Time Frame

• Iowa State University suggests starting petunias from seed indoors 10 to 12 weeks before the last average frost date for your area.
  
  

Preparation

• Prepare planting containers by washing with soap and water and spraying with a solution of one part bleach and nine parts water. Fill pots with a lightweight seed germination mix, and submerge in water until the soil surface is wet.
  
  

Seeding

• Sprinkle the tiny petunia seeds on the soil surface and gently press into the soil. The seeds require light for germination, so leave them uncovered. Water the seedlings with a fine mist.
  
  

Germination and Planting

• Place containers in a location with bright, indirect light and temperatures of 75 to 80 Fahrenheit. Cover pots with plastic wrap to maintain constant moisture and mist when necessary. Petunia seeds usually germinate in 10 to 12 days. Grow seedlings in a sunny window or under lights, and pot up when plants have two true leaves. Once danger of frost has passed, harden petunias off outside for a few days before planting in the garden.