Tomatoes (Lycopersicon esculentum) are frost-tender plants, and they're usually grown as an annual crop. Sowing tomato seeds indoors before frosts are over in spring prolongs the growing season, but the warm temperatures and low light levels indoors cause tall, thin, weak tomato seedlings. Starting tomatoes outdoors in shady areas also causes seedlings with those conditions. These leggy seedlings grow into thin-stemmed plants that break under the weight of a heavy crop, but you can fix leggy tomato seedlings when you transplant them into your garden.
Burying Seedling Stems
Burying tomato seedling stems when transplanting the seedlings into the garden helps cure legginess. Tomato stems can develop roots. Young roots appear as small, white bumps at the stem bases, but roots also can develop higher on the stems when they're buried in moist soil. Burying tomato stems when transplanting leggy seedlings leads to large, healthy root systems and does no harm.
Tomato seedlings must be transplanted into an area where they receive at least eight hours of direct sunlight every day, or else they will grow leggy. Wait until after your location's final average annual frost date before transplanting tomatoes outdoors.
Burying leggy tomato seedlings in long, shallow trenches provides the best results. Tomato roots grow best in warm, moist soil that contains plenty of oxygen. Burying the seedlings vertically places their root balls deep in cold soil with low levels of oxygen, but placing the seedlings horizontally in shallow trenches keeps the roots near the warm, airy soil at the ground's surface.
Dig a trench 2 to 3 inches deep and as long as a tomato seedling's root ball and stem are to the top two or three sets of leaves. Pinch off the leaves below the top two or three sets, and place the tomato seedling in the trench. Angle the stem area that will remain above ground away from the soil surface. Cover the rest of the stem with soil that you removed while making the trench.
Caring for Transplanted Seedlings
Leggy tomato seedlings need moist soil to develop new roots and grow strongly. Water transplanted seedlings immediately so that their soil is moist to a depth of 8 to 10 inches, and water them again each time the soil surface is dry. Don't water the seedlings so much that the ground becomes soggy.
The above-ground portions of transplanted seedlings' stems straighten and grow vertically over time. You can encourage them to grow straight by pushing a stake, or cane, into the soil next to each plant and loosely tying the stems to the canes with twine.
Avoiding Leggy Seedlings
Sowing tomato seeds at the right time, in good light and at cool temperatures prevents leggy seedlings. Short, stocky, thick-stemmed tomato seedlings become the best mature plants.
Sow tomato seeds five to six weeks before the final average annual frost date, and place them in a cool, bright indoor area, such as a south-facing window. Provide them with temperatures warmer than 32 degrees Fahrenheit at night and 60 F during the day but not warmer than 65 F. When the seedlings appear above the soil, turn their pots or seedling trays every day to help prevent the seedlings from leaning toward light. Pots and seedling trays for tomato plants must have bottom drainage holes.