Petunias are an easy to care for and colorful addition to the summer garden. Petunias are annual plants that do well in containers and in the garden. There are two types of petunias, according to horticulturists at the University of Rhode Island: grandiflora and multiflora. Grandiflora petunias, with fewer but larger flowers, are better suited for container growing than the multiflora. Petunia seeds germinate quickly and can be transferred to the garden as soon as temperatures remain 60 degrees F or above.
Fill the flat to within 1/4 inch of the top with seed starting mix and place it in a pan of water. Allow it to remain until the soil is completely wet.
Mix together 1/2 teaspoon of seed fungicide and 1 gallon of water and pour it over the soil in the flat. Allow the soil to drain completely. Use your hands to pack down the soil to remove air pockets.
Sprinkle the petunia seeds over the surface of the soil. Do not cover them. Place the flat in a plastic bag and seal it.
Place the bagged flat on the heat mat, set to 70 degrees F. The seeds need light to germinate, but not direct sun. If the soil appears to be drying, mist it with a misting bottle to keep it moist. The seeds should germinate in three to five days. At that point, remove them from the heat mat and take the flat out of the bag.
Place the flat in an area that receives bright light, but no direct sun. Allow the soil to dry slightly between watering. Stick your finger in the soil and if it feels dry to 1/2 of the surface, it is time to water.
Fertilize the seedlings with starter fertilizer diluted to half the strength recommended on the package.
Transplant the seedlings to small, individual pots when they have two sets of leaves. Place them in an area that remains at 65 degrees F. Fertilize the seedlings every two weeks with 1 ounce of 20-20-20 fertilizer, diluted in 1 gallon of water.
Transplant the young petunia plants into a sunny spot in the garden after all danger of frost has passed and the temperature is at least 60 degrees F. Add 2 pounds of 5-10-5 fertilizer to the soil in the planting area and mix it to a depth of 8 inches. Dig holes the same depth as the pots in which the petunias have been growing, and twice the diameter. Remove them from the pots and place the roots into the hole. Backfill with soil.
Water deeply (6 to 8 inches deep) when the soil is dry. Stick your finger into the soil, and if the top 2 inches feels dry, it is time to water.
Place a 2-inch layer of mulch around the base of the petunias. This will help keep the soil moist and the roots cool.
Pinch back the petunia plants if they become leggy. Remove flowers as they fade to encourage the plant to continue to bloom.