Answer: You can plant container grown perennials any time you can dig the soil, although spring and early Spetember are the best times for planting most things. If you plant in mid summer, make sure to water them well while they are becoming established.
Soil preparation is the key to success with perennials, so take time to do that right. You might also want to look at a book or two about perennial gardening to help you get off to a good start. "Perennials for Dummies" by Marcia Tatroe ISBN 0-7645-5030-6 is a straightforward guide and includes planting plans and plant lists. It also details soil preparation and site/microclimate analysis.
Enjoy your new garden!
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