Understanding and Growing Rhododendrons

  R. smirnowii beneath Austrian pines
R. smirnowii beneath Austrian Pines - Heritage Drive.

Rhododendrons and azaleas are long-lived plants, provided that the site selection and the soil preparation closely 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 acidic, largely organic, moist and well drained.


Basic Requirements - Some sun, or at least bright light is necessary to encourage bud formation and compact growth. They should be planted where sheltered from the prevailing winter winds which desiccate the foliage. Shade during the hottest part of the day in summer is appreciated. Shade in late February and through March, when the ground is frozen, is especially important as the strengthening sun will scorch the leaves.

Sites to Avoid
  • Do not plant too close to a reflecting wall or under an overhang
  • Avoid planting beneath shallow rooted trees such as maples
  • Do not plant in windy areas where snow blows away, leaving bare ground
  • Avoid areas of very heavy, dark shade

    Best Location - Overhead dappled light for most of the day is ideal for large, smooth-leafed forms. Scaly- leafed alpine forms and deciduous azaleas need more sun. 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 north west. The best companion trees are pines and oaks which are deep rooted and do not compete with surface feeding rhododendrons. Their annual leaf shed also provides excellent mulch which, in turn, decays to create acidic compost.


    pH - Acid soil is required, ideally 4.5 to 5.5 pH. Simple testing kits are sometimes available at garden centres and will provide an approximate range of acidity. A more accurate analysis can be done by laboratories through agricultural extension services, universities, or other horticultural organizations. Where alkaline soil exists, keeping the root system above the normal grade level is absolutely necessary to diminish the risk of rising pH levels in the soil, caused by the movement of sub-surface soil water. Line the planting bed with sulphur and incorporate sulphur in the mixture around the roots.

    Texture - Soil should be loose and crumbly with a very high percentage of humus. A minimum of 50 percent extra coarse chunky peat is recommended, plus other organic material such as composted oak leaves, shredded pine bark, decaying wood chunks, etc.; literally "forest junk". This material helps to maintain the 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 in drought conditions. Never over water. Mulch over the roots helps to retain moisture, but should not be so thick that it prevents a light rain from penetrating. Do not pack mulch close around the main stem of the plant as this can cause suffocation.

    Soil Drainage - Sharp drainage is critical. Rhodos are mountain plants, growing where there is always down grade 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 mound or berm, or a raised planting bed using a retaining curb 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.

    Air Drainage - Rhododendrons are best on slopes where cold air can drop away from the plants. Never plant them in a hollow and avoid flat areas where possible. Soil should slope away from the hot south sun so that the root system is kept as cool as possible.


    In preparing the planting bed, 50-60 cm (20-24") is an adequate depth. Loosen the compacted garden soil, removing at least 10-20 cm (4-8"), especially if it is a clay soil. Dig in coarse material to improve drainage. The finished mound should be high enough to keep most of the root system above the grade level as shown below.

    Recommended Panting Bed for Rhododendron

    In the highly organic mixture, dig a shallow hole, slightly deeper and wider than the root, add some peat and water 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 medium. Remove any covering around the root. If the plant has been container grown the root clump must be scratched and loosened on the sides and bottom in order to break any pot-binding and to expose the root endings. Dropping or sharply banging the root will also help to aerate the root clump. Set the plant so that the top of the root ball is at or even above the surface of the soil, never lower. This is especially important in the case of container grown plants which should have as much as one third of the root system above the soil surface. Backfill around the sides of the root to steady the plant but do not pack the soil too firmly.

    Flood the plant in with a strong jet of water form 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 8 cm (3") of mulch over the roots. Use pine needles, oak leaves, bark chips, etc. Ground covers or small pebbles can be used. Do not use pure fine peat as a mulch as it dries out to form a crust which prevents rain from penetrating to the roots.

    After Care - Do not allow to dry out, (especially the first season) but do not over water. Do not disturb the root surface; hand weeding is advised. Add extra leaf mulch in late fall. Extra winter protection such as pine boughs from Christmas trees will help to retain snow and afford wind protection and shade. Remove thick mulch when spring comes, to allow the ground to thaw, but retain a layer throughout the year, as the slowly decaying material provides nutrients and helps to retain moisture.

    Rhododendrons benefit from light applications of an acid-based fertilizer in early spring. Do not over feed, and avoid nitrogen feeding after mid June, as this may encourage late season growth which will not mature and harden before winter. A balanced fertilizer containing nitrogen in the form of ammonium sulphate or sulphur coated urea is recommended.

    Additional Tips - Do not plant rhodos within the root area of a Black Walnut tree, as the Walnut roots exude a toxin in the soil. Do not use aluminum sulphate to acidify the soil as the aluminum ions will build up and become toxic over a period of time. Ferrous sulphate can be used to lower pH quickly but does leach out. For longer lasting amendment, lift the plants and renew the soil with added sulphur and coarse peat.

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