Cultivation of Vireya Rhododendrons|
Vireya Rhododendrons, like any other plants, have specific
cultural requirements if they are to flourish and give pleasure
to the grower. Fortunately, their demands are relatively modest
and providing these basic needs are met they will provide
enjoyment for many years. General guidance notes are given
for each of the categories listed below but local growing
conditions should also be taken into account. It is worth
bearing in mind that the aim is to re-create, as far as possible,
the environment experienced by the plants in their natural
In the wild, the majority of vireyas are found growing either
as epiphytes, clinging to branches and trunks high in the
tree canopy, with only small deposits of moss and humus covering
their roots; or terrestrially, often in crevices on steep
cuttings. Both of these aspects provide excellent drainage
after the frequent downpours prevalent in these habitats and
this is undoubtedly the most important consideration when
growing plants in cultivation.
There are many "recipes" used by growers today
but a suggested growing medium that is suitable for a wide
range of vireyas can be made up from equal parts of coarse
peat, fine grade bark and pumice. This will produce a compost
of open consistency that will hold sufficient water to meet
the plants immediate needs but without risk of water-logging
and at the same time allow air to circulate around the roots.
Other ingredients can be substituted where appropriate, including
perlite, tree-fern fibre or bracken, coarse river sand or
grit (a type suitable for ericaceous plants) or leaf mould.
Ideally, the mix should be slightly acidic at around pH5.5
and magnesian limestone can be added if required to lower
the pH level.
Vireyas growing in their natural environment receive rain
almost daily although most of this drains rapidly away and
little is held in the small amount of compost in which the
plant is rooted. In cultivation, plants should be thoroughly
watered, preferably with rainwater, and then the compost allowed
to become barely moist to the touch before watering again.
During the cooler winter period when plants are less active
the compost should be kept somewhat drier.
The weight of a potted plant is often, with experience, a
good guide as to whether or not water is needed however, if
unsure, it is better to withhold water rather than risk water-logging
which could lead to root rot and the subsequent demise of
the plant. If a plant becomes too dry the surface of the leaves
may turn dull. In this case, the foliage should be misted
and the plant kept in humid, shady conditions and watered
only sparingly until there are signs of recovery. Do not soak
the compost as the roots will already be damaged from drying
out and excess moisture will cause them to rot, thereby killing
Vireya growers have implemented many different feeding regimes
over the years using a wide variety of fertilizers and whilst
there have been varying degrees of success, there are several
points upon which nearly all agree. Regular light applications
of fertilizer are most beneficial, rather than high dosage
fast-acting feeds, and it has also been noted that vireyas
respond well to regular foliar feeding.
Care must be taken not to overfeed as this can cause unsightly
leaf-tip burn and also damage the root system of a plant.
If there is a build-up of salts in the compost from heavy
feeding, the pot should be thoroughly soaked so the salts
can leach away.
Proprietary acid plant foods should be avoided, growers instead
ideally using a fertilizer that is high in nitrogen, calcium
and magnesium but with low potash, potassium and phosphorus
Pinching & Pruning
'Pinching' refers to the practice of removing the growing
tip of a branch to encourage the formation of two or more
new growth breaks from that point. If carried out regularly,
particularly when a plant is young, it can lead to a more
densely branched and compact bush. Whilst pinching may delay
the onset of flowering, the numbers of blooms produced will
be increased as there will be more branches on which buds
can form. Feeding the plant at the time of pinching out the
emerging growth usually helps the plant to produce more new
Some vireyas can be rather "leggy" in habit but
most plants will eventually fill out from lower down a bare
stem given time and space to develop. Pruning should be undertaken
with some caution and if severe should be carried out over
a period, allowing the plant time to recover. Feeding beforehand
is often worthwhile to ensure the bush is growing strongly
and afterwards the compost should be kept slightly drier than
usual until new growth emerges.
Temperature & Humidity
When grown in a temperate climate, vireyas will not generally
stand freezing temperatures although if exposed to only a
short, light frost there may be some re-growth from the base
of the plant. Lowland species and their hybrid progeny ideally
prefer a minimum night temperature of 15° Celcius (59°
Fahrenheit) although most will cope with less. If a plants
leaves turn red in the winter months it is often a sign of
stress due to the temperature being too low.
