Perennials grown in containers may be planted any time during the growing season. Ideally, plant them so they can become established before dormancy and the onset of winter. In Colorado, bare root perennials are best planted in the spring.
Inspect plants prior to purchase. Look for plants whose new roots are fleshy and creamy white, not root bound or discolored, and whose stems are not leggy. Remove the container before planting. If the soil ball is entangled with masses of roots around the outer edge, carefully make shallow cuts (1/2 inch or less) in several places along the outside of the root ball to encourage roots to grow out into the surrounding soil.
To minimize root drying, don’t remove plants from their containers until just before planting. Store plants in a cool, shady area until planting time. Place them at the same soil level as they were in the container. Avoid deep planting because plant roots often suffocate when planted too deeply. Once plants have been placed in the hole, backfill around the soil ball. Water well, watching for soil settling. Fill spaces as needed after initial watering.