One of the jewels of our garden is the Rhododendron Walk and the rhodos blooming in the Sino-Himalayan Garden. Along the Rhodo Walk, Roy planted species in alphabetical order, along with their hybrids, but this classification is no longer enforced.
There are over a thousand species of this plant, mostly found in Asia, but several are North American natives. In fact, the state flower of Washington State is Rhododendron macrophyllum. The genus name is from the Greek for 'rose tree.'
Updates 2012 from Gardener Walks Beside the road (Bed 122) is R. forrestii which is 45-50 years old. A few years ago the gardener in the Sino-Himalayn Garden removed 110 layers and has planted some along the bank. They will be a mass of colour in five years. Rhodos from a nursery take time to adjust; layers that have grown on their own adapt better. In the mid-1980s to the 1990s we used herbicides, and the plants declined. Four years after we stopped, the soil rejuvenated and we now see many seedlings
The gardener also noted that another form of protection for rhodos is indumentum (the fuzzy hairs on the underside of the leaf) which also reduces insect damage. Hybrids are attacked more by weevils than species are.
The smaller-leaf rhodos can take more exposure to sun if there is enough moisture. They are buried in snow in winter and have snow melt in summer. The larger the leaf, the more shelter the rhodo needs.
Rhodos are very surface-rooted; the lower half of the root ball is dead and can be removed when moving a large plant, but the weight of the root ball is necessary for balance. This is why some of the larger plants obtained from other collections lean.
Erica's Original Notes Rhododendrons are a fascinating ancient family of plants which belong to the Ericaceae. Most are found in the Northern Hemisphere; over half the varieties are native to China. In 1740, when Linnaeus was cataloguing plants, there were only 11 known varieties of rhododendron; now, more than 200 years later, there are over 10,000 new varieties.
The western world became excited about rhododendrons when the early botanist-explorers returned from Asia with the lovely large-flowered plants, alas too tender to survive without a greenhouse. However, in 1850 the small-flowered but hardy Rhododendron catawbiense was discovered in North America; then the fun began, crossing this hardy soul with its tender Asian counterpart, resulting in the wealth of glorious hardy varieties we have today. Our Garden collection consists of hybrid rhododendrons arranged around their parents and planted alphabetically. Our sister university botanic garden collection consists of rhododendron species only, with no hybrids.
In the Sino-Himalayan area of the Garden, we have an interesting local collection grown from Himalayan expedition seed. There are three rhododendrons native to B.C.: R. macrophyllum, R. albiflorum and R. lapponicum.R. albiflorum grows 5 to 6 feet, with creamy-white flowers, abundant in the mountains, and also on the west coast of Vancouver Island at 800 feet elevation. R. lapponicum (Lapland Rosebay) is a smaller, compact plant with purple flowers found in the north. R. macrophyllum, one of our finest native shrubs which is protected, has large leaves and pink flowers and is seen along the highway in Manning Park.
It is the showy flowers of these plants that attracts people, but to the specialist it is the leaves that come in all sorts of shapes, colours and sizes that are just as important.
On the north side of the main rhododendron path, we have a grouping of azaleas. At one time they were considered to be in a different genus. There are two major groups of azaleas. The semi-evergreen or evergreen azaleas were received originally from Japan when E. H. Wilson introduced the beautiful kurumes. Other azaleas are deciduous, some of which have two growth spurts, one in the spring and the other in the summer, when the leaves are smaller and coarser. The azalea planting is arranged in such a manner as to show how the hybridization of these plants developed, starting at the beginning with some of the ancestral species, such as R. viscosum(swamp honeysuckle), R. nudiflorum(Pink Star), and R. japonicum; the last one is a major influence in the development of the modern azalea; it is usually found with a harsh orange colour, with tubular-shaped fragrant flowers, and is the parent of R. mollis. The following is a list of the well-known hybrids:
GhentHybrids are a popular group of azaleas, arising from hybridization originally done by Belgian tradesmen 150 years ago and also in England. They are distinguished by their usually fragrant, long-tubed, honeysuckle-like flowers.
Knap Hill or Exbury Hybrids were developed by the nurseryman Anthony Waterer of Knap Hill in England and also by Lionel de Rothschild at Exbury; these are characterized by their trumpet-shaped, usually scentless flowers in a wide range of colours.
Mollis Azaleas originated in 1873 from selections of R. japonicum by Belgian Louis van Houtte and later crossed with R. molle. Their flowers are large, scentless trusses of intense colouring.
Occidentale Hybrids are mainly the result of crossings of R. occidentale with R. molle and others; these are recognized by their later flowering and delicate pastel colours.
Rustica Hybrids are a group of double-flowered azaleas produced by crossing double-flowered forms of Ghent azaleas with R. japonicum. They are of compact habit with attractive, sweetly-scented flowers.
More notes from Erica: Azaleodendron is an oddity first created by Dean William Herbert (second son of the Earl of Caernarvon who had time on his hands in those leisurely days!); he crossed R. maximum with an azalea, R. viscosum. The result is a sterile hybrid which doesn't know if it is a rhododendron or an azalea, especially when you look at the leaves!
R. Ponticumwas used for grafting until recent times. It was used extensively in Britain as a shelter belt but has naturalized.
Indumentumis the woolly or hairy covering of foliage, usually on the underside.
Lepidotesare the orange-brown scales carried on leaves and also at times on stems and buds.
Stamens are one of the main identifying points between rhododendrons and azaleas, the former having 10 or more stamens and the latter usually have 5 stamens.