Caring for your perennial plants starts when you put them in your car. Avoid extended exposure to extreme heat or cold - minimize the amount of time they are in your car trunk. Prevent wind damage by covering your plants if they are in a pickup truck or trailer for transport. Monitor the moisture level in the container when you get it home. Keep the soil moist enough that the plants do not wilt, but do not keep the soil soggy.
Plant them in the ground* as soon as you can and check the soil moisture around the roots regularly during the first few months. The composted bark mixture in which they have been grown in the container can dry out very quickly, even if the surrounding soil is still moist. Most perennial plants will also benefit from a 1"-2" layer of mulch over the root zone to reduce weed competition and to moderate temperature and moisture variations.
*Prepare the soil by digging up the area where you plan to plant. Dig an area that is at least twice as wide as the container whenever possible to loosen the area for growing roots. Incorporate compost or other organic matter into the entire planting area, if needed, and mix well. Add a small amount of starter or slow release fertilizer to the bottom of the planting hole and cover with about an inch of soil. Loosen the root ball to expose the roots and trim any that are broken before placing the plant into the hole. Soak the hole and the root ball and then replace the soil around the roots and firm it in place. Water again to settle the soil and minimize air pockets. The level of your plant when you are finished should be the same in the garden as it is in the container, or slightly higher, if drainage is not good.
Remove broken or damaged portions of plants and cut off old blooms to prolong the beauty and bloom season of your plants. If plant foliage becomes unsightly during the growing season, cut plants back to healthy leaves and stems or to within an inch or two of the soil, fertilize lightly and water well to promote new growth. Plants that have become too tall or rangy may also be cut back to promote fuller growth, although this will delay flowering if done before they have bloomed. Remove old growth after plants have gone dormant in late fall or in early spring to make way for new spring growth.