Are you pushing the hardiness zone of the plant, or does the plant typically get damaged in winter in your area?
Yes -> plant in spring (ideally by early June)
No -> Do you have plans to be away from home for any prolonged periods this summer, especially during the hottest part of the season?
Yes -> plant spring-early summer, or in early autumn (by mid September)
No -> plant any time from spring through mid September; pay extra attention to water needs if planting during hot and/or dry weather.