Petunias have been one of the most popular flowering annuals for years. Their popularity can be attributed to several desirable traits. Petunias are easy to grow, bloom reliably all summer, and are available in a wide range of colors, flower forms, and growth habits.
Most gardeners buy petunias at their local garden center or greenhouse in spring. However, petunias can also be started indoors in late winter. Sow petunia seeds indoors about 10 weeks before the intended outdoor planting date. (Petunias should be planted outdoors after the danger of frost is past.) Late February or early March is an appropriate sowing date in Iowa.
A commercially prepared medium, such as Jiffy Mix, is a good germination medium. Containers used for starting seeds should be clean and have drainage holes in the bottom. Previously used containers should be washed in soapy water and then disinfected by dipping in a solution containing one part chlorine bleach and nine parts water.
Fill the container to within 1 inch of the top and then firm lightly. Next, moisten the medium by partially submerging the container in water. When the surface becomes wet, remove the container, allow it to drain for 15 or 20 minutes, and then sow the seeds. The medium can also be moistened with a fine spray from a sprinkler.
The seeds of petunias are very small. (There are approximately 250,000 to 300,000 seeds per ounce.) For many home gardeners, sowing the fine petunia seeds is difficult. Fortunately, pelleted petunia seeds are often available. Pelleted seeds are seeds coated with a material to make them larger and easier to handle. Carefully sow the seeds on the soil surface, and then gently press the seeds into the germination medium with a pencil or a small block of wood. Since petunia seeds require light for germination, don't bury them in the germination medium or cover them with additional material. After sowing, thoroughly moisten the medium with a sprinkler or by partially submerging the container in water.
Uniform medium temperatures and moisture are required for optimum seed germination. Place the container in a warm (75 to 80 degrees F) location in bright light. To maintain uniform moisture levels, place a piece of clear plastic food wrap or plastic dome over the container. Do not set the covered container in direct sunlight. The high temperatures which may develop in direct sunlight may inhibit or prevent germination. Petunia seeds should germinate in 7 to 10 days.
Remove the plastic food wrap or dome as soon as germination occurs. Then place the seedlings under fluorescent lights or in a sunny window. Fluorescent lights should be no more than 4 to 6 inches above the plants and should be left on for 12 to 16 hours. Temperatures should be 60 to 65 degrees F. Transplant the seedlings into plastic cell packs, peat pots, or other containers when the seedlings have 3 true leaves. To produce stocky plants, continue to grow the plants in a cool location under fluorescent lights or in a sunny window. Allow the potting mix to dry between waterings. If using a commercial potting mix containing a slow-release fertilizer, fertilization shouldn't be necessary. An application of a dilute fertilizer solution every 2 weeks should be sufficient for those potting mixes that don't contain a slow-release fertilizer. Finally, harden the seedlings outdoors a few days before planting into the garden.