Although most people buy petunias as young plants, starting petunias indoors for later transplantation in your garden is not difficult. Start your petunias indoors three to four weeks before you plan to plant the seedlings outside. Plan to plant outdoors as soon as the risk of hard frost has passed. That date will vary depending on your climate zone and other climactic factors.
Fill your peat sprouting pots to within an inch of the top with a commercial sprouting medium. Although simple things like peat, pearlite, or vermiculite may work, commercial sprouting soils have nutrients to help your petunias after germination.
Moisten the medium by submerging the containers in water until they become wet. Allow the containers to dry for 15 to 20 minutes before sowing the seeds.
Sow the pelletized petunia seeds as instructed on the seed package. Petunia seeds are very small and are very difficult to properly sow unless pelletized.
Moisten the pellets with a sprinkler and cover the pots with plastic wrap.
Place the pots in a warm location with light. Petunias require light to germinate; however, do not place the pots in direct sunlight. They germinate best in 75 to 80 degrees F.
Remove the plastic wrap as soon as the petunias germinate.
Place the petunias on a sunny windowsill where the temperatures will not drop below 60 degrees F. Keep the soil in your petunia pots moist, but not soaking wet.
Move the plants outdoors during the day a few days before planting to harden them off for outdoor planting. The young plants should be ready to plant outdoors after a few weeks of growing on the windowsill.
To plant outdoors, prepare your garden by digging down about 12 inches and turning the soil.
Add a couple inches of compost. Work the soil with a hoe and rake until the largest clump of soil is about the size of a pea.
Plant the young petunias by digging a hole about the size of the peat pots and dropping the pots into the hole. Space your petunia plants about a foot apart.
Backfill the hole and mulch around the petunias to discourage weed growth and help the soil retain water.
Water your petunias whenever the top 1/4 inch of soil feels dry.