When planting perennials keep two things in mind -- newly transplanted perennials don't tolerate extreme temperature changes and the plants need constant, even moisture to overcome transplant shock and produce new growth. The ideal planting time, then, is when daytime temperatures are mild, and the soil is cool and moist.
Hardy perennials, such as roses, lavender, daylilies and coneflowers, can be planted in the spring as soon as the ground is soft enough to work, although it's best to wait until after the last hard frost. Wait until May or June to plant more tender perennials. Planting perennials in the spring and early summer allows them to develop a strong root system before the soaring temperatures of midsummer.
Nurseries sell potted perennials throughout the growing season, and offer deep discounts in late summer and early fall. Fall planting is very effective in Southern climates with mild winters. In northern climates, you run the risk of freezing the plants before they've developed adequate root systems. Plant perennials no later than six weeks to one month before the first expected frost. Otherwise, perennials may not survive the first winter. (ref. 2, 3)
Spring and fall are the preferred times to plant perennials. High temperatures and dry conditions during the summer cause excessive transplant shock and growth is usually stunted for a few weeks. If you must plant perennials in the summer, plant them in the evening when temperatures are cooler. Water the plants thoroughly and keep them well-watered for the first two to three weeks. Mulch the soil with wood chips to keep it cool and conserve moisture.
When you plant perennials depends in part on whether you are planting bare-root or potted transplants. Bare-root perennials are dug from a green house or nursery field and shipped without soil. Keep the roots moist and plant bare root perennials in spring as soon as the soil is soft enough to work, but before new leaves emerge. Container grown plants are more forgiving, and can be planted throughout the growing season with proper care.