Journal ARS Article
Vol. 47: No. 2: Year 1993
Tropical Rhododendrons for the Home and Greenhouse: Vireyas in Cool Climates
Richard Chaikin, DDS, MScD
It has been said that if you have owned five houseplants for as long as one year, then you can grow vireyas. That is, if you know how.
The plants all do very well in pots year round. We use only white standard pots in order to keep the root ball as cool as possible. The pot size is chosen so that the roots will not remain too moist for a long time. The stock plants and freshly rooted cuttings are grown in a south-facing greenhouse. However, my private collection was recently grown in a high-rise apartment building in the Boston area where the windows all face east-southeast. We have had as many as fifty-five vireyas there in one room at the same time.
David Leach once wrote that vireyas "provide an astounding vivid array of flower shapes and colors. There are single bell-shaped flowers that grow in incandescent, scarlet cascades; 8-inch trumpets that resemble gigantic Easter lilies, some in color combinations one could only describe as psychedelic; spidery clusters of white tubes that flare abruptly into little stars at their tips; and trios of long curved cylinders. Many are fragrant, and virtually all can be grown in 6-inch pots for at least several years. In cultivation, they tend to bloom most heavily from November to April, when the flowers are especially welcome, and they tolerate night temperatures as low as 45°F - an energy saving dividend for windowsill or greenhouse.
"The vireyas are noted for the lapidary purity of the color of their flowers, unflawed by any admixture of blue, and for their bold, Oriental contrasts. There are plangent oranges and yellows, some with purple accents; red and pink combinations; crimson and apricot. The whites are immaculate and enveloped in some of the most enticing of all floral fragrances. The range of reds, from the pale pinks through rose and scarlet to deep crimson, is complete. An additional attraction of these equatorial rhododendrons is the intensification of their colors as the flowers age. Instead of fading, an appealing yellow with an orange flush, for example, becomes a strong, saturated orange" (Vireya Rhododendrons," David G. Leach, Horticulture, The Magazine of American Gardening, Jan. 1987, 30-35).
Some vireyas have very strong colors and so are difficult to place in an indoor landscape effectively. On the other hand, the pale pastels of 'Pink Delight', 'Star Posy', 'Robert Bates' or 'Kisses' blend very well. They vary quite a bit in size from the 6-foot 'Narnia' to the 3 to 4-inch Rhododendron anagalliflorum or R. womersleyi. The length of bloom varies, but many varieties have twelve months of bloom; 'Pink Delight' is an example. Vireyas are never in full bloom like standard rhododendrons. You may have flowers, new growth and dormant branches all at the same time on the same plant. Therefore, cuttings may be taken at any time. The trusses of flowers can be from 2 inches to 8 inches in diameter, and the flowers can be from 1/2 to 8 inches.
Vireyas are terrestrial by choice, but are also epiphytic by necessity. If there is inadequate light they will end up as epiphytes on tree ferns, which I have replicated in my greenhouse. They can also grow on top of logs lying on the ground, as I have also replicated. It is a myth that vireyas are shade loving, super delicate tropical greenhouse plants. I have seen a nursery flat that holds fifteen 4-inch pots containing six potted vireya plants that were 5 feet tall!
As for classification, these plants are lepidotes, having scales on the leaves. The seeds have tails or wings on each end. In the original classification by Dr. Sleumer, the classification is based on four scale types on the underside of the leaves. It should be noted that the best way to see these scales is with a 70x magnifier. The scales differ and may be disk-shaped, star-shaped or various lobed shapes. There are also differing characteristics in the shape and color of the flowers. Thus seven subsections were named. One section was further subdivided into seven series based on the differing leaf shapes and sizes.
The latest classification by Dr. George Argent is based on two scale types (large and small centers), on the nature of the rim of the bud bracts (scaly, hairy or smooth) and, lastly, on whether or not the seed capsule has an outer peeling layer.
To successfully grow vireyas use 40% coarse perlite, 40% conifer bark mini-nuggets and 20% coarse peat moss for the potting mix. These plants do quite well when the roots become crowded; therefore, do not move up in pot size prematurely. We like to fertilize every four months with orchid fertilizer of liquid fish emulsion, together with chelated iron and Peters trace elements. Thorough watering is done once per week, allowing full drainage (not wet feet) in a dry apartment, but can be as infrequent as every ten days in a solarium or greenhouse full of plants and humidity. You must pinch and prune often to encourage a more compact growth in any vireya, especially when the plant is young. We advocate pinching any solitary growing branch, even when the plant is only a few inches tall, until the vireya is three years old. Thereafter, pinch the new center growth as soon as the leaves start to develop and you know it is not a flower bud. The beginner can wait until the buds starts to grow either as leaves or flowers before attempting to differentiate between the two. Flower buds start at three to five years of age so after that the vireyas must be pruned ruthlessly. After all, Mother Nature frequently does this in the wild by freezing the top growth down to the ground.
Prune the leggy types; don't bother to pinch them. You see, lepidotes have dormant adventitious buds all over so they will sprout out after pruning. Just remember to prune evenly back to the lowest set of leaves. The apical dominance theory works here. If the plant is not pruned evenly, the topmost branch will be the one that grows at the expense of the lower branches. After the plant is four years old, you may cut back one-third of the plant each year.
Once a month, rose fungicide must be sprayed on all leaves until it runs off to combat powdery mildew. Mildew can build up tolerance; therefore, it is best to rotate between the mildew fungicides. Use 1 teaspoon per gallon of water, or 1 drop in 1/2 quart for hand sprayers. Always be alert for this disease and spray immediately if you see it.
Red spots are caused by a magnesium deficiency, and yellow leaves are from chlorosis or from a calcium deficiency. Use dolomite or gypsum, chelated iron or liquid iron. You may use ammonium sulfate (never the nitrate). Black leaf tips are from salt burn. Flush out the pot to cure it.