The height quoted for rhododendrons is the height after 10 years so that is about the best predictor of growth rate that is available. Fortunately the American Rhododendron Society has a database where you can find the colors, hardiness, etc. as well as the height after 10 years for your area.
ARS Rhododendron Database
The tallest rhododendron plants it lists are 7 feet tall in 10 years:
alutaceum var russotinctum
Professor Hugo de Vries
Cool, thanks rhodyman! Curious that the database gives you a choice to search for 8 foot plants, but none are found!?
Since most rhododendrons are hybrids and new ones are introduced almost every week, there is a chance there may be some 8-footers before too long.
Rhodyman, a question for you: About 10 years ago on the web I recall being on the website of a German rare/subtropical plant enthusiast. He had a long article, that had been translated into English, about efforts to bred "hardy" versions of various subtropical plants. After discussing the likes of "hardy citrus", big-leaf rhododendrons came up and there was mention of someone in the US state of NJ actively trying to bred a "hardy" big leaf that would be derived from the tender big-leaf species like R. sinogrande or R. macabeanum but be able to stand the heat & cold of NJ. (people who haven't seen pics of these monsters, check out the link) At the time, which I guess was around 1999, the article spoke as though these effects had been ongoing for some years.
Any idea who was working on this in the 1990s? Hank Schannen of Rarefind is an obvious candidate, and the next time I see him I plan to ask him about this. (I missed him on my spring trip to the nursery this year). But I know there have been other hybridizers active in the Mid-Atlantic, I thought you might have some idea, or know who to ask.
Here is a link that might be useful: http://www.chimacumwoods.com/macabeanum.htm
Hank loves R. sinogrande and the other big leaf species. There are several people who are trying to breed species leaves on good garden plants. I haven't heard of any great successes. If there are any, Hank would know who and what.
Years ago, Al Reid hybridized the Linwood Hardies in Linwood, NJ. They were hybrids of the best greenhouse azaleas and hardy stock. They resulted in the beauty of the greenhouse azaleas but with good hardiness.
Rhodyman, you are a great source of info. Thanks. The name "Linwood" definitely rings a bell, but it's hard to remember something I read 10 years ago.
Both times I've been at Rarefind, I've seen many spectacular rhodies but I don't remember seeing something that looked like a big-leaf planted in the open, so you must be right that there haven't been great successes yet.
Speaking of "big" except big plants this time, not big leaves, do any of the 50% R. arboreum crosses (most quite old from the 19th century) do well on the East Coast? It seems at least a couple of them must. 'Rhododendron Portraits' lists R. 'Vesuvius' as being very hardy, but it's not in the trade. Van Veen's & Hammonds though have a couple that are from this grex that I want to try. (R. arboreum X US East Coast native rhody) I'd like a plant that resembles the amazing R. arboreum plants I've seen in pictures from the Himalayas & the UK, but would be hardy here since the pure species is not hardy enough, and blooms too early. Red blooms, even if it's a muddy, off-red, and getting as tall as possible.
I have a R. arboreum X R. yak hybrid from Greer that has been very tough through heat and dryness so far. (I'm a deadbeat waterer if there ever was one). But with parentage like that, summer isn't going to be the problem as R. arb. is heat tolerant and R. yak is somewhat drought tolerant. The problem is going to be winter and we haven't had one to test it yet.
I spoke to one of the owners of Roslyn on the phone many years ago and they said R. arboreum might persist a couple winters on Long Island but would never make it through a cold winter. So with just a little more hardiness from an East Coast native these hybrids must be able to survive - that is my thinking at least.
'Lady Eleanor Cathcart' is an R. maximum X R. arboreum hybrid described as 'compact' so that won't work for me, but at least I know these hybrids do grow well. (I also don't really go for the color.) Rarefind is selling it.
Here is a link that might be useful: http://www.rosebay.org/chapterweb/brooksgt33.htm
I have a question about the R.macabeanum which I received as a gift from a friend several years ago. The plant is reaching 7 foot tall now and the leaves are about a foot long. Still haven't seen any flowers, but since it is a big guy I am thinking that it might need to be 15 or 20 years old before it flowers the first time.
Anyone have any experience with this beauty? I'd like to know if I will still be alive to see it flower the first time.
Since it only grows in zn 9 coastal areas of California & Oregon there aren't going to be many people who have experience with it. You might also try the California & Pacific Northwest forums. I have read they take about that long, too.
As I mentioned in this Northwest Gardening forum thread, I bought a Rhododendron macabeanum today. In response to Still Kris's question, the nursery tag said that one of the nice features of R. macabeanum is that it can bloom when it's 2' or 3' tall. One of the other bigleaf rhodies said on the tag that it doesn't bloom till it's 10-15 years old.