The low-maintenance petunia provides dependable colour to flowers beds and containers from spring until fall frost. Petunias come in nearly any colour desired and they continue to flower even if ignored. The plants survive drought-stress and require minimal fertilisation. These annual flowers require replanting each year, from either seeds or purchased seedlings.
Indoor Seed Starting
Petunias take six to 10 weeks to grow enough for outdoor transplanting, so sow the seeds at least six weeks before the last expected spring frost in your area. Seeds germinate within two weeks but the young plants grow slowly. Petunia seeds require moist soil and light for germination. A location in a sunny window or under fluorescent grow lights provides the seeds and seedlings with the necessary brightness. The seedlings require water and light fertilisation during their long indoor growing period.
Transplanting Homegrown Seedlings
Preparing the petunias for transplanting approximately a week before your desired transplant date helps the plants adjust to outdoor conditions. Once frost danger is past, set the potted seedlings outdoors in a protected area. Gradually move them into direct sunlight over the next seven days. This process, called hardening-off, prevents sunscald and transplant shock to the young plants. Petunias are ready for transplanting once the plants are hardened-off and after the soil temperature reaches 15.6 degrees Celsius.
Transplanting Nursery Seedlings
Nursery grown seedlings are available at garden centres from spring through fall. You can plant these petunias at any time after frost dangers is past and once soil temperatures reach 15.6 degrees Celsius. It's preferable to plant in the cooler early summer so the newly planted petunias don't suffer shock from summer heat, but plants kept well watered usually survive the heat with minimal damage. Plant nursery-grown petunias through late summer. Once fall arrives the temperatures cool rapidly, so late-planted petunias are short-lived.
Petunias grow well as container plants. Hanging baskets and pots of petunias typically make an appearance at garden centres several weeks before the optimum planting time. Displaying the containers of flowers outdoors too early can lead to frost damage or plant death. Both home-planted and nursery containers displayed outdoors during warm spells in spring survive cold snaps only if they are brought indoors for protection. If you plan to transplant these mature petunias to a garden bed, do so when you would normally plant seedlings outdoors.