- 1). Fill a planting tray with moistened potting mix to within an inch of the top. Lightly firm the contents of the tray. Sprinkle seeds sparingly over the potting mix. Water the seeds with a fine mist from a spray bottle and cover the containers with plastic wrap. Set the trays in a bright place out of direct sunlight where the temperature remains between 70 and 85 degrees F.
- 2). Keep the plastic in place until the seeds germinate, which should take about 10 to 12 days, according to Iowa State University Extension. Once you see seedlings emerge, remove the plastic wrap and set the tray near a sunny window or under fluorescent lights and keep them at a temperature between 60 and 65 degrees F.
- 3). Transplant seedlings into individual peat pots once the seedlings have three true leaves. Allow the potting mix to dry between waterings. Fertilize petunia seedlings every two weeks with a diluted liquid fertilizer.
- 4). Harden off the seedlings for about a week prior to planting outdoors. Set them outside on warm sunny days but bring them in for the night, recommends the University of Minnesota Extension. Petunias are ready for planting outside when the danger of frost has passed and soil temperatures have warmed to about 60 degrees F.
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