A Guide to Growing Seedlings Indoors

Get an Early Start on Gardening by Germinating Seeds Indoors

A Guide to Growing Seedlings Indoors

By Nancy Pierson Farris – If you garden north of the Mason/Dixon line, your last frost date may come after Easter. To harvest mature tender vegetables before fall, you need to start growing seedlings indoors about six to eight weeks before your last spring frost.

I garden in low-country South Carolina. My last frost comes in March; but by July, daytime temperatures top 95 degrees. That kind of heat stalls the bees, blasts the blooms, and scalds fruit on the bush. If I’m to harvest Fourth of July tomatoes (Burpee), they must bear in June. That’s why I start my garden activities soon after I pack up the Christmas decorations.

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How to Start Growing Seedlings Indoors

To have well-developed plants ready to set out after the last spring frost, I begin germinating seeds indoors in my greenhouse in January. By mid-February, I can set out cold-tolerant seedlings.

I find the ideal way to create the best garden soil mix is to make my own. I rake back leaf mulch or pine straw underneath trees on my property. Below the fresh layer, I find a partially rotted layer. I scoop that up and sift it through hardware cloth into a 2-1/2-gallon bucket. In my wheelbarrow, I add an equal amount of good garden soil and two quarts of perlite (which improves drainage).

Nancy displays some well developed seedlings ready to go into the garden.

When the soil is well mixed, I shovel it into 18″ x 20″ x 4″ flats, which my husband built with scrap lumber. Then I heat water in a tea kettle and a couple of large pots. When the water boils, I pour it slowly into the soil in the flats. I use about a gallon of boiling water per flat.

Sterilizing soil in this way does not destroy good bacteria, but it kills fungus that causes damp-off. Nothing discourages me more than coming to my greenhouse one morning to find seedlings which were green and growing the day before lying down, their stems rotted off at ground level. Pepper seedlings, snapdragons and periwinkle are especially vulnerable to damp-off.

After sterilizing the soil, I allow it to dry until it is just damp (usually overnight).

Pour boiling water through soil to kill fungus.

Meanwhile, I prepare labels and gather seed packets for each flat. I place these into numbered bags and include a list of what will be planted in each flat. With a permanent marker, I write a number on one end and one side of each flat.

With everything ready, it takes only a few minutes to mark the rows, place the labels, then plant each flat. I use the handle of my trowel to make a shallow furrow, then water lightly before I plant the seeds. I cover seeds according to their size; the soil on top should be about as thick as the seed itself.

My #1 flat will contain cold-tolerant plants: broccoli, cauliflower, cabbage, lettuce, snapdragons, nasturtium. I plant coleus in the #1 flat because I want them to have time to make good growth after I pot them up. In this flat I start three or four plants of Fourth of July tomato, which will be set into the garden early, under protective covers.

My #2 flat will have a heating cable (available at most garden centers). Into this flat, I will sow eggplants, peppers, and flowers which need bottom heat for germination. Even after four decades of gardening experience, I still double check the seed packets for requirements of each crop. Some need warmth, some—like petunias—need light to germinate (don’t cover those seeds).

After I sow my flats, I place a sheet of clear fiberglass on top, or wrap the flat in polyethylene. This holds the moisture in, but does not shut out the light.

I will sow my #3 and #4 flats two weeks later. These have main crop tomatoes like Rutgers, Roma, and Better Boy. Here I also plant flowers and herbs such as impatiens, celosia, salvia, petunias, sage, basil, and several kinds of marigolds.

When I plant similar varieties in the same flat — such as cauliflower, broccoli and cabbage — I find it expedient to plant lettuce or snapdragon in the rows between. Watering sometimes causes seeds to float and all cole crops look alike in the seedling stage. In spite of my precautions, sometimes a plant in the broccoli row forms a head, revealing that it is a cabbage.

As soon as most seedlings are up, I uncover the flats, and mist them daily to keep them moist. Cole crops germinate in about six days; most other things sprout in seven to 10 days; pepper seedlings may not show themselves for 15 to 18 days.

As seedlings develop second and third leaves, I pot up most so they can develop good root systems. Since my flats are deep, broccoli, cabbage, and nasturtiums remain in the flat for four weeks and are set directly into the garden. Cauliflower performs better for me if I pot it first and wait until seedlings are six or eight weeks old and severe cold weather ends.

Tender plants like tomatoes, peppers, and eggplants stay in the greenhouse until frost danger has passed. Exceptions exist, such as some cold-set tomatoes. I put out the Fourth of July tomatoes two weeks before the last frost. For protective covers, I use clear plastic jugs emptied of juice, with the bottoms removed. On sunny days, I remove the jugs because temperatures inside can get too warm. At night, I put the jugs back over the plants.

For potting, I have used peat pots, but now I use cheap paper or Styrofoam cups. With a permanent marker I write the name of a variety onto a paper cup, then I poke a drainage hole or two in the bottom. I use the same soil mixture with slow-release fertilizer added.

When I transplant to the garden, I break away the bottom of the cup and slide the top rim up around the plant stem, right at ground level. There it serves as a collar to keep cutworms from attacking my newly set seedlings.

While the plants are in the greenhouse, I use fluorescent lights to insure 10 or 12 hours a day of light. The fixtures are hung on chains so that I can reposition them as the plants grow, keeping the tubes about two inches above the leaves.

For the first two weeks after beginning the process of growing seedlings indoors, I water every other day with a fish emulsion mixture.

In the greenhouse, marigold seedlings set among other plants will repel white flies. In the garden, dwarf marigolds inter-planted with snap beans ward off bean beetles. Marigolds near okra and tomatoes may reduce the nematode population. Marigolds brighten my flowerbeds all summer. They even find their way into the kitchen: like nasturtiums, marigold blooms are edible and a bloom or two can be used to garnish a salad or vegetable tray.

Wherever you live, you can get your garden off to an early start by growing seedlings indoors. If you don’t know when the last expected frost date comes in your area, check with your local extension agent. Then count back eight weeks, and get your flats ready.

Originally published in Countryside and Small Stock Journal January / February 2009 and regularly vetted for accuracy.

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