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How to Grow Seedlings Indoors
By Bountiful Gardens Staff

Seedling Flat with Celery Starts

Start your seedlings indoors as early as March in some regions.
PHOTO CREDIT: Bountiful Gardens Staff

Tomatoes, peppers, eggplant, and onions need to be started at least two months before you want to put them outside.

Melons, cucumbers, squashes, basil, okra, and sunflowers (as well as chia and quinoa in some places) can be started approximately a month before planting out. Germinate seeds at 65 to 75 degrees F. Tomatoes, peppers, melons, eggplant, and basil will germinate best with additional heat, such as the special plant heat mats you can get at a garden store. The top of a refrigerator, gas stove with a pilot light, or any electric appliance that is on for a long period will often add just enough warmth. You don't need anything above room temperature once the leaves appear. Use a good-quality soil mix or make one from good garden soil and compost in roughly equal parts. The price of potting soil is usually a good indicator of quality. Soil should be moist but not wet.

A good habit is to sow only part of your seed at first, saving the rest to plant a week or two later. That way, if the seeds come to harm in some way, you still have some. And when harvest time comes, you won't have all your produce at once, but can pick over a long period. This is referred to in books and catalogs as succession-sowing.

Garden crops need much brighter light than most houseplants. A sunny window will work, if all the seedlings can get light—you may have to turn the flat often. If the window "almost works" but the light is not quite enough, consider hanging a light just over the plants. A compact florescent (larger size) in an inexpensive droplight could be just the boost they need.

Many gardeners will need to use mostly artificial light. A florescent shop light or grow light fixture is not very expensive, uses little power, and works well as long as it is close to the plants—light loses strength quickly with distance. Hang your light on an adjustable chain or cord so that it can start 4–6" from the soil, and raise it as the plants grow (or start by putting the plants on blocks and lower them). Plants that are tall and thin, with lots of stem between leaves, need more light. If a plant gets bleached leaves, the light is too close.

Growth should be continuous—if it slows, feed the seedlings with compost tea or other mild liquid fertilizer. Check to see that the roots have not filled the flat or pot. Once the seedlings are as tall as the pot is deep and have several pairs of leaves, they should be pricked out to a larger container or transplanted into the garden.

If seedlings appear withered at soil level and fall over, they've been attacked by fungi and will not recover. This is called damping off. Here's how to prevent it:

  • Use a loose soil mix that drains well and doesn't compact.
  • Use Mycogrow or other mycorrhizal inoculant to prevent disease (the good fungi fight the bad fungi).
  • Maintain good air circulation.
  • Plant larger seeds in a hexagonal pattern so they are not crowded—use chicken wire as a guide for spacing. Smaller seeds can be broadcasted, but not too thickly.
  • Keep temperature around 65 degrees F.
  • Avoid overwatering—VERY IMPORTANT!

For about a week before transplanting to the garden the seedlings need to be hardened off. Set them outside an hour or two at mid-day at first, gradually lengthening into the cooler morning and evening hours. Make sure they don't dry out and protect them from wind and critters. Or put them in a cold frame and open the lid longer each day.

For an easy way to harden off seedlings and get a jump on the season, you can also use glass or plastic jugs with the bottom cut out placed over the plant. These are called cloches. You can also make little plastic tents over the plants.

Much greater protection from frost and cold is attained by using water to store the day's heat and warm the plant at night. You can use plastic bottles of water to surround your seedlings, or put bigger containers of water inside your cold frame, hoop house, row cover, or other enclosure. It's quite amazing how much more growth plants can make with water around them to store the day's heat and radiate it at night.

Related topics in this issue:
Redwood Seedling Flat Kits: get the same flats we use at Ecology Action.
DIY Seedling Flats: step-by-step instructions on how to put together a seedling flat kit
Ecology Action's Garden Calendar: a rough guide on what to do, and when to do it!
A free, online Seed-Spacing Calculator from Bountiful Gardens

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