Normally, for our region, we can plant around May 10-15, but it may be a good idea to look at the weather over the next two weeks. If you have already planted and they predict cold nights, a good watering will protect you from the cold.
Ask an experienced garden centre or gardener friend about the frost-free date in your area. Depending on the cold tolerance of the flowers you intend to plant, use this last date as an indication. Cold season annuals such as pansies, snapdragons and potted plants can be planted several weeks before the last frost date. Tropical plants that are extremely sensitive to cold, such as impatiens and basil, must wait a few weeks after the frost-free date.
Keep an eye on the weather; some years, the weather warms up a little less than expected. As a safety measure, be prepared to cover your flowers in the event of unusual late frost. Remember also that the soil must warm up so that the annual plants can grow well. If you have raised beds – which allow the ground to warm up faster than the ground of the dug beds – you can plant earlier.