Start tomato seeds indoors about 6 to 8 weeks before the last frost in spring. Transplant tomato seedlings to the garden just after the last frost in spring.
Here are important tips to keep in mind when you start tomatoes from seed.
Tomato Seed Starting Temperature
- The optimal seed starting temperature for tomatoes is between 70°F to 80°F
- The seed will germinate best where the bottom temperature is about 85°F.
- Seedlings grow best when day time temperatures are between 70°F and 75°F during the day and between 60°F and 65°F during the night.
Tomato Seed Starting Soil
- Use a sterilized planter mix. A sterilized, pasteurized soil or commercial seed starting mix will be free of weeds and fungi that cause seedlings to collapse and die.
- You can pasteurize your own garden soil: bake the soil in a shallow pan at 200°F for an hour or two (the smell will be bad). If you place a potato in the oven with the soil, the soil will be sterilized when the potato has baked.
- To make your own seed starting mix, combine equal parts sand, vermiculite or perlite, and peat moss.
Sowing Tomato Seeds
- Tomato seeds can be started in pots, peat pellets, or flats. Be sure there are holes in the bottom for drainage.
- If you sow seed in 3- to 4-inch clay or peat pots, fill the pots to ½ inch from the top.
- Thoroughly moisten the seed starting mix before sowing to make sure it is not dry below the surface.
- Sow seed ¼ to 1/3 inch deep; space seeds 1 inch apart. As a general rule, sow seed three times the depth as the width of the seed.
- Firm the seeds into the soil with a piece of wood or flat object; then add a thin layer about ¼ inch of moist soil mix over the seeds and level it and firm again. This will bring seeds into good contact with the soil which is important for germination.
- Sow twice as many seeds as you need plants so that you can later choose from the strongest seedlings.
- Do not sow seeds too deeply otherwise the seeds may rot before they germinate.
Care Before Tomato Seedlings Emerge
- Keep the seed starting mix or soil moist but not wet after sowing; gently water with a fine spray being careful not to wash away the seed.
- Seeds can be started in a bright window or under lights set about 2 inches above the plants.
- Maintain soil temperature between 75°F and 85°F; seeds will germinate in 7 to 10 days. To maintain an even soil temperature, use a propagating mat or rubberized “electric blanket” placed under starting pots, flats, or trays.
- Place the seed-starting container inside a clear plastic bag or tent or cover the container tightly with a plastic sheet to retain moisture and warmth until the seeds germinate. You can also lay a piece of newspaper on the containers until seedlings emerge. To germinate seeds require warmth and moisture, not light.
- Remove the plastic bag or sheet when seeds emerge.
Tomato Seedling Care
- When seedlings emerge, give them full sunshine or place them under grow lights. Use two 48-inch, 40-watt fluorescent tubes placed a few inches above the seedlings. These can be supplemented with a couple of incandescent bulbs to provide the “red light” that growing plants need.
- Keep lights on 16 hours a day when using fluorescent lights.
- Plants must have lights off at least 8 hours a day to grow strong.
- Set tomato seedlings one or two inches below the fluorescent tubes and maintain that distance as the plants grow. If plants are too far away they will stretch and develop thin, weak stems.
- Check seedlings every day as they grow. Be sure to keep grow lights at a constant distance above the seedlings.
- Seedlings growing in a window should be turned every day so that they do not develop a permanent lean–heliotropism means growing towards the sun.
- Keep seedlings near a constant 70°F; temperature greater than 70°F may produce tall, spindly sprouts. Windowsills may get hot during the day and cold at night; seedlings grow best if kept at an even temperature.
- Keep the soil moist, but not wet. Water or mist plants gently to avoid washing them out. Use room temperature water.
Feeding Tomato Seedlings
- Do not fertilize seedlings right away; wait a week or two or until the first repotting, then mix in a small amount of balanced, water-soluble fertilizer in the plants’ water.
Starting Tomatoes in a Cold Frame
- Tomato seeds can be started in a cold frame–a glass- or plastic-covered box, heated by solar radiation as long as the temperatures remain warm enough for germination and growth.
- Seedlings growing in a cold frame or greenhouse should receive 12 hours of sunlight each day.
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