When is the best time to plant butterfly bushes. I live in Cobourg.
Butterfly bush, Buddleia davidii, is a pollinator magnet, and so a welcome addition to the garden. Like most deciduous flowering shrubs, spring is the best time to plant, followed by autumn, before the first frost. Your part of the province is considered a Zone 6a, so you are on the edge of the hardiness zone of many of the Buddleia davidii cultivars available, although there is a good variety, with some hardy to Zone 5a. A butterfly bush hardy to Zone 5a would have a better chance of withstanding a harsh winter.
There are a few important considerations to keep in mind. Buddleia davidii, originating in Asia, has a well-deserved reputation as an invasive species in many parts of North America. If you are committed to planting a butterfly bush, you should consider one of the many newer cultivars available that are sterile, or nearly sterile, and so do not aggressively self-seed like older varieties. There is now a range of blossom colour available, and a range of sizes. These newer cultivars are far preferable in the garden.
Many of the larger nurseries have websites with detailed descriptions of the cultivars available including bloom colour and time, shape, size, and other features.