success with rhododendrons

By Ron Knight

WHAT'S THE SECRET OF SUCCESS with rhododendrons? Make them think they're growing in the Himalayas!

Most rhododendrons in cultivation are happiest when you provide them with the conditions that exist in the mountainous regions of Asia where their ancestors lived. These conditions include:

Lots of moisture and well-drained soil

Rhododendrons in the Himalayas live in a cloud forest, often on steep slopes. During their active growing season; they receive a nearly constant supply of moisture, yet water does not pool around their roots, driving out air. Coarse, well-drained soil allows rhododendron roots to absorb the large amounts of oxygen that they need for healthy growth.

Acidic soil

In areas where there is plenty of rainfall, the water solution between the soil particles is usually acidic. Rhododendrons must be grown in a soil like this, where the pH is between 5 and 6. In this pH range, dissolved nutrients are in a form that can be absorbed efficiently by the root hairs.

A mulch covering over the root zone

In the Himalayas, the soil under rhododendrons in covered by a layer of dead leaves, flowers, twigs, etc. This loose mulch helps keep the roots cool, prevents them from drying out, protects them from sudden changes in temperature, particularly in winter, and blocks out weeds. Hot, wet conditions around the roots, on the other hand, make rhododendrons more susceptible to fungus attacks.

Lightly filtered shade and wind protection

Small-leafed, low, clump-forming rhododendrons can survive on sunny, windy slopes above the tree line. However, although larger rhodos like good light, they need protection from strong winds.

The Sunshine Coast of British Columbia is rhododendron heaven because most of these environmental factors exist naturally. For example, at Caron Gardens, a rhododendron display garden in Pender Harbour, the native forest soil is acidic, with pH around 5. At least 70% of any soil sample is composed of organic matter and sand, which provide superb drainage even in areas that are not on hillsides. Silt and clay occur in very small amounts. Moss and a mulch of coniferous tree debris keep roots cool. Filtered shade and wind protection are provided by towering Douglas-fir and hemlock trees. In fact, the only environmental factor that is lacking is a constant supply of moisture. Even though the Sunshine Coast receives plenty of rain, it resembles a cloud forest (thankfully) for only a short time each year. Particularly during the summer, there can be several weeks in a row that are sunny and dry. Such conditions put stress on rhododendrons at the very time they are most in need of water for the production of new leaves and the next year’s flower buds. If you understand these environmental needs of rhododendrons and the conditions in your area, it is not difficult to grow superb specimens.

A newly planted rhododendron bed is in the foreground. Notice that the pink rhododendron is planted slightly above grade and is surrounded by mulch from a Sunshine Coast dump.

Image © Ron Knight

Larger leafed rhododendrons such as 'Burnaby Centennial' do well in filtered shade but prefer good light.

Image © Ron Knight

Rhododendrons thrive in Sunshine Coast woodland gardens.

Image © Ron Knight

Smaller leafed, low-growing rhododendrons such as R. russatum can tolerate sunnier and windier locations.

Image © Ron Knight