JARS v39n2 - Experiences with Growing Tropical Rhododendrons

Experiences with Growing Tropical Rhododendrons
Pat Halligan, Langley, WA

Plants were dying left and right on me. After enjoying a balmy summer and rainy autumn outdoors, my vireyas were now indoors. Soon after being moved inside, they began to look peaked, then downright sick. I took emergency cuttings, and then watched the parent plants die.

Two years later, with good sized plants from the cuttings, I was again able to leave the plants outdoors for the frost free season. This time I noticed that the plants looked well during the dry months when I controlled the watering, and sickly when mother nature became overly exuberant with the rain. I suspected root rot, so I un-potted all the plants and found all but R. laetum and R. 'Pink Delight x jasminiflorum with dead root systems in the bottom halves of their pots. One plant growing in a specially made decorative pot I had designed to provide perfect drainage did not have a single brown rootlet.

I decided that the best pot was no pot at all. I planted my sick vireyas in a raised bed in the greenhouse. The "soil" consisted of rotted wood collected in the woods from buried stumps and logs. "The use of natural rotted wood carries a risk of introduction of diseases. Although I have thus far enjoyed success with this medium, others have had disease problems and were forced to change to another medium." Almost immediately the plants perked up. I examined the root systems after a few months and found that they had expanded greatly, and were now free of the brown rootlets so characteristic of root rot. The yellow leaves turned green and the plants produced flower buds.

I found that the plants suffered in the overheated greenhouse in the summer, so I prepared a bed outdoors and transferred them there for the summer. The extremely light soil permitted me to transfer the plants without damage to the roots. Transplanting the vireyas did not seem to slow them down one bit. Now I have a new greenhouse from which the plastic is removed in late April and replaced in early October, allowing the plants to grow outdoors in summer without moving them.

Last winter, during the cold snap, the temperature inside the greenhouse fell to at least 25°F (That's the lowest temperature I saw on the thermometer, which was placed in one of the warmer areas of the greenhouse.) and the soil actually froze solid. Amazingly, most of the vireyas actually lived. R. commonae growing in the coldest part of the greenhouse was totally oblivious to the cold. Nearby, most of the cuttings of R. laetum survived but were damaged. R. christianae x laetum was killed and R. 'Pink Delight' x jasminiflorum was badly damaged. Flowers of R. laetum were badly distorted this summer, but those of R. 'Clorinda' #2 and R. konori x zoelleri were perfect. Flower buds on other vireyas were killed.

In addition to vireyas I have also been growing "Maddeniis" using various techniques with varying degrees of success. It didn't take long to realize that "Maddeniis" suffer when their pots get heated by the sun. Soon shades for the pots were made and the plants grew happily. However, the shades were bulky and bothersome, so I buried the pots in the ground (a very fast draining sandy soil). The plants grew very nicely until hot weather hit. Then the larger root bound plants would wilt every few days if not watered regularly. To give these plants space to grow, and to ensure perfect drainage, I drilled ½" holes in a 2" grid in the bottom and sides of the pots. Soon the plants were growing even better. Examination of the root systems showed that the occasional areas of browned rootlets found along the large expanse of flat bottom were eliminated. The roots grew out of the holes into the surrounding soil to grab moisture during dry periods. Severing these roots to move or display the plants did not seem to affect the plants at all.

A lack of pots for my burgeoning population of growing seedlings forced me to go one step further. I heeled large numbers of plants into beds of rotted wood (same as my potting "mix"), and moved them whenever the season or lack of space dictated. These plants seemed to show no interruption of growth by transplanting. In fact, they now look better than their potted counterparts, which are now much too large for their three gallon pots.

I have found that my "Maddeniis" revel in full sun. Having decided that I had entirely too many plants of certain crosses, I decided to put them in a bed in full exposure to wind and sun, to force them to flower. Surprisingly, the plants ( R. nuttallii hybrids) responded by growing even better than they did under more reasonable conditions. Now they look like a huge tomato patch, and they are setting buds.

A small R. crassum of mine flowered the year following a third degree sunburn. That summer I kept it in a more typical place, and it responded with nice leaves but no flowers. This summer I went for broke. I put it on a southwest wall "to burn some flowers out of it." It now has flower buds, but no burnt leaves. It loves being cooked. (Word of warning: If that plant had been in a pot, it probably would have ended up looking like a shish-kabob due to a constrained root system.) I've seen larger plants of R. crassum that their owners said had never flowered. I suspect that this species needs full sun to flower well.

The two greenhouses that I have built have given me a good example of the importance of ventilation for preventing Botrytis blight. My old greenhouse had manual outside ventilation below 70°F. Because I was gone three days a week, I had to keep the greenhouse closed three days a week. Botrytis blight was rampant. I had to keep the closed air at virtual hurricane velocity with the fans to keep the fungus under partial control.

Later I made another greenhouse with forced air ventilation using the same two fans, one at an intake vent and the other at the exit vent. Air circulated whenever the temperature was above 35°F. Botrytis virtually disappeared.

Over the past several years growing tropical rhododendrons has given me many setbacks and failures, and still does. But when the ledger card is complete, the joys of growing these plants far outweigh the tribulations. Even without flowers, just watching my "corn stalks" ( R. nuttallii x taggianum ) grow with their perky blue green leaves is a joy. And then they flower, too. My advice to would-be growers is to buy some "Maddenii" seed from the ARS seed exchange. Nurse the little plants along for a year. Then stand back and watch them choke out the weeds and stink up the whole place with their flowers.