Perennial plants that come back year after year can be added to the garden any time of year if the ground is not frozen and if the plants are purchased growing in containers. During the hot summer months it is more difficult to add new plants as you must keep the roots moist but you can still have a lovely perennial garden if you wait until June or even July to get started. Improve the soil first. Then chose plants for the area based on the sun exposure and type of soil you have. A local nursery can help you with this. Next remove plants from the pots they were growing in at the nursery and place into planting holes at the site. Water, add some Osmocote fertilizer and keep the soil moist as the new plants send out roots. You can watch some YouTube videos that I have made with PlantersPlace to learn how to properly plant and design a perennial garden. Just put my name and gardening topic into the search bar under YouTube. Welcome to gardening with perennials and Keep Growing, Marianne Binetti