It's indeed a nice thing to dream about. Much easier to do in a rainy, maritime or subtropical climate where fast growing varieties can be accommodated.

http://www.alamy.com/stock-photo-rhododendron-arboreum-subsp-delavayi-tree-lined-avenue-path-red-flowers-62698377.html

Those are in County Wicklow, Ireland.

Maybe a Himalayan variety would work? https://adventurebeforeavarice.files.wordpress.com/2015/03/rhodo1.jpg https://adventurebeforeavarice.files.wordpress.com/2015/03/rhodo5.jpg https://adventurebeforeavarice.files.wordpress.com/2015/03/rhodo4.jpg https://adventurebeforeavarice.files.wordpress.com/2015/03/rhodo2.jpg https://adventurebeforeavarice.com/2015/03/30/the-blooming-himalayas/

https://goo.gl/images/dpvz5f

I wasn't able to open those links, but no arborescent Himalayan rhododendron is anywhere near zone 5 hardy. As Mainegrower notes, there aren't many that grow big, quickly, that are that hardy at all. I might cautiously try a few small, inexpensive plants of something like 'Lady of June', which Rarefind rates as zn 5 hardy...but as is the case with any nursery, that isn't a true guarantee. http://www.rarefindnursery.com/index.php/rhododendrons/rhododendron-elepidote-large-leaved/rhododendron-lady-of-june.html

It is 1/4 Himalayan. Close enough?

Actually the Cox encyclopedia vaguely implies 'Lady Eleanor Cathcart' might be more reliable in a 5b climate. It is a parent of Lady of June. Mainegrower might have some experience with this varieties up in the north. The older hybrid is arboreum X maximum. One really wonders how they could make such hybrids in 1850, but somehow they did...perhaps the vagaries of English weather meant sometimes an arboreum was open at the same time as a maximum.

To the degree I was looking for them on my limited travels in the area...the paucity of huge, non-native rhody hybrids north of NYC in the interior northeast makes me wonder if cold winters have periodically wiped them out. But again Mainegrower would be much more informed on that to me. The biggest ironclad rhodies I remember seeing in say, Amherst, MA, were 5 footers, in contrast the 15 footers you sometimes spot in places like suburban Bucks and Mercer counties. Doesn't mean there might not be a 15' ironclad in Amherst, just means they are less common.

correcttion, hirsutum lists 'Lady Eleanor Cathcart' as maximum X arboreum, which is more plausible. Still that's 1/2 Himalayan, and could be hardy enough for you.

In addition to the problems listed above, getting the soil environment suitable for a rhododendron used as a street tree, and maintaining that environment, would be so difficult as to be impractical.

Mike

Mike, generally agree though it depends how wide the median/road edge is. Northern road crews have to use a lot of salt, but if you kept the plants a sufficient distance away it should be ok. I remember seeing rhodies in New England gardens that were not too far from the road.

I was thinking more along the lines of soil compaction. All sorts of equipment have been driven on the soil, and then you have continued foot traffic. Gardeners don't call the area between the sidewalk and street a Hell strip for nothing.

Could be...does the rocky soil of New England compress? I just know I remember seeing in western MA, that sort of landscaping where people put evergreen hedges out by the road to block the view of their front yard. It's more common up there than some other parts of the country.

I spent a week in western MA. near the town of Stockbridge back in 2001. It's the home of the Norman Rockwell Museum and Alice's Restaurant. I saw my first Japanese Beetles in the botanical garden there. I noticed the hedges too.

Mike

In recent years, most if not all New England and other northern tier states have switched from a mixture of sand and rock salt to salt brine for winter road treatment. This is 25 or so percent mixture of salt or other chemical with 75% water. It's often spread before actual snow or icy precipitation. It uses less total salt, but it becomes an aerosol as traffic constantly causes it to become air born. Despite assurances to the contrary by state highway departments, any garage or body shop will tell you that it causes much more rust than the previous road treatments because it seeps into any crevice and is much more difficult to rinse off. Brine also accelerates the deterioration of concrete to an alarming degree. Roadside plantings also suffer much more from brine than they ever did from the salt and sand mixture. By April, the yellowing of roadside white pines and other vegetation due to the desiccation is a common sign of spring. Other plantings, including hedges are similarly effected and the damage extends much further back from the road than before. Highway departments love brine because it works, is cheaper because of the reduction of salt used and requires less labor to spread. The actual cost, though, is rusted out vehicles, spalling concrete, and dead or severely damaged plantings.

Rosebay Rhododendron (Rhododendron maximum) | American Forests

PA Big Trees

I'm actually surprised there are not somewhat bigger ones in New England or the northeast. The trusty old Wyman's Encyclopedia gives a size range of "12'-36'" for that species.

David, there are some quite large ones, but open grown as they would be if used as street trees, it would be quite a bit of work to maintain them in tree form. Based on my experience the dormant buds along the stems want to sprout to return it to large bush form, and it would be difficult to work around 15' wide shrubs along the road margins in most settings where one wants street trees since it would get in the way of pedestrians and parking.

Agree Babs, though it doesn't look like the cemetery example I posted above is particularly hard to maintain as a tree. It could vary by seedling clone, I presume. The largest maxs I've seen in person were the ones at Swarthmore that are the sort of unappealing haggard blobs you are probably thinking about.

Thanks for all the advice. I haven't found any US nurseries online offering Lady Eleanor Cathcart. yet. We have some blueberries not too far from the road that I regularly apply soil acidifier to, so maybe that would be enough to counteract road salt effects.

I have never ordered from this one, but they are known for carrying a wide range of the older varieties. You might as well inquire.

http://www.rhodiesrus.com/

I hate to be negative about this project, but rhodendron as a street tree in Z5 is just not a practical undertaking. Road verges are open and exposed. Sub-zero temperatures plus bright sun plus wind exposure will do in almost any rhododendron pretty quickly. This would be particularly true of r. maximum or hybrids thereof. R. maximum needs a good deal of shade. The problem with salt and other ice melting chemicals is not soil acidity, it's dessication. Any evergreen exposed to salt mixtures will suffer. Those with broad leaves like rhododendrons will suffer more than most and die. The soil used on road edges is usually not suitable for growing much of anything, let alone a species that has specific requirements for pH, organic matter and lots of oxygen around the roots. What is commonly found is poor subsoil that has been deliberately and extensively compacted during the process of road building.

If growing rhododendrons is to be a worthwhile municipal project - and it most certainly could be - the time, money and effort involved would be much better spent on public areas where the rhododendrons would have a chance to thrive. Parks, public buildings such as a library or town offices, schools and perhaps even private property ought to offer much better potential sites than road verges.

Obviously I defer to mainegrower's expertise. But I still wonder if there's some wiggle room with the definition of roadway "verges". Some municipalities have that those sidewalks that are well displaced from a road, and in some cases they are also elevated by a retaining wall. Do I think spending $10000s or even $100000s on many mature R. maximum (or whatever you can find) is a good ideal for a northeastern town, to line their roads? No. Do I think putting a few inexpensive starter plants in places that are owned by the municipality but relatively sheltered from road spray (but still visible from the road) could be an acceptable idea? Yes. