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 germination medium/potting mix, containers, proper temperature and moisture conditions, and adequate light.

Flower and vegetable seeds can be purchased at local garden centers. They're also available from mail-order companies. Mail-order sources include Stokes Seeds, Box 548, Buffalo, New York 14240 (; Park Seed Company, 1 Parkton Avenue, Greenwood, South Carolina 29647 (; Burpee Seeds, 300 Park Avenue, Warminster, Pennsylvania 18974 (; Harris Seeds, Box 24966, Rochester, New York 14624 (; and many others.

The germination medium should be lightweight, porous, and free of disease pathogens. Excellent germination media are commercially prepared soilless products, such as Jiffy Mix or Redi-Earth. Use a high quality, well-drained potting mix when transplanting seedlings into individual pots or cell packs.

Various containers can be used to germinate and grow transplants. Gardeners can purchase flats, trays, cell packs, 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 and their germination requirements largely determine 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, are commonly germinated in flats. However, they can also be sown directly into individual containers, thereby eliminating the need to transplant the seedlings. (As a point of reference, there are approximately 2,000,000 wax 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 are often available for fine-seeded annuals, such as begonias and petunias.

When sowing seeds in flats or trays, fill the container with the germination medium to within 1 inch of the top. Firm the medium, water thoroughly, then allow it to drain. Fine seeds and those seeds which require light for germination are sown on the surface of the medium and then lightly pressed into the germination medium. Cover all other seeds with additional medium to a thickness of one to two times the seed's 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. The medium can also be moistened with a rubber bulb sprinkler. The fine mist from the rubber bulb sprinkler will not disturb the seeds or 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 crop time (number of weeks from sowing to planting outdoors) for several popular flowers and vegetables are as follows: 10 to 12 weeks - geranium; 8 to 10 weeks - petunia and impatiens; 6 to 8 weeks - marigold, pepper, and eggplant; 5 to 7 weeks - tomato, cabbage, broccoli, and cauliflower; 3 to 4 weeks - cucumber, watermelon, muskmelon, and squash. If unsure of the correct sowing date, check the seed packet.

To insure a uniform moisture level during germination, cover the container with clear plastic food wrap. Flats can also be covered with clear plastic domes.

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 degrees F is adequate for the germination of most flowers and vegetables. Remove the plastic food wrap or dome 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 or cell packs when the second pair of "true" leaves appear. Containers with 2 or more seedlings 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. If using a commercial potting mix containing a slow-release fertilizer, fertilization should not be necessary. An application of a dilute fertilizer solution once every 2 weeks should be sufficient for those potting mixes that don't contain a slow-release fertilizer. Finally, harden or acclimate the seedlings outdoors for several days before planting them into the garden.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Horticulture and Home Pest News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on January 9, 2008. The information contained within may not be the most current and accurate depending on when it is accessed.