David J. Williams, M.D.
Article revised from one published in the "Rosebay", a Massachusetts Chapter publication.
Many articles have appeared in this and other horticultural journals concerning the pitfalls and disappointments experienced by competent growers when they attempted to grow rhododendrons in containers beyond just a few years. While not professing to have an expert's license or knowledge about rhododendrons or their growing habits, I have managed to grow some plants in containers for several years. I attribute this small margin of success to beginner's luck.
When I first started developing an interest in rhododendrons, I never ran across an article denigrating container plantings of them, so I was blissfully ignorant about the many hazards others had experienced. Being a person who has always dreamed about owning ALL of the species, I quickly filled my available garden space with many varieties of this most interesting plant. But, I yearned for more and more plants. Thus I developed the idea of growing them in containers; a chance to grow more plants was now open! Naturally, no one can own ALL the species, but I could see that many more than I had growing in my yard could be enjoyed if containers were utilized.
The following ideas were gleaned from years of error and trial. Take them for what they're worth - just one man's ideas. What I'm going to say isn't gospel - just maybe a bit more than gossip.
The first important fact of life for your container plants is - WATER! In Oregon, where I live, we have natural water from the heavens, beginning in late October and continuing through late May. We get about 40 to 50 inches a year, sometimes more, but rarely less than 30. Even so, there are dry periods when one must water the plants, especially those in containers. Sometimes I have to water during a drier than usual winter, though winters here are rarely dry to the degree gardeners experience in the Eastern U.S. Remember, sub-freezing temperatures during the winter dormant time is nature's greatest desiccator of the roots of your plants. During the Oregon summers, it is not unusual to have temperatures in the high nineties to low one-hundreds and to have 8 to 12 weeks without any rain. Then it is mandatory to water daily and sometimes twice daily. The hotter the temperature, the more water your plants need.
I don't use overhead sprinklers for my containers, preferring to flood the roots by hand held hose. I use city water and care nothing about the mineral content of this water. I assume the possible adverse makeup of the water will be more than outweighed by the water content. My ignorance about the chemical make-up of Eugene's city water hasn't hurt my plants - as far as I know! Remember! Ignorance is Bliss!
I try to use redwood containers because they are light weight, water resistant, and reasonably attractive. As the plant grows, I usually change containers to a larger size. Redwood planters are light enough to move about fairly easily; clay pots are heavier and with soil and plants therein become quite formidable. In Oregon, redwood containers are easily found, though still costly. At the present time, a planter that measures 18" high, and is 18" in diameter, having 6 or 8 sides costs about $25. Not cheap; I conserve the old ones for future use, carefully washing them out after their use and storing them.
When deciding upon the growing medium or soil makeup, I've used mostly the commercially available mixtures. I really can't tell whether one brand is better than another. But, I believe what all the really accomplished gardeners write about; namely, that good drainage is paramount. Be sure the drainage holes are big enough. I cover them with nylon screening, to keep out the ubiquitous slug, then use bits of broken clay pots to insure the holes will not be clogged with debris. Any soil should work, but if you think your choice is "too rich", add sand or perlite to increase the aeration. I doubt you can overuse either product, so don't hesitate to add them if you have any doubts.
Set your containers on strips of wood, either 1" x 2" or 2" x 2", to keep the base of the container from being constantly moist. I learned the hard way that if your planters are sitting on a deck, the continuously wet decking will rot, even if it's cedar. Redwood rots too, but much more slower than other woods. The deck will go first, believe me!
Finally, set your plant into the soil of the container, keeping the soil surface 2" below the top of the container, so that the water will stay inside when you apply it. And, remember to apply water liberally. I don't worry about leaching out the useful minerals. I do worry about dehydration. More of my plants have died of thirst than ever suffered chlorosis.
As for fertilization, I'm in luck. I live in close proximity to a world renowned nurseryman, who orders fertilizer with special additives for his nursery. I figure that his ideas are good enough for me, so I purchase fertilizer from him. I usually add fertilizer four times a year, on a haphazard schedule, based on the season and weather, rather than the calendar.
Hail, a good but temporary ground cover
Photo by David J. Williams
Now - what are my successes? Well, I'll tell you; sometimes, it's the PITS! But, I've a few plants in containers that I'm willing to discuss. I've a Cedars of Lebanon ( Cedrus libani ) 4' high, that's been in the same container (30" x 10" x 8") for 18 years. It's my "large bonsai", so to speak. And, I've a R. yakushimanum ssp. yakushimanum (Exbury form) in a container for about 25 years, as nearly as I can tell. And, an 'Impi' ( R. sanguineum ssp. didymum x 'Moser's Maroon') is in its same 18" diameter container after 15 years. Also, I've a 15' tall Alpine Fir in a 3' container that is thriving after 15 years.
R. 'Elizabeth' grown as a standard
Photo by David J. Williams
Like all species nuts, I've had the most failures growing the rhododendron species, due mainly to my own inadequacies, I'm sure. But, I have enjoyed expanding my yard. At present, I have about 25 plants in various sized containers; plants like Pinus thunbergii (Japanese Black Pine), Stewartia pseudocamellia (Japanese Stewartia), Acer palmatum 'Shishigashiri', R. campylocarpum , R. cinnabarinum v. roylei , 'Hurricane', ( R. yakushimanum ssp. yakushimanum x 'Tumalo'), R. metternichii v. metternichii , R. keiskei 'Yaku Fairy', 'Point Defiance', R indicum 'Balsaminaeflora', R. adenophorum , R. recurvoides , R. paradoxum , R. iodes , R. shweliense , R. houlstonii , R. lyi , ( R. yakushimanum ssp. yakushimanum x R. crinigerum ), R. strigillosum , 'Elizabeth' on standard, 'Hotei', 'Princess Ann', R. metternichii v. kyomaruense - that's enough! There are a few more, but you get the idea.
R. metternichii var. kyomaruense
, and Stewartia
Photo by David J. Williams
So, if you have a patio or balcony, try highlighting the area with container grown rhododendrons. You can choose from a variety of color, leaf sizes, fall foliage, or whatever you want. Just be leery of long sub-freezing winters. I've no experience with that type of weather, so I cannot help you there. But, I do know you can have a good time planting a new form in a container, and you will certainly enjoy the bloom within reach of your guests and friends. Probably, you'll even be thought of as an "expert" in rhododendron horticulture. But, be careful to whom you are boasting; there are a lot of real experts out there - and maybe they know more than you!
One word of general warning; if you can't grow the plant in your yard, I doubt you can grow it in a container. So, spend your time and effort on more productive plants. I think you'll be happy.
Dr. Williams is a General Practitioner in Eugene, Oregon. He has been growing rhododendrons for over thirty years. A member of the Eugene Chapter, he has served in all of the chapter's executive capacities at one time or other. Presently, he is the Editor and author of the monthly newsletter, "The Hybrid", a position he has occupied for the last 10 years.