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.

Success With Rhododendrons

by Ron Knight
Images © 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. 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.

Larger leafed rhododendrons such as 'Burnaby Centennial' do well in filtered shade.

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.

Filtered shade and wind protection

Small-leafed, low, clump-forming rhododendrons can survive on sunny, windy slopes above the tree line. However, most rhododendrons grow best underneath taller plants that provide partial shade and protection from strong winds.

Rhododendrons thrive in Sunshine Coast woodland gardens.

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. Here are 7 steps that will ensure your rhododendrons receive the four basic requirements discussed above:

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

1. Thoroughly soak the rhododendron before you take it out of its container.
2. Dig a shallow yet wide hole, preferably in an area with partial shade. Cedar trees are not the best choice to provide shade because their roots are invasive and their leaves form a very dense canopy.
3. Prepare a soil mix of approximately equal parts of bark mulch (free from the Sunshine Coast dumps), steer manure or topsoil or compost, and peat moss. Stir these components together with the native soil in the hole and then add water.
4. Use a knife to score the root ball vertically in several places around the edge if the roots have started to grow in circles. Otherwise, just fluff up the roots gently.
5. Plant the rhododendron slightly above grade and surround the root ball with the soil mix. Do not stamp down the soil around the plant. (If you’re on rock or clay, make a raised bed so that the rhododendron will be planted in a mound of the new soil mix above the hard, poorly draining material.)
6. Cover the area to within a few inches of the stem with about 3 inches of loose mulch. Almost any material (bark, pine needles, leaves, straw, gravel, etc.) will do. Don’t use sawdust or commercial peat moss, however.
7. Water the area frequently during the first growing season, ensuring at least one inch of water per week. If you have a large garden, a drip irrigation system will save hours of hand watering and ensure that moisture goes directly to the roots and not to the weeds.

Planting and watering your rhododendrons is not the end, by any means. In order to maintain superb-looking specimens, you will need to learn to prune, transplant, deadhead, fertilize, and protect them from pests and disease. Consider taking workshops such as those offered by Caron Gardens: 'How to Grow Great Rhododendrons' and 'How to Select and Design with Rhododendrons'.

Ron Knight is a retired biology teacher, past-president of the Vancouver Rhododendron Society, and a confirmed rhodoholic. Ron and his wife Carla are the creators of Caron Gardens where they display over 450 different rhododendrons and companion perennial plants in a natural woodland setting on Hotel Lake. They also sell award-winning rhododendrons and use their garden and classroom to offer workshops which are advertised in the Pender Harbour Community School newsletter and the SCRD magazine. Detailed information on several upcoming workshops is available online. You can contact Ron at 604-883-9807 or by e-mail at carongardens@telus.net.