How to Grow Petunias
Decide if you want to try and germinate some petunia seeds indoors to get an early start on the growing season. (Germinating petunia seeds can be challenging due to their minute size.)
To start indoors, buy some petunia seeds (preferably purchase pelleted seeds, which are easier to work with).
Approximately one month before spring, start germinating your petunia seeds. Fill up each planting cell with seed starting mix, and water thoroughly, saturating the soil.
Press a few petunia seeds into the soil with your fingertip, being careful not to remove the seeds as you press them into the soil. Do not cover with soil, instead use plastic wrap and place it over the planting cells; this aids in keeping both warmth and moisture maximized.
Place the planting cells in an area that receives plenty of light and warmth, (approximately 70 to 80 degrees F) but not in direct sunlight. Germination time varies, but generally 4 to 7 days later you should see seedlings emerge.
Take off the plastic wrap as soon as you see seedlings emerge. Place them in an area that is about 10° cooler (60 to 70 F). Deborah Brown, at the Department of Horticultural Science at the University of Minnesota Extension, suggests that as soon as petunia seedlings emerge, place the seedlings 6 inches from a fluorescent light for 16-18 hours a day, until they are ready to be transplanted outdoors.
Transplant your petunia seedlings when they're approximately 3 inches high. Brown also suggests that before planting your petunias outdoors, you should "harden" them off; this is a process by which young seedlings are slowly acclimated to being outdoors by placing them in the sun each day, for a few days prior to transplanting them permanently outside.
Once you're certain there will be no more frosty cold nights, choose the location in your garden where you want to plant your petunias, or if you want to plant them in hanging baskets or in containers, like barrels. Petunias like a lot of sun, and depending on the variety you are growing can require a lot of room to grow (such as spreading petunias). So choose an area which is conducive to the variety you are growing.
Turn over the soil with a shovel or a trowel. The University of Rhode Island Landscape Horticulture Program suggests working into the soil 2 pounds of 5-10-5 (5% nitrogen, 10% phosphorus, 15% potassium) fertilizer or one pound of 10-10-10 (10% nitrogen, 10% phosphorus, 10% potassium) dry fertilizer per 100 square feet, to ensure both vigorous growth and plentiful blooms. After you've turned over the soil, water the area thoroughly.
Depending on the variety you are growing, space hybrid petunias (such as hybrid grandiflora and hybrid multiflora) 12 to 18 inches apart. Dig holes that are slightly larger than the planting cell. Fill up each hole with water, letting the water drain.
Remove a petunia from the planting cell by pushing upward from the bottom. Petunias do not like their roots disrupted so use care when placing the petunia into a hole. Replace the soil you removed when you dug the hole, gently firming the soil as you go. Carefully water each seedling thoroughly. To keep moisture in, place garden mulch around each one of your Petunias to the depth of approximately 2 to 3 inches.
To care for, and maintain, your petunias, the University of Rhode Island Landscape Horticulture Program also suggests that when your petunias appear dry, water them to a depth of 6 to 8 inches. This, however, depends on growing conditions such as soil, weather and how much mulch you have spread around your petunias.