1. Don’t crowd seedlings
If you are starting tomatoes from seed, be sure to give the seedlings plenty of room to branch out. Crowded conditions inhibit their growth, so transplant them into their own individual 4 in. pot, shortly after they get their first true leaves.
2. Provide lots of light
Tomato seedlings need strong, direct light. Days are short during winter, so even placing them near a very sunny window may not provide them with sufficient natural light. Unless you are growing them in a greenhouse, your best option is to use some type of artificial plant lighting, for 14-18 hours every day.
3. Put a fan on your seedlings
It seems tomato plants need to move and sway in the breeze, to develop strong stems. That happens naturally outdoors, but if you are growing your seedlings inside, provide a breeze by turning a fan on them for 5-10 minutes, twice a day.
4. Preheat the soil in your garden
Tomatoes love heat. Cover the planting area with black or red plastic a couple of weeks before you intend to plant. Those extra degrees of soil warmth will translate into earlier tomatoes.
5. Bury them
Plant your tomato plants deeper than they come in the pot, all the way up to the top few leaves. When planted this way, tomatoes are able to develop roots all along their stems. And more roots will make for a stronger plant.
6. Mulch later
Hold off on mulching until after the ground has had a chance to warm up. While mulching does conserve water and prevents the soil and soil born diseases from splashing up on the plants, if you put it down too early it will also shade and therefore cool the soil.
7. Remove the bottom leaves
Once your tomato plants reach about 3 ft. tall, remove the leaves from the bottom 1 ft. of stem. These are the oldest leaves and they are usually the first leaves to develop fungus problems.
8. Pinch & prune for more tomatoes
Pinch and remove suckers that develop in the crotch joint of two branches. They won’t bear fruit and will take energy away from the rest of the plant. But go easy on pruning the rest of the plant.
9. Water the tomato plants regularly
Water deeply and regularly while the plants are developing. Irregular watering, (missing a week and trying to make up for it), leads to blossom end rot and cracking.
10. Getting them to set tomatoes
Tomatoes are vines, after all, and indeterminate tomatoes reach for the sun. They like to grow tall before they start setting fruits. If you’re impatient, pinching off the tips of the main stems in early summer will encourage them to start putting their energy into flowering.