The easiest way to get started with perennials is to purchase nursery-grown plants at local outlets or through mail-order or online catalogs. You can be reasonably certain that local outlets will sell plants when it is a good time for you to plant them in your garden. Mail-order nurseries usually ship plants at the right time for planting in your area. (Mail-order outlets ship dormant roots of some species instead of live plants.)
The best time to plant perennials varies from species to species. In general, spring is a good time to plant perennials that flower in summer and fall. Late summer and fall is a good time to plant perennials that flower in spring and early summer. Some plants are fussy about when you plant them. Others are more forgiving.
Before planting, remove any weeds that may have cropped up in your previously prepared planting bed. Loosen the soil where you will plant the perennial and dig a hole large enough to accommodate the root system.
If the roots are pot-bound -- closely pressed against the side of the container and growing in circles around the inside -- tease them out so they can grow outward into the surrounding soil. If you don't, they may keep growing in circles and the seedling will remain stunted. In severe cases, you may need to use a pruning shears or a knife to make three or four cuts into the rootball. This will stimulate new root growth out into the soil. Pinch back any flowers and flower buds that have formed on the plant and perhaps a little of the foliage to compensate for the root damage.
It is important to plant most perrenials at the same depth as their container depth. Planting too deep or too shallow stresses the plants. If planting dormant roots, carefully follow directions concerning the proper depth of planting.
Water plants thoroughly after transplanting. Don't let the soil dry out for the first week or two while the roots get established. Check daily to make sure that the soil around the transplants is moist below the surface and that the plants aren't wilted. Mulching can help soil retain moisture.
Be sure to space plants properly. Refer to the plant label about specific spacing suggestions for the variety you are growing. (Use spread information in plant database as a guide.) It may take perennials several years to reach their full size. Consider growing annuals in between them until they fill out.
Fellow gardeners are often a good source of perennial plants, especially when they have extras to share when they divide plants. Make the most of gift plants by making sure that you get enough information from your friends about what the plant needs to thrive. Even if they don't know the genus and species, at least find out what soil and sun conditions the plant enjoyed before it was divided.
You can also grow perennials from seed, either starting them in pots or flats or sowing them directly in the garden. Compared with annuals, many perennials are slow or difficult to germinate, grow slowly when young, or have other special requirements. For example, some need to be sown in summer and exposed to the cold over winter before they'll germinate. Keep in mind that seed saved from your own perennials may not breed true. They may produce plants that have remarkably different flower color, size, or other characterisics from their parents. For more information on seed starting, see How to Grow Annuals.