by BRIAN MINTER
April 10, 2006
Rhododendrons - they really are one of the finest broad-leafed evergreens in our gardens. They are also a plant about which folks have many misconceptions. When we think of Rhododendrons, we usually conjure up images of lush foliaged varieties smothered in huge trusses of blossoms. These types are indeed the most popular and for the most part, thrive in zones six to eight where the temperatures seldom drop below -12°C. However, according to the respected rhododendron authority in Oregon, Ted Van Veen, over 800 species and over 10,000 named varieties grow within 800 miles of the North Pole and as far south as the sand dunes of Florida. The well-known Rhododendrons growing in Manning Provincial Park are just one example of the ruggedness of some varieties. Although some species may survive in many parts of the province, it's really the West Coast which provides the best home for these lovely plants.
Rhododendrons are easy to grow in the Pacific Northwest, but far too many folks have problems because they really do not understand their needs. As another renowned Oregon rhododendron expert, Harold Greer, points out, Rhododendrons are foraging plants, and they won't tolerate some situations.
First, according to Greer, they must have a constant supply of moisture. Plants set out under huge trees often lose the battle for water and die because few folks realize that large trees can draw as much as 300 gallons of water a day during periods of warm weather. Rhododendrons need access to moisture, especially during cold or hot weather to prevent desiccation of their lovely foliage.
Secondly, Rhododendrons need well drained soil. Slow draining soil or high water tables are the kiss of death for Rhododendrons. Phytophtora, or root rot, is a serious concern in Rhododendrons, and little can be done to save a plant once this disease sets in. This is not to say Rhododendrons must be grown in sandy or gravelly soil, quite the contrary. They love soils rich in organic matter, peat and bark, but any water must drain away quickly.
The third requirement for Rhododendrons, and this is a very important one, is aeration. It is essential that rhododendron roots have access to oxygen. I always like to mix fir or hemlock bark mulch into the planting hole to provide adequate aeration.
Once you understand the soil requirements of Rhododendrons, it's then up to you to find a plant to suit your particular location. Most Rhododendrons have a hardiness rating. This rating, often given in terms of zones, indicates the minimum temperature a well matured plant can take without serious injury. In the eastern Fraser Valley, for example, the cold northeast winds dictate that H2 and H3 varieties are more likely to survive than some of the less hardy ones. Varieties like 'Vulcan', 'Trude Webster', 'Rocket' and 'Cosmopolitan' will all tolerate temperatures of -26°C with minimal winter damage. Similarly, the small leafed varieties like ‘PJM’, ‘Ramapo’ and ‘Purple Gem’ are very hardy.
Folks are still confused about the amount of sun a rhododendron can tolerate. I always use this rule of thumb: the larger the leaves, the more shade it will tolerate; conversely, the smaller the leaves, the more sun it will tolerate. Unless a rhododendron has exceptionally large leaves, it has no business being in the shade, otherwise it becomes leggy and really quite unsightly. When you hear that some Rhododendrons do not like the sun, it means that they should not be planted against a south or west facing wall with intense reflected heat, unless this situation can be alleviated by planting a small shade tree to act as a screen between noon and 3:00pm to protect the rhododendron from the sun's hottest rays. Out in the open, however, Rhododendrons do just fine, especially if they are surrounded by complementary plantings.
When planting new Rhododendrons, it is essential to very gently loosen the roots of container grown plants to encourage them to take off in the new soil. Field grown, burlapped Rhododendrons should have the sack left on to prevent the rootball from falling apart.
As the many varieties of Rhododendrons are now coming into bloom, it's a golden opportunity to visit your local garden shops to choose some of your favourites. Remember: Rhododendrons always look best in groupings, either with other complementary plants or planted by themselves in groups of threes or fives. From the tiniest miniature 'Impeditum' to the huge 'Anna Rose Whitney', I'm sure your landscape will benefit from these wonderful plants.