Growing Rhododendrons in a Northern Garden

Northern Exposure

By Barbara Rayment | Image: Barbara Rayment

The thing I miss most about Vancouver in the spring is the glorious and luxuriant blooming of rhododendrons. This, more than the common affliction of zone denial, is probably why I persist in experimenting with rhodos in the north-central interior, where growing conditions are just a little bit different.

Those of us who justify our winters with "yes, but it's a dry cold" have to work a bit harder to overwinter broadleaf evergreens in our gardens. Although there are a number of hardy rhododendrons, they are all extremely specific about their microclimate and soil needs. Most require soil with a high organic content - something not found in our relatively young glacial soils.

The rhododendron family is huge, comprising hundreds of species and probably thousands of named hybrids and selections. While most are evergreen, there are also deciduous species, including many of the familiar azaleas. They all have similar growing requirements; the main difference, for northern gardeners, is the deciduous varieties don't need the same degree of protection from desiccating winter winds.

What they all have in common is a need for consistently moist soil, well amended with organic matter. Rhodos and azaleas are shallow rooted, so moisture-retentive (but not water-logged) soil, regular watering and a good layer of organic surface mulch are all essential. They also prefer a very acidic soil; this means that peat is the ideal amendment, as it lowers our already mildly acidic pH levels to a suitable level while increasing organic matter. Look for sustainably harvested peat, to help preserve natural wetlands.

Compost is also an excellent soil amendment, of course, but the pH can be too high when used alone. Coco fiber is another sustainable alternative which adds organic matter, but it doesn't change soil pH, and there is a cost to the environment in processing, packaging and shipping it halfway around the world. Pine needles, harvested and baled for use as mulch in some parts of the U.S., would also serve the purpose, but their removal may be harming the ecosystem they were harvested from, and - again - they have to be baled and shipped long distances.

The pH of the soil can be lowered by use of sulphur, either in pellet form or in combination with other ingredients. It is not recommended that aluminium sulphate be used to acidify soil for rhodos as it is for blue spruces, because the aluminum can be toxic to the rhodos.

When planting rhodos, the soil for an area at least their mature width, and preferably twice that, should be heavily amended with the organic matter of your choice, to create a high percentage of organic matter and a low (acidic) pH. In heavy clay soils, it's necessary to

add gypsum (to break up the clay) and create raised beds. Rhodos simply will not survive if planted into heavy clay soil - I can testify to this from my early attempts and a misguided determination to grow them where I wanted, and not where they wanted.

Because rhodos are shallow rooted, regular watering is essential. An organic surface mulch, 10 to 15 cm (4-6 in.) thick, of aged wood chips, fine bark mulch, or some combination of this will keep the surface cool and the moisture in the soil where it belongs. The thick leathery leaves of the evergreen varieties take a while to show drought stress, which can be confusing to gardeners who have learned to read the more immediate feedback from soft deciduous leaves. It is often too late by the time drought damage becomes evident. (It should be pointed out that evergreen rhodos, like all evergreens, don't keep all their leaves forever - they shed some percentage of their oldest leaves every year, and this is quite normal, not a cause for panic.)