The Secret of success with Rhododendrons


By Ron Knight, Caron Gardens.

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 moving through 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.

  • Acidic soil

In areas where there is plenty of rainfall, the water solution between the soil particles is 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 the optimal chemical form for efficient absorption 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.

  • Protection from strong winds

Small-leafed, low-growing, clump-forming rhododendrons can survive on windy slopes above the tree line. However, most rhododendrons in the cold Himalayan climate grow best underneath tall trees that provide protection from strong winds.

The Pacific Northwest is rhododendron heaven because most of these environmental factors are easy to duplicate. 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 forest soil sample is composed of organic matter and sand, which provides 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. The garden is in zone 7 and in this milder kind of climate, over 95% of the rhododendrons benefit from maximum sunshine, which results in a higher percentage of blooms and a bushier plant habit. For the remaining 5% of rhododendrons (big-leafed and tender varieties) 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 Pacific Northwest 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, hot 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 optimal growing conditions:

1)      Thoroughly soak the rhododendron root ball for a few minutes in a wheelbarrow full of water.

2)      Dig a shallow yet wide hole. Try to stay as far away as possible from cedar trees because their roots are invasive and their leaves form a very dense canopy.

3)      Prepare a soil mix of approximately equal parts of mulch, steer manure, topsoil or compost, and peat moss. Stir these components together with the native soil in the hole and then add water. Never add mushroom manure because it is alkaline and full of salts. One of the best investments ($60) you can make in your garden is to mail a soil sample to Pacific Soil Analysis inRichmond  (604 273 8226) and have them send you recommendations for amending your particular soil.

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, grass clippings, or commercial peat moss. Moreover, bone meal is unnecessary and you need not add fertilizer for the first year.

7)        Water the area frequently during the first growing season, especially during the summer months. 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 doing some reading from the internet or taking a workshop offered by Caron Gardens, entitled: “How to Grow Great Rhododendrons”.