While growing the adult petunia is relatively easy, beginning your plants from seeds can be a challenge. The key is to provide enough water, and enough light, to encourage your seeds to germinate and grow. Begin this project 10 to 12 weeks before you plan to plant your petunias. The seeds take a long time to develop.
Choosing Seeds and Planting
Look for seeds that are 1 year old. Seeds that sit on shelves for years do not tend to germinate well. Petunia seeds are incredibly small and difficult to handle, so make sure to do your planting in a well-lit area. Choose a plastic pot or tray with several holes in the bottom for drainage. Fill the pot with standard potting soil up to ¼ inch from the rim. Water this soil until it is damp, but not soggy. Sprinkle your seeds on top of the soil, and mist them with water. This water will help them work themselves down into the soil. Cover the pot with plastic wrap, and place it in a bright location. Do not place the pot in direct sunlight, but choose a bright room. This room should also be warm, between 75 and 80 degrees, for the best results.
In about 7 days, your seeds should be sprouting. Remove the plastic wrap, and place the trays in direct sunlight, in a slightly cooler room (about 65 degrees). The plants have no root system at this stage, so you'll need to water them daily. When each plant has three to four leaves, break the petunia cover into several smaller parts. Plant each portion in its own pot, with potting soil. Thin each stand so each petunia is 1 to 2 inches apart. Add a standard flower fertilizer to your water, and water these plants once per week. Gradually, the plants will grow tall and strong enough to be transplanted into your outdoor garden.