Container-grown plants may be transplanted into the garden at almost any time during the growing season, or they may be set aside for a few weeks if they are watered and protected from excessive sun, heat, and cold. The best time to plant perennials is spring or fall, when the weather is cool and moist. Perennials in larger (quart-to-gallon) pots are ready for the garden, while plants in smaller pots benefit from being repotted until they are larger and strong enough to withstand the transplant into the garden.
1. To plant, dig a hole in a well-prepared bed just deep enough to accommodate the container, and about twice as wide. With a new bed, add soil amendments as you turn the soil; when adding a plant to an existing bed, mix compost and a slow-release organic fertilizer into the soil and backfill.
2. Slip the plant from its container, and gently tease apart the roots. If the plant is root bound -- the roots are circling round the outside of the soil -- cut four 1-inch-deep slices down the sides and across the bottom of the root ball.
3. Set the plant into the hole at the same depth it grew in the pot. Refill the hole with compost-enriched soil, patting the soil into place so no air pockets remain. Water thoroughly.