Starting Seedlings Indoors

For many home gardeners, it's fun to get a head start on the upcoming garden season by starting seedlings indoors. Growing quality seedlings indoors requires high quality seeds, a well-drained, disease-free growing medium, containers, proper temperature and moisture conditions, and adequate light.

The growing medium should be porous and free of disease pathogens. Home gardeners can use commercially prepared soilless media, such as Jiffy Mix, or prepare their own by mixing equal parts garden soil, peat, and perlite. Homemade soil mixes should be prepared in the fall before the ground freezes and then stored in a plastic bag for use in winter. Homemade soil mixes should also be pasteurized before use. Pasteurization destroys weed seeds, insects, and disease pathogens in the soil mix. To pasteurize, lightly moisten the soil mix, place in a shallow baking pan, and bake in the oven at 180ûF for 30 minutes. Allow the soil mix to cool before using or storing in a plastic bag.

Various containers can be used to germinate and grow transplants. Gardeners can purchase flats, trays, pots, compressed peat pellets, and other commercial products. Cut-off milk cartons or plastic jugs, paper cups, and egg cartons can also be used to start seeds. Previously used flats, trays, and pots should be cleaned and disinfected before use. Wash the containers in soapy water, then disinfect them in a solution of one part chlorine bleach and nine parts water. Holes should be punched in the bottom of milk cartons, jugs, paper cups, and similar containers to allow for drainage.

The size of the seeds largely determines the type of container and sowing method. Fine seeds, such as begonias and petunias, are typically sown in flats or trays. After germination, the seedlings are transplanted into individual containers. Large seeds, such as marigolds and tomatoes, can also be germinated in flats. However, they are often sown directly into individual containers, thereby eliminating the need to transplant the seedlings. (As a point of reference, there are approximately 2,000,000 begonia seeds per ounce, while there are 10,000 marigold seeds per ounce.) For ease of handling and planting, some seed companies offer coated or pelleted seeds. Pelleted seeds may be available for fine-seeded annuals, such as begonias, petunias, and impatiens.

When sowing seeds in flats or trays, fill the container with the growing medium to within 1/2 to 1 inch of the top. Firm the medium, water thoroughly, then allow it to drain. Fine seeds are usually dusted on the surface of the seedbed, then lightly pressed into the surface of the growing medium. Large seeds should be covered with growing medium to a thickness of one to two times their diameter. After sowing the seeds, water the medium by partially submersing the container in water. When the surface becomes wet, remove the container from the water and allow it to drain. Watering from below prevents the washing of seeds on the surface of the medium.

When sowing seeds into individual containers, plant two or three seeds per container (peat pots, pellets, soil blocks, etc.). Place the containers in a flat and water.

The correct indoor sowing dates for several popular flowers and vegetables in Iowa are: late January geranium; late February impatiens and begonia; early March cabbage, broccoli, and cauliflower; mid-March pepper, eggplant, petunia, and salvia; late March tomato, marigold, and zinnia; and early April muskmelon, watermelon, squash, and cucumber. If unsure of the sowing date, check the seed packet.

To insure a uniform moisture level during germination, cover the container with clear plastic wrap or place in a clear plastic bag. Poke a few holes in the plastic to allow for some air circulation.

Set the container in bright light, but out of direct sunlight. Extremely high temperatures may develop if the covered container is set in direct sunlight. These high temperatures may adversely affect germination. A medium temperature of 70 to 75 F is adequate for the germination of most flowers and vegetables. Remove the plastic covering or bag as soon as germination occurs.

Once the seeds have germinated, move the seedlings to an area with slightly cooler temperatures and direct sun or place under fluorescent lights. Transplant the seedlings growing in flats into individual containers when the second pair of "true" leaves appear. Large-seeded plants that were sown two to three seeds per container should be thinned to one seedling per container. Destroy the weak seedlings by cutting them off with a razor blade.

Short, stocky, dark green seedlings are the best quality transplants. For best results, grow seedlings under fluorescent lights. It isn't necessary to have "grow lights" or a fancy light stand. A standard fluorescent shop fixture with two 40-watt tubes (one cool white and one warm white) works fine. The fluorescent lights should be no more than 4 to 6 inches above the plants. They should be lit 12 to 16 hours per day. Plants grown in a window often become tall and spindly because of inadequate light.

Thoroughly water the seedlings when the soil surface becomes dry to the touch. Fertilize weekly with a one-quarter strength houseplant fertilizer. Finally, harden or condition the seedlings outdoors for several days before planting them into the garden.
This article originally appeared in the January 16, 1998 issue, pp. 2-3.


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