Petunias are one of the most popular flowers, often grown as annuals. Common name(s): Petunia Flower colours: Varies Bloom time: Spring, Summer, Fall Max reacahble height: Up to 10 inches Difficulty to grow:: Easy to grow Planting and care You start the sowing indoor in the late summer. Sprinkle evenly on the seed potting-mix. This potting mix should be soil-less. Press them gently without covering completely. Seeds will need sunlight to sprout. Spray some water and cover with the plastic to keep soil moist.