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How to Grow: Rhododendron
full sun, part sun, part shade
Bloom Period and Seasonal Color
Spring in colors such as white, pink, red, purple, and yellow
Mature Height x Spread
3 to 10 feet x 3 to 10 feet
native, attracts beneficials, fall color
Rhododendrons are stalwarts in the New England shrub border. They are one of the few broadleaf evergreen shrubs that have brilliant, spring blooming flowers and are hardy in your climate. I talked about deciduous azaleas (also in the rhododendron family) in a previous entry. The bright flowers come in a broad range of colors such as purple, pink and red. They are generally grouped as large-leaved (and sized) and small-leaved (and sized) types. In our climate they grow well in full sun as well as part shade. The dark evergreen leaves provide color in winter to contrast the browns and whites of that season. Some varieties have large, almost tropical plant sized leaves, while others have small leaves that turn a brilliant, reddish fall color.
When, Where and How to Plant
Rhododendrons can be hardy to zone 4 or 5. Select varieties hardy for your area. Purchase plants from a local garden center. Plant in spring to summer on moist, well-drained soil, humus-rich, acidic soil. Rhododendron flowers best with filtered light. Avoid hot, dry, windy sites. They may need some winter protection if grown in windy locations. Space plants 3 to 6 feet apart.
Keep rhododendron plants well watered. Apply mulch, such as pine needles, wood chips or peat moss, to keep the soil acidic, cool and moist. Fertilize in spring to lower the pH to 5.0 with an acidifying fertilizer meant for rhododendrons.
Regional Advice and Care
Prune rhododendrons after flowering in spring to shape the shrub and reduce the height. Don’t prune into old wood. Rhododendrons are slow to recover from severe pruning. Remove dead, diseased, or broken branches at any time. Protect exposed plants in cold climates by driving four stakes around the shrub and wrapping burlap around the stakes to buffer the drying winds. Don’t let the burlap touch the foliage or it may dry the leaves out.
Companion Planting and Design
In a mixed shrub border plant large-leaved rhododendrons, which are also large plants, with other evergreens such as mountain laurel. Plant these large shrubs on the corner of a building so they don’t block a view from a window. They grow well on the north side of garages and buildings. Plant the smaller sized and leafed varieties as foundation plants or in a mixed shade border with hosta, astilbe and ornamental grasses.
For large-leaved varieties that can grow to 10 feet tall, try the lilac-purple ‘Grandiflora’, white flowered ‘Catawba White’ or red flowered ‘Roseum Elegans’. For small leaved varieties the ‘PJM’ is probably the most popular. This shrub grows 3 to 6 feet tall and wide with light purple colored flowers and nice fall foliage color. Variations include ‘Olga’ which has pink colored flowers and ‘Molly Fordham’ has white flowers. These variations may not be as hardy as the traditional ‘PJM’.
Excerpted from my book, New England Getting Started Garden Guide.