You may need to transplant a rhododendron for a number of reasons: seedlings are ready for the garden, container rhododendrons are rootbound, or your original garden plants have spread out immensely and are crowding other flowers and shrubs. Follow these tips to ensure successful transplanting of your rhododendrons.
Step 1: Select a Prospective Location to Transplant
If you are dividing an already-growing plant, pick a location within 6 feet. If you are introducing rhododendrons to your garden for the first time, choose a shady, well-drained location that faces directly north or east, or to the northwest. Avoid due south or southwest light lines as they can scorch leaves and flowers of your rhododendrons.
Step 2: Plan the Timing of Your Transplant
Transplanting should be done in the early spring before new leaf growth begins, or in the early fall after blooming so that plants can settle and stabilize in the new location before the arrival of frost and winter weather.
Step 3: Test the Soil at the Location
Do a soil test before transplanting to determine the acidity of the soil. Rhododendrons thrive in soil that tests as strongly acidic, with a pH reading of no higher than 5.5. The soil can be rendered more acidic by adding organic compost including oak leaf mold, peat moss, and pine needles. Mix the soil well with the organic material for each individual potting hole you dig and let it rest for an hour for more effective blending. If your soil is clay-heavy, construct a raised bed for the rhododendrons, and fill its base with gravel to promote effective drainage.
Step 4: Use Wide Planting Holes and Provide Adequate Space Between Plants
Broad planting holes are particularly important when moving plants from a container into the garden, and for transplanting maturing rhododendrons in the garden. The roots need to spread out immediately to acclimatize to the new soil and moisture conditions. Plant seedlings at least 3 feet apart, as they will grow speedily in the first season. Dig the hole at least 1 inch less deep than the rootball's measurement from crown to root tips. The root crown should remain above the soil surface.
Step 5: Disentangle and Soak the Root Ball Carefully
Disentangle some of the newest root growth gently from the rootball, so it can seek water and nutrition. Soaking the rootball after removing the plant from the soil or container will help it unknot and expand effectively. If the rootball is very dry, completely submerge it in water for half an hour in a suitably-sized container.
Step 6: Cover the Root Mass and Mulch
Press soil back in firmly around the root mass to help support the rhododendrons in an upright position. Place a light mulch over the surface, but a finger-width away from the root crown which needs air to continue growing. Level the mulch so it does not slope downward toward the root crown. If water fills the root basin and remains there, root rot will damage the root and breed toxic mold infections.