Toronto Master Gardener Factsheet    

Rhododendrons and azaleas are a diverse group of evergreen and deciduous acid-soil- loving plants. They vary in height from dwarf azaleas for the rock garden to full size rhododendrons at almost 6 metres in height.

Botanically all azaleas are called rhododendrons with no botanical or cultural difference between them. However, a working definition of azaleas is that they are mostly deciduous with funnel-shaped flowers; rhododendrons are usually evergreen with larger bell-shaped flowers in terminal clusters. There are exceptions to these rules.

Site Requirements

• soil which is acidic (pH 4.5 to 5.5)
• very organic, moisture retentive and well drained soil
• some sun to encourage flower bud formation and a well-shaped, compact growth habit - dappled sunlight is ideal
• shade during the hottest part of the day in summer and when the soil is frozen in winter, especially in late February and March when the warming sun can scorch the leaves
• shelter from prevailing winds which will desiccate the leaves or in winter blow away the snow leaving bare soil.

Note: many new rhododendron and azalea introductions are more tolerant of alkaline soils and sun than the older types.

Planting

The best trees to plant near are pines and oaks which are deep-rooted and will not compete for moisture and nutrients. Their needles and leaves help acidify the soil when they decompose. Avoid shallow-rooted trees like maples. If the soil is very alkaline, add powdered or granular sulphur to the planting area. Ferrous sulphate is good but avoid aluminum sulphate which is toxic to rhododendrons.

As rhododendrons have shallow root systems with many fine hair-like roots in a compact clump, a garden bed depth of 18 inches to 2 feet (45 - 60 cm) is adequate. The planting mix should consist of at least half peatmoss to help provide acidic conditions, allow air to penetrate to the roots and excess water to drain away. Loosen the side and bottom of the root ball to break any pot-bound roots and to free the root endings: it is important to be sure that these root tips are thoroughly in contact with the soil. Set the plant so the top of the root ball is at the surface of the soil, never lower. Water well. Use pine needles, oak leaves or bark chips to mulch the entire area of the root system.

After care

Do not allow the soil to dry out (especially the first season) but also do not overwater. Hand weeding is advisable to avoid disturbing the shallow roots. Add extra leaf mulch in the late fall. Extra winter protection from evergreen boughs helps to collect snow and provide needed wind protection and shade, or make a tent of burlap and fill it with leaves. Remove the winter mulch in early spring but retain a layer throughout the year. Apply a light application of a balanced fertilizer in the spring. Do not overfeed and stop using a high nitrogen fertilizer after June 30. Nitrogen is better in a sulphate rather than a nitrate form because the sulphur adds acidity too.

Further reading

• Rhododendrons in the Landscape. Sonja Nelson
• The Smaller Rhododendrons. Peter A. Cox

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Date revised: This factsheet was created prior to May 2005.