Petunias are a popular and versatile flowering annual that can be used in planting beds or a variety of planting containers. The question of when to plant petunias can depend on a multiplicity of factors, including the plant hardiness zone of your garden, and whether you intend to grow your petunias from seed.
Growing Petunias from Seed
Petunias are popular with many gardeners as they are easy to grow from transplants, but can prove more difficult if started from seed. Petunia seeds are infinitesimally small, and unlike many types of seed must be spread on top of the growing medium and need light to germinate. Germination takes seven to 10 days, and care must be taken to maintain optimum temperature and humidity conditions as the young seedlings emerge.
Ideally, the soil temperature should be held constant between 70 and 80 degrees Fahrenheit. Once the seedlings emerge and begin to become established, the growing medium should be allowed to become slightly dry between watering, but not so much as to allow the seedlings to wilt. Proper watering is critical at the seedling stage of development for petunias.
Petunia seedlings can be transplanted into trays or individual containers once they develop two to three leaves. This is usually 14 to 21 days after germination. When transplanting at the seedling stage, you should take care to handle the young plant by their leaves and avoid touching the stems or root cluster. After transplanting into individualized containers or trays, fertilize every two weeks and maintain the plants at 65 degrees Fahrenheit, if possible. Once the transplants become established, you can begin to “harden” the young plants to the outdoor environment by placing them outside on warmer days to acclimate them. You can move your petunias outdoors once all danger of frost has passed and the soil temperature in your flower beds is at least 60 degrees.