If Vireya rhododendrons have started blooming, it’s the cool seasons in the subtropics. Vireyas are a subgenus of rhododendron, they are tropical shrubs originating from SE Asia to Australia. Many hybrids have been produced, some are very fragrant, and most are ideal for container growing on a balcony, on (or under) a tree, or in a shadehouse. In my experience, treating Vireyas as you would an epiphytic orchid, or Medinilla, really helps. Vireyas make good cut flowers and buttonholes (remember them?). If you start a collection, you can have them blooming over many months.
If you have never grown a Vireya rhododendron before, probably the easiest way to picture how to care for them is to picture epiphytic (tree dwelling) plants such as orchids or Medinilla growing in a shade-house, or an epiphytic cactus under the dappled shade of a tree.
Wild Vireyas are epiphytic or lithophytic (growing on rocks), just as many orchids and Medinilla do. Vireya rhododendrons have glossy leaves, are long flowering period and there are many with unusual colours. Many occur in highland forest – Australia’s indigenous Vireya, Rhododendron lochiae grows in typical conditions – cloud forest in the wet tropics where the conditions are cool to almost hot, damp and humid.
Vireya is a fascinating and distinctive section of Rhododendrons. They are mostly geographically separated from the rest of the genus, having only a few species growing on the SE Asian mainland, and they have diversified greatly in tropical highland and sub-alpine habitats from peninsular Malaysia, Indonesia, New Guinea, the Philippines, the Solomon Islands and Australia.
A few species of moth and butterfly have been identified as Vireya pollinators in the wild, but the most frequent pollinators are bats and birds. Since most Vireyas live in highland tropical forest and cloud forest, these environments are too cool for many insects to be active and so bats and especially birds have taken on the role as pollinators*. It is believed that this bird-rhododendron association is why there are a number of red and orange-flowered Vireyas at high altitudes. This bird-plant association also seems responsible for the recurrent bursts of flowering in Vireyas – their pollinators are active and need food all year round.
Insect pollinators, bees especially, are rather inefficient pollinators. Many gardeners find surprising, but the reason for this is bees are very efficient in grooming pollen off their bodies in order to harvest it for food. So there’s less left over to transfer to another flower for pollination to occur.
Birds feed on nectar but they do not harvest pollen and so large quantities can be transferred to another flower. Not only are birds efficient pollinators, they also have the ability to keep warm in the cool, highland conditions where Vireyas occur.
Since the last period of glaciation was just 10-11,000 years ago, the tree line we see in the tropical highlands where these rhododendrons grow has only been at its current elevation for the same period of time. Not only have birds and Vireyas co-evolved, they have done so in a relatively short geological timeframe.
Vireyas are being successfully grown around Australia, evidence they adapt well to cultivation in a wider range of conditions than in nature. Essential conditions include a frost-free climate, an acidic growing medium, excellent drainage, dappled shade, mulching when grown in the ground, and an occasional prune to shape. Like orchids, Vireyas respond to regular but dilute fertiliser during the warm seasons.
The ideal position has dappled shade protected from hot western sun and heat radiated by walls and fences, and shelter from hot, drying winds. Highland areas, which are cool, but not freezing suit most varieties, and in coastal regions protection from salt-laden sea wind is important.
Well composted, deep, loamy soil is wonderful for growing conventional rhododendrons, but with Vireyas their root systems need aeration as much as drainage. In this respect, vireyas more resemble epiphytic orchids and cacti than conventional rhododendrons.
Bark based orchid and Vireya potting mixes break down into fine particles which impede aeration and drainage after a year. This is also a problem with bark mulches. Mulching with pine needles, hoop pine bark mulch, pine bark fines or leafmould works.
Repot container grown Vireyas each years as you would an orchid. Shake off the fine particles before they gather to form a sludgy layer. Repot using fresh orchid mix. Perlite and charcoal are excellent additions to both potting and planting mixes because they are porous and provide the aeration and drainage necessary for Vireya root systems. Unlike bark, perlite and charcoal do not decompose nor do they lose their porosity.
Like all rhododendrons, Vireyas need regular moisture, but if the ground doesn’t drain or becomes stagnant the roots will rot. Buy a pH test kit so you can confirm the soil – or the growing medium – are acidic, between pH5 and pH6. (how to do this is explained in the kit). If your soil is alkaline or sodic (saline), you will have to grow Vireyas in containers.
Equal parts of acidic garden soil, leafmould, horticultural sand, coarse perlite and charcoal make a good planting mix. If you don’t make leafmould, use potting mix prepared for acid-loving plants (aka ‘Ericaceous’ potting mix).
Plant no deeper in the ground than they are in the pot and plant into mounds 45cm high, or a raised bed.
Stand potted Vireyas on wire mesh benches, or on gravel. Raised beds and mounds will improve drainage, but it is the perlite and charcoal that enhance aeration so plants produce their extensive, shallow, fibrous root systems.
In my experience, treating Vireyas as you would an epiphytic orchid really helps:
* Don’t pot them any deeper than they were in the last pot;
* Do put heavily chlorinated tap water into a watering can and allow the chlorine to outgas overnight before use;
* Provide bright but filtered light, shelter plants from desiccating winds or salty breezes;
* Use orchid potting mix, slow release fertiliser sold for orchids, and grow them in terracotta orchid pots and you’ll be off to a good start.
Here are some basic tips for in ground cultivation of Vireyas: just don’t forget the long term value of adding perlite and charcoal for encouraging root development.
A good source of vireyas in SE Queensland is Camellia Glen Nursery, but you can find even find them in hardware stores from mid-winter onwards.
6th July 2020
* Wilson P, Wolfe AD, Armbruster WS, Thomson JD. 2007. Constrained lability in floral evolution: counting convergent origins of hummingbird pollination in Penstemon and Keckiella. The New Phytologist176:883–890.
Muchhala N, Thomson JD. 2010. Fur versus feathers: pollen delivery by bats and hummingbirds and consequences for pollen production. The American Naturalist175:717–726.