Choosing Rhododendron Plants
Choosing rhododendrons is largely matter of deciding on size,
flowers, foliage and flowering season, while always considering your growing
conditions. Sometimes other aspects may also be important. For instance, you may
want fragrance or a special growth form, but mainly it is size, color and
foliage that matter most.
Deciding on the size of plant is not just a matter of knowing its height and spread; the general growth
habit is equally important. Densely foliaged plants often look better than open growers but open plants
allow light to penetrate and allow the planting of woodland perennials and small shrubs near them.
Consider the overall effect -height, width and foliage cover -when deciding if a plant is suitable.
- Flowering season
- Apart from the vireyas, most rhododendrons have a fairly set flowering season. The exact flowering time
of a particular species or cultivar will vary with the latitude and from year to year, but the progression
from early- to late-flowering plants through the course of the season is much the same every year.
Temperate-climate rhododendrons develop their flower buds in the fall, overwinter in bud, then
flower from late winter to early summer depending on the plant. Flowering is initiated by temperature
and day length. This is most evident in the evergreen azaleas: the early-flowering Belgian Indica
hybrids will flower as soon as their buds are mature and often show the odd bloom in the fall, but
Satsuki azaleas are more day-length dependent and show few blooms until the spring days are over 12 hours long.
If you live in a frost-free climate, early-flowering azaleas and rhododendrons can be relied upon to add
color from mid-winter. But very early-flowering rhododendrons are difficult in frosty areas. The
flowers of even the hardiest plants are tender and few will tolerate lower than 28.4°P (-2°C). Unless
you can protect them, it is best to avoid very early flowerers.
Most rhododendrons will have finished flowering by the time of the summer
solstice. Indeed, in mild areas the flowering season will largely be over within six weeks of the spring equinox.
The later the flowers open, the more likely it is that their display will be shortened by the
increasingly strong spring sun. Some shading, at least from the hot afternoon sun, is vital if the blooms are
to last. Later flowers also have to compete for attention with the
roses and early summer-blooming perennials.
By choosing rhododendrons with flowering seasons that correspond with the amount of shelter
you can provide from late frosts and early summer sun, it is possible, even in reasonably cold areas, to
have a four-month flowering period. In frost-free climates this can be extended to up to eight months
because, provided they escape frost, the Indica azaleas will start flowering in the fall and continue
on through winter.
- There are rhododendrons in almost every shade. All that is lacking is a true blue. There are plenty of
mauves and purples that masquerade as blues but there are no gentian-blue rhododendrons.
Certain colors and styles of marking tend to predominate in each of the main divisions. Red,
white, mauve and soft yellow are the predominant colors of the true rhododendrons; deciduous azaleas
tend towards yellow, orange-red and red; while white and deep pink to magenta-purple are the most
common colors among evergreen azaleas.
There are no yellow evergreen azaleas nor are there any deep blackish-purple azaleas, evergreen or deciduous.
Rhododendrons may be single colors or flushed and/or marked with secondary colors. They often
have conspicuous contrasting spots in the throat of the flower that seldom develop into a conspicuous
flare. Occasionally the flowers are edged with another color.
Hybridizers have not failed to notice these differences and over the years many attempts at
crossing between the groups to extend the color range have been made. While we now know that it
is genetically impossible to produce yellow-flowered evergreen azaleas by hybridizing with yellow
deciduous azaleas, much has been achieved and genetic manipulation offers the prospect of an even
greater range of varieties.
- 'Fragrantissimum' is probably the best known of the scented rhododendrons. Obviously, the name and
the age of the cultivar (it was raised before 1868) have a lot to do with it. Unfortunately 'Fragrantissimum' does not really deserve such widespread recognition. It is a straggly, untidy plant that is really
only suited to growing as an espalier unless severely trimmed each year, yet it does have a distinctive spicy scent.
It is one of a group of rhododendrons known as edgeworthii hybrids, after R. edgeworthii, a species
found in northeast Burma, southeast Tibet, northern India and southern China. Rhododendron edgeworthii
is similar to 'Fragrantissimum' except for a felty indumentum and a slightly stockier growth habit.
The flowers are white flushed with pink, as are those of most of its offspring.
Most of the R. edgeworthii hybrids were raised between 1860 and 1890 and they tend to be similar
to one another. Among the most common are 'Princess Alice', 'Countess of Sefton' and 'Suave'.
There are many other fragrant rhododendrons, mostly species in the Maddenia subsection, such as
R. johnstoneanum, R. maddenii, R. nuttallii, R. lindleyii and R. dalhousiae.
While the fragrant rhododendrons (mentioned above) tend to be rather frost tender, the royal family of
fragrant rhododendrons, the Loderii hybrids, are considerably tougher. These magnificent plants will
tolerate temperatures down to around -4°F (-20°C). However, they grow to at least 8 ft (2.5 m) high, so
allow plenty of room. As with most of the fragrant
rhododendrons, white to light pink shades predominate in this group.
Many deciduous azaleas are fragrant, most particularly those with R. occidentale parentage, such as
'Delicatissima' and 'Exquisita'. While few evergreen azaleas have any noticeable scent, those with a
'Mucronatum' background, such as 'Alba Magnifica' and 'Fielder's White' are usually lightly scented and
can be very effective when they are mass planted. The colors are usually white to mauve.
There are also several fragrant vireya hybrids, most of which have R. jasminifiorum somewhere in
their parentage. These usually have long, tubular, white to light pink flowers.
- Flower buds might seem an unusual aspect to consider, but often they are attractive and as they are
usually clearly visible right through winter they ought to be considered when choosing plants. Buds
can be rounded, pointed, tailor squat; they can be smooth, dusty or covered with fine indumentum; and
best of all, they carry the promise of spring flowers.
Even the most spectacular rhododendron's flowers are only open for a relatively brief time. In terms of
year-round appearance, the foliage is far more important than the flowers. Rhododendron foliage is
every bit as variable as the flowers and can be just as beautiful too, so take the time to consider the foliage
when choosing your plants. Of course, it is hard to ignore the foliage unless you are buying a deciduous
azalea out of leaf but it is surprising how often poor foliage is ignored for the sake of a month of pretty flowers.
- Most rhododendron leaves are roughly oval in shape with a rounded or slightly pointed tip. Leaf size,
however, varies enormously, though it is usually related to plant size, rainfall and temperature. Tiny alpine
species, such as R. intricatum, have leaves as small as 1/2 in (10 mm) long, but that is not surprising as they
are only very small plants and couldn't support large leaves. Also, large leaves wouldn't last long in the
bitter cold and high winds of the alpine zone.
The largest leaves belong to R. sinogrande, native to areas of very high rainfall. Measuring up to 28 in
(75 cm) long, these leaves are leathery, glossy, deep green and heavily veined. Its rounded yellow flower
trusses are very attractive but they pale into insignificance beside the foliage. Several other
members of the Grandia subsection -R. grande, R. macabeanum, R. magnificum, R praestans
R. protistum -have very similar foliage, but it is seldom more that just over half the size of a mature
R. sinogrande leaf.
- Sun tolerance
- It is often said that you can tell the sun tolerance of a rhododendron by the size of its leaves. To a large
extent that is true. Small-leafed plants will with-stand more sun and often need some exposure, but it
doesn't mean that you have to restrict yourself to small-leafed plants in sunny positions.
Sunburn usually appears as a browning in the center of the leaf and is most common in very
sheltered sites or if the soil or atmosphere is dry. Keeping the root zone cool and moist is paramount
if you want to grow large-leafed rhododendrons in sunny positions, and this will help to raise the
atmospheric moisture too.
If you suffer from high summer temperatures combined with low humidity, large-leafed species
should be grown in light shade.
- Rhododendron leaves come in all shades of green, often with purple, red or bronze tones. Alpines tend
to have glaucous (bluish) or purple-tinted leaves
while those from mild, wet climates usually have the deepest green foliage.
The best colors are often seen in the new growth, which may be quite different from the mature
foliage. This coloring, especially the silver tones, is often caused by a fine indumentum that wears away
as the leaf ages. The reddish-brown pigment seen in young leaves is thought to afford tender foliage some
protection from sunburn. R. protistum var giganteum and R. nuttallii offer striking examples of red new growth.
- Fall color
- Deciduous azaleas often display brilliant fall foliage color. This starts as a yellowing and progresses
through orange to bright red and finally deep blood red. The intensity of the colors is largely
dependent on the fall climate: warm days and cold, but not freezing, nights promote the best color.
The spring leaves of evergreen azaleas also color before they drop. Some fall at the yellow stage but
others turn red before dropping. The foliage that remains over winter is often intensely colored and
can be quite a feature. Azaleas with R. kaempferi backgrounds usually show the best winter color.
- Indumentum adds to the character of a plant and tends to be a feature that is more prevalent in the
species than the hybrids, although R. yakushimanum hybrids are often well covered. Species with
indumentum that could be considered a feature include: R. bureavii, R. degronianum, R. pachysanthum and R.
lndumentum is often restricted to the undersides of the leaves where it is seldom seen. Such
undersurface indumentum is more conspicuous with plants that can be looked up at from below. The
Grandia subsection species, such as R. grande, R. macabeanum and R. sinogrande, all have very large
leaves with a conspicuous, silvery buff indumentum.
Some rhododendrons have aromatic foliage. This may not be noticeable unless the foliage is crushed,
but on damp or warm days the scent, often cinnamon-like, can be quite strong. Rhododendron campylogynum, R. cinnabarinum, R. glaucophyllum and R. hippophaeoides
have noticeably aromatic foliage.
A few rhododendrons have attractive reddish brown, peeling bark. It is rarely a feature to rival
their flowers or foliage, but it is an extra, adding to the appeal of plants that might otherwise look
rather drab in winter. Rhododendron barbatum, R. cinnabarinum, R. hodgsonii and R. thomsonii have
interesting bark, and R. arboreum has distinctive
gnarled bark that sometimes peels to reveal a reddish undersurface.