Answer: If your tomato seeds sprout and begin to grow but then die, they may be showing signs of damping off, a fungal disease. Too much water and poor air circulation will make the environment perfect for development of this disease. Here are some guidelines for growing healthy tomato seedlings - follow the advice and I'm sure you'll have better results with your little plants:
Starting tomato seeds is quite easy. Follow these basic steps for growing healhty transplants.
1. Start with a bagged seedstarting medium or fluffy potting mixture. (Heavy potting mixes may stay too moist and reduce germination rates.) Moisten the mix to the dampness of a well-wrung sponge, fill your seedling tray or individual containers, and, and firm the surface.
2. Sprinkle the seeds on top of the soil, about 1/2 inch apart in flats (where they can be scattered over the surface or placed in rows), or sow two to three seeds per individual container.
3. Firm the seeds into the soil with a small piece of wood or other flat object. Then put a thin layer - about 1/4 inch - of moist soil mix over the seeds and then firm it again. This brings the seeds into good contact with the soil, which is important for germination.
4. Place the container inside a plastic bag or cover it with a sheet of plastic or a plastic tray lid to keep the soil mix from drying out.
5. Put the container in a place where the temperature remains steadily around 70oF. Warm soil is more important than warm air for optimum germination, so providing heat from below really helps. Many gardeners find that the top of the refrigerator is an ideal spot for germinating seeds.
6. The seedlings will begin to emerge in a few days. Check daily and remove the plastic at the first sign of green, then move the container to a well-lit location. A sunny window will work, but fluorescent light is better. If you use lights, set the plants an inch or two below the tubes and maintain that distance as the plants grow. If the distance is too great, the plants will stretch towards the lights and develop thin, weak stems. Keep the lights on 14 to 16 hours a day, but turn them off for the night. Plants need a rest, too!
7. Provide your seedlings with daytime temperatures in the range of 60oF to 75oF, and night temperatures in the range of 60oF to 65oF to encourage sturdy, stocky plants. Too much heat can encourage leggy, weak growth. Even flourescent lights can create quite a bit of heat, so check daily to make sure temperatures don't get too extreme for seedling health. If you need to cool things down, set a fan on low speed near seedlings. (The "wind" also helps seedlings to grow stockier.)
8. Keep the soil moist, but not wet. Water gently so you don't disturb the soil and expose seedlings' roots. Use room-temperature water if possible.
9. Don't worry about fertilizing the seedlings right away. Some commercial seed-starting mixes have fertilizer mixed in that will take care of the seedlings' nutritional needs until after they've put on their second set of true leaves. Wait at least a week (or even until after the first repotting) before feeding seedlings. Then apply a fertilizer diluted to label directions.
Many gardeners transplant their tomato seedlings to larger containers at least once while they continue to nurture them indoors. This makes for larger and stronger root systems. Transplant out in the garden when soil and air temperatures warm.
Best wishes with your tomato seedlings~
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