the sooner the seeds are up and growing, the sooner I'll sink my teeth into a garden-ripe tomato, right?

Not necessarily.

Earlier planting leads to better harvests only when tomato seedlings have consistent, near-perfect growing conditions. Even with a greenhouse, such conditions are not easily created. And the earlier tomatoes are planted, the harder it is to give them what they need.

SLOW AND STEADY

The ideal tomato seedling (also called a "transplant" or "start") plods along, growing steadily, making a seamless transition to the outdoors when transplant time finally comes. Consistently moist soil and regular feeding, both easily provided, are part of this prescription. Keep tabs on your watering by periodically poking your finger or an electronic water meter down into the potting soil, or by lifting the container to check its weight.

There are a couple of ways to feed your seedlings. One is to add soluble fertilizer to the water; use a fertilizer formulated for this purpose and follow the instructions, because too much fertilizer can be as harmful as too little. Fish emulsion is a good, soluble organic fertilizer.

An even easier way to feed is to mix into the potting soil some insoluble fertilizer that slowly but steadily releases nutrients. No need for high-tech, "slow release" fertilizers here, although they will do the trick. My potting mix includes one-quarter, by volume, compost, and this, along with a smidgen of soybean meal, steadily feeds my potted plants in sync with their needs. Cottonseed meal, alfalfa meal and feather meal are other organic, slow-release fertilizers that can be added to a potting mix.

POT ON

To avoid any hesitation in the plants' growth, they need to be shifted to larger pots as they grow. Don't start those tiny seedlings in large pots, because they'll just sulk in a large volume of wet soil. Ideally, repot whenever plants grow taller than one-half to two-thirds the height of their container. Like watering and feeding, repotting is not difficult, but does demand attention. The only problem is all the window space that lots of large pots eventually gobble up, even moreso the earlier seeds are sown.

So much for the easy part of keeping tomato seedlings that were planted early growing happily. The plants also need abundant light and relatively cool temperatures — ideally around 65 degrees Fahrenheit, and this is a combination not easy to provide on a windowsill or, without care, in a greenhouse. A sunny window in a cool room is ideal.

Artificial light is another option. Use a fluorescent light and keep adjusting its height so it's within inches of the plant. Many seedlings can bask under a double fixture of two 4-foot-long fluorescent bulbs. The light from an incandescent bulb isn't the right spectrum, and the heat the bulbs give off if hanging close above the plant will scorch the leaves.

A LITTLE STRESS IS GOOD

One more thing a tomato seedling needs for good growth is stress. It sounds harsh, but a bit of stress indoors prepares the plants for buffeting wind, pelting rain, bright sunlight and cooler temperatures (early in the season, at least) outdoors. What's more, stress can, to some degree, make up for insufficient light and too much heat. Brush your hands over the leaves or shake the plants one or two times a day and they'll develop into stocky, dark green youngsters.

Then, a gradual introduction to outdoor conditions is beneficial. "Harden" the plants for a week by setting them outdoors in a spot protected from the full brunt of wind and sun before planting them in the garden. Bring them indoors if frost threatens.

Don't let anxiety over "perfect growing conditions" keep you from growing your own tomato seedlings. Doing so gives you the choice of the tastiest varieties. (I suggest Sungold, Belgian Giant, Rose, Brandywine, Amish Paste and Carmello.) Also, tomatoes are forgiving plants. The worst-case scenarios for early sown plants given less than ideal growing conditions is an early crop that peters out, or a delayed first crop followed by tomatoes right up until frost.

Blame those first warm breezes of spring for our tendency to sow tomato seeds indoors too early. The time to transplant tomato seedlings outdoors is a week after the average date of the last killing frost in your area (the date is available from your local Cooperative Extension office). The time needed to grow a reasonably sized seedling is about six weeks, so count back from that last frost date and hold back sowing seeds indoors until about then.

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