Plant petunias either as seedlings or start from seeds. If you start them from seed, start them indoors at least 10 to 12 weeks before the spring planting date. This means during winter, start them indoors to make it in time for planting them in spring.
They thrive in fertile, moist but well-drained soil. Use plenty of organic matter like well-rotted manure, garden compost, or other soil improvers. If your soil does not drain well, you can add some sandy soil to improve.
Growing Petunias Annuals And Perennials – Planting