PORTLAND, ORE. — Not all rhododendrons prefer cool climates. Plant collectors are getting excited about a gorgeous group of semitropical rhododendrons called vireyas, which make up about a third of the genus Rhododendron. More than 300 species of vireya rhododendrons grow mainly in New Guinea. There are also some on other South Pacific islands and two that are native to Australia.
Those aren't the only places to see vireya rhododendrons, though. White Smith, Lucie Sorensen-Smith, and George Watson, owners of Bovees Nursery in Portland, Ore., have one of the largest collections of them in the United States, if not the world.
The evergreen shrubs' main attraction? The bright blooms - and the fact that the plants flower more than just once a year.
Rhododendron laetum blooms three times yearly, says Ms. Sorensen-Smith. "Others bloom almost constantly throughout the year." The slower months for vireya flowers are July, August, and September.
Sorensen-Smith began collecting vireya rhododendrons in 1980. "Collector friends would drop by and give me vireya cuttings and convinced me to start growing them," she says. "I obtained [many] from a friend, Bill Moynier, who had ordered them from collectors in Australia and New Zealand."
She then met Mr. Smith, a retired parks superintendent from Washington State, who had the second-largest collection of these rare rhododendrons in the Northwest. When they married seven years ago and joined their collections, the number swelled to more than 400.
The scaly leaves of vireyas vary in size from half an inch to a foot long. But the beautiful blooms are the real reason to grow these gems. Rhododendron stenophyllum from the Mt. Kinabalu area of North Borneo has delicate orange flowers erupting from red buds. Rhododendron kawakamii, the only vireya native to Taiwan, has small yellow flowers.
"As each plant blooms, we carefully evaluate it for the quality of flowers," says Sorensen-Smith. "We are constantly adding many new wonderful hybrids that are being developed mainly in Australia and New Zealand."
Vireyas can be grown outdoors in raised beds all year in Florida, southern California, and other areas with mild winter temperatures. "All other places you need to bring them into a small greenhouse, sunroom, or your living room during the cold months," she says. Then, "they can be placed in containers on your patio after the last frost."
In their native habitat, vireya rhododendrons are often found on high, moss-covered boughs of trees.
"The average customer doesn't have the same growing media as [in] the wilds of Borneo, so we [recommend] a soilless growing medium consisting of one-third organic matter, such as a chunky orchid bark and peat moss mixture," she says. The other two-thirds of the potting medium can be inorganic materials such as pumice or perlite, both of which provide fast drainage.
"It is similar to orchid culture in that you don't want to overwater," says Sorensen-Smith. "Our rule of thumb is that we lift the pots to see if they are light in weight, and [if so] water them thoroughly. In the summer it's every five days, but in the winter, we water once a month."
Another similarity in the care of vireya rhododendrons and orchids is that neither likes an overabundance of plant food. She fertilizes vireya cuttings - and plants younger than a year old - only three times a year using one-fourth to one-half the recommended amount of a water-soluble plant food, such as 20-20-20. Plants more than a year old are fed every three or four months with a specially made fertilizer that contains slow-release nitrogen, plus superphosphate, dolomite limestone, iron chelate, and trace elements. of minerals.
Sorensen-Smith suggests that gardeners encourage vireyas to grow bushier by pruning and pinching them from the time they are only a few inches high until they are two to three years old.
After they reach that age, she advises, avoid pruning and pinching until after blooming. Otherwise, the plant may not flower the following season. "Flower buds often wait for the second or third flush of growth," she says.
Most of the yellow vireyas (they'll have laetum, zoelleri, or aurigeranum in their names) grow upright. The pinks, reds, and whites are usually compact, bushy types. Careful pruning of older plants can help taller types remain three to four feet high. A good rule of thumb for pruning plants more than four years old is to cut one-third of the older stems back to ground level every year.
One of the few problems gardeners have with vireyas is mildew, a fungus that can spread from regular rhododendrons growing in the yard. An occasional aphid or weevil may also put in an appearance, says Sorensen-Smith. But mostly vireyas aren't difficult plants.
Vireyas were discovered in the South Pacific in 1843. Thomas Lobb, of Veitch Nursery, introduced five species of vireyas into Britain in 1845. Charles Curtis, another collector, added two other species a short time later. From this group of seven vireya species, more than 500 hybrids were eventually raised by Veitch Nursery.
After World War II, Dr. Herman Sleumer collected many vireyas from New Guinea and introduced many new species to Britain, New Zealand, Australia, and the US, giving plant collectors - and especially plant breeders, who have created countless hybrids - a new enthusiasm.
For more information see "Vireyas: A Practical Gardening Guide," by John Kenyon and Jacqueline Walker, or these websites:
Vireya Rhododendrons, www.vireya.net
The Bovees Nursery, www.bovees.com
Rhododendron Species Foundation, www.rhododendron.org/display/Species Foundation.htm
Pacific Island Nursery, www.pacificislandnursery.com