While garden centres and nurseries offer petunia bedding plants ready for planting in spring, it's possible to start petunias by planting seeds indoors. Plant petunias seeds in late winter, 10 to 12 weeks before the last expected frost date in your area. Depending on your climate, petunia seeds are usually planted between mid-February and early March. It's important not to start the seeds too early, as petunia seedlings kept indoors too long become leggy and unhealthy.
Any fine-textured, loose, well-drained planting media is appropriate for planting petunia seeds. Good quality commercial potting soil containing a mixture of fine bark, sphagnum moss and perlite are good choices, or you can opt to use those materials to make your own mixture. Avoid garden soil, which is too heavy, will become compacted, and won't drain.
Any sterile container, including margarine containers or plastic milk cartons, can be used to plant petunia seeds, as long as the containers have a hole in the bottom so moisture can drain. Celled plastic trays are convenient, as one plant is grown in each cell. Peat pellets or peat pots are useful because the pellets or pots are planted directly in the soil, which reduces stress to petunia seedlings.
Plant the seeds
To plant petunia seeds, moisten the planting media lightly, then sprinkle the seeds carefully on the surface of the planting media. The seeds aren't buried, but are pressed gently into the planting media, as soil will block light from reaching the tiny seeds. Spray the seeds lightly with water.
To germinate the seeds, place the container in bright light, but not in direct sunlight, which will scorch the seedlings. Normal room temperatures between 23.9 and 26.7 degrees Celsius (75 and 80 degrees Fahrenheit) are best for successful germination. Cover the container with clear plastic until the seeds germinate in seven to 10 days. At that time, remove the plastic and move the container to a sunny window, or place the container under a florescent light. If the seeds weren't planted in celled trays or peat pots, the seedlings are transplanted into celled trays or peat pots when each seedling has three leaves.
Care for seedlings
Once the petunia seedlings are transplanted, the soil is allowed to dry slightly between waterings. Petunia seedlings benefit from a water soluble fertiliser applied every other week. Dilute the fertiliser to half of the strength recommended on the label. Move the seedlings outdoors gradually after all danger of frost has passed, increasing the time spent outdoors every day. Once the petunias are acclimated, it's time to plant the petunias in their permanent homes.