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Rhododendron Growth and Care in Canada
Rhododendrons and azaleas are long-lived plants provided that the site selection and soil preparation recreate the conditions that exist where they grow in the wild. Their basic needs are protection from drying winds and hot sun, and a soil which is acid, largely organic, moist, and well-drained.
SITE REQUIREMENTS:
Some sun to encourage bud formation and compact growth. Shelter from prevailing winds which desiccate the foliage. Shade during the hottest part of the day in summer. Shade in winter when the ground is frozen, especially late February and March when the warming sun can scorch the leaves.
SITES TO AVOID:
Do not plant too close to c reflecting wall or under an overhang. Avoid planting close to shallow-rooted trees such as maples. Do not plant in windy areas where snow blows away, leaving bare ground. Avoid very heavy, dark shade.
BEST CHOICE:
Overhead dappled light for most of the day is ideal; or choose the north side of the house, fence, or other plantings, where they will get good early-morning light from the east, shade from the south during mid-day, both summer and winter, and with wind screening on the west and northwest. The best companion trees are pines and oaks which are deep rooted and do not compete with surface-feeding rhododendrons.
SOIL:
pH - Acid soil is required, ideally 4.5 to 5.5 pH. A sample of soil can be analysed by a government laboratory or a nursery.
TEXTURE:
Soil should be loose and crumbly with a high percentage of humus. A minium of 50% extra-coarse peat is recommended, plus other organic material such as composted leaves, shredded pine bark, decaying wood chunks, etc.; literally, "forest junk." This material helps to maintain acid conditions and the open texture allows air to penetrate to the root run after excess water has drained away. Oxygen in the root system is a vital factor in maintaining healthy rhododendrons.
MOISTURE:
Soil must be consistently moist but never soggy. Adequate humus should hold enough moisture between rains, but watering may be necessary during drought conditions. Never overwater. Mulch over the roots helps to retain moisture but should not be so thick that is prevents a light rain from penetrating. Do not pack mulch up around the main stem of the plant, as this can cause suffocation.
SOIL DRAINAGE:
Sharp drainage is critical. Rhododendrons are mountain plants, growing where there is always downgrade for excess water to drain away. If existing garden soil is very heavy and/or poorly drained, keep the entire root system above grade by creating a raised mound or a planting bed using a retaining wall such as logs, timbers, or rocks. A layer of gravel or other coarse material is advised under the planting mixture to carry excess water quickly away from the roots.
PLANTING METHODS:
In preparing the planting bed, 50-60cm (20-24") is adequate depth. In the highly organic mixture, dig a shallow hole, slightly deeper and wider than the root, add some extra peat , and water to the hole to create a wet slush. Rhododendrons have shallow root systems with many fine, hair-like roots held in a compact clump. It is important to be sure that the root tips are thoroughly in contact with the planting mix. Remove any container, burlap, or plastic, scratch and loosen the sides and bottom of the root in order to break any pot-binding, and expose the root endings. Dropping or sharply banging the root of a potted plant will help to aerate the root clump. Set the plant so that the top of the ball is at, or even above, the surface of the soil - never lower. This is especially important in the case of pot grown Rhododendrons. Backfill around the sides of the root but do not pack soil too firmly.
Flood the plant in with a strong jet of water from the hose in order to blend the soil from the root with the new planting medium. Allow the water to settle, then adjust the level of the root as necessary. Apply 3cm (1") of mulch over the root system. Use pine needles, oak leaves, bark chips, etc. Do not use pure peat as a mulch, as it dries to form a crust which prevents rain from penetrating to the root.
AFTERCARE:
Do not allow to dry out (especially the first season) but do not overwater. Do not disturb the roots. Hand weeding is advised. Add extra leaf mulch in late fall. Extra winter protection such as pine boughs from Christmas trees help to retain snow and afford wind protection and shade. Remove thick mulch when the spring thaw comes to allow the ground to thaw, but retain a layer throughout the year as the slowly decaying material feeds the plants.
Rhododendrons benefit from light applications of an acid-based fertilizer in early spring. Do not overfeed, and avoid nitrogen feeding after late June, as this encourages late-season growth which will not mature before winter. A balanced fertilizer containing nitrogen in the form of ammonium sulphate or sulphur-coated urea is recommended.
Vol. 97 no 27
© Murray's Garden and
Horticultural Services
Last updated:
March 6, 2008