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Starting Seeds Indoors

submitted by Don Janssen, UNL Extension Educator

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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 also be pasteurized before use.

Various containers can be used to germinate and grow transplants. Gardeners can purchase flats, trays, pots, compressed peat pellets, and other commercial products. Previously used flats, trays, and pots should be cleaned and disinfected before use.

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.

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. Place the containers in a flat and water.

The correct indoor sowing dates for several popular flowers and vegetables 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;
  • 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. A medium temperature of 70 to 75 degrees 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.

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 resource was added January 2005 and was updated February 2013. It appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement


University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office

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