Species from the cool montane forest regions are the least
demanding and will thrive with a minimum night temperature
of 8° Celcius (46° F). Again, two or three degrees
lower during the winter will not adversely affect the health
of a plant, particularly if reasonable daytime temperatures
are achieved, although an extended cold spell may cause it
to become practically dormant until the arrival of warmer
On the whole, varieties with larger, shiny leaves are more
heat tolerant than the small-leaved, higher altitude species
that find it hard to cope with high summer temperatures, making
them difficult to grow in many areas.
As far as possible, temperatures should be kept below 27°
Celcius (81° F) to avoid stress, although given high humidity
vireyas readily cope with higher temperatures and at such
times regular spraying is most beneficial, at the same time
ensuring there is good air circulation around the plants.
Light & Shade
In their natural habitat in the tropics, vireyas experience
equal 12 hour periods of day and night with very little seasonal
variation throughout the year. Such consistently high light
levels are certainly not a feature of the temperate climate
in which many vireya growers live, with extended periods of
low light during the winter months increasing rapidly at the
onset of spring. In these conditions it is sometimes necessary
to provide temporary shading on particularly warm, bright
days to give the plants an opportunity to acclimatise.
It is generally considered good practice to position plants
where they will receive full sun in the morning but be given
the protection of dappled shade to avoid the full intensity
of the afternoon sun, especially during the summer months
when it is possible the leaves may be scorched. Conversely,
too little light can have an adverse effect on the production
of flower buds. Botanic institutions often use full-spectrum
high intensity discharge lights suspended above plants in
glasshouses to supplement the amount of light they recieve
during winter and similar, smaller models are now available
for use by the enthusiast that may prove beneficial in certain
Pests & Diseases
Vireyas, in common with other rhododendrons, are generally
pest and disease free providing their day-to-day needs are
met. Mildew can be a problem when temperatures are high and
should be dealt with quickly by spraying with fungicide. If
possible, the growing environment should be adapted to ensure
there is good air movement around the plants. Small orange
spots of rust can occur on the underside of foliage and infected
leaves should be removed to avoid spread. Mealy bug, scale,
aphids and thrips have all been known to attack vireyas but
a healthy plant should withstand their attentions. If the
outbreak is severe, spraying with an appropriate insecticide
should bring control.
Occasionally a plant may collapse for no apparent reason
and this can most often be attributed to either the presence
of vine weevil larvae in the compost, seen as small white
grubs that feed on the roots of the plant and eventually prevent
it from taking up water and nutrients; or alternatively may
be due to an outbreak of Phytophthera, a type of fungus that
attacks the root system - the first visible symptom being
wilting foliage that does not respond to watering. This problem
is more prevalent in high temperatures and by the time it
is evident, it is too late to save the plant.
Beds, Pots & Baskets
Local conditions will to a large extent dictate the way plants
are grown - frost-free areas allowing outdoor display beds,
whilst for many growers pot culture is the only practical
method. Fortunately, a restricted root run does not present
any difficulty for vireyas as they are often found in the
wild growing with only a shallow covering of soil, perhaps
in a rock crevice or a fork in the branches of a tree.
Whichever method is adopted, excellent drainage is of paramount
importance (see 'Growing Medium') and when planting in a raised
bed the plant should be placed on the surface or in a shallow
hole and the compost drawn up just sufficiently to cover the
root ball. It is essential to ensure that the plant is positioned
no deeper than it was in its container as this invariably
proves fatal. This applies equally when re-potting.
Growing in pots has the advantage of portability however
vireyas should not be over-potted as there is a danger of
the compost turning sour, with a consequent detrimental effect
on the health of the plant, whereas under-potting carries
few risks. With regard to the type of container to use, plants
in clay pots may need watering more frequently than those
in plastic pots however are less likely to be over-watered.
Taller specimens are perhaps better suited to clay pots, benefiting
from the extra stability provided by the weight of the pot.
Whatever is chosen, ample provision should be made for drainage.
Hanging baskets are particularly good for displaying those
vireyas with a more sprawling or pendulous habit as the branches
can be encouraged to hang down, echoing their epiphytic tendencies.
A wire basket lined with moss and filled with a suitable potting
mix is also a most attractive way of presenting many of the