Growing Tomatoes From Seed: Why Would the Leaves Turn Green and Die?

Disease

• Diseases, such as damping off, are the most common reason tomato seedlings die. Damping off sometimes occurs before the seeds ever germinate when the seeds rot in the soil. At other times, infected seedlings develop dark green or blackened stems, which wither and fall over. Most other plant diseases cause spotted or blotchy leaves. To prevent plant diseases, use only new or clean containers, sterilized by adding one part chlorine bleach to 10 parts warm water. Use a lightweight potting soil, rather than garden soil or a heavy potting soil. Garden soils can harbor diseases and insect pests. Both garden soil and heavy potting soils don't provide good drainage and contribute to damping off.

Inadequate Light

• Young seedlings need lots of light to grow well. When they don't get enough light, the leaves become pale green and the stems become thin and leggy. If the plants don't get enough light for an extended period, they might even die. Keep seed trays in a warm location, such as the top of the refrigerator, until they germinate. Once the seedlings emerge, move the seed tray to a bright sunny window or place the seedlings under lights. A 40-watt bulb or fluorescent light tubes 4 inches above the plants provides enough light. Run the lights for 12 to 16 hours each day, but check the seedlings frequently to make sure they're not getting too hot.

Fertilizer or Water Problems

• Too much fertilizer or water probably won't kill the seedlings outright, but they might contribute to potentially fatal issues, such as damping off. Tomato seedlings don't need fertilizer until they've developed their second leaves, according to the National Gardening Association. Excess watering can also cause problems. When the soil is constantly soggy, young seedlings can't access oxygen. Water only to keep the soil evenly moist and allow the soil to dry out slightly between waterings. Chronically soggy soil can cause damping off.

Plant Selection

• In addition to proper growing techniques, careful plant selection can also increase the chances of healthy seedlings. Heirloom tomatoes are often valued for their flavor and interesting colors and shapes. Unfortunately, many of them are more prone to disease problems. Try growing disease-resistant tomato hybrids instead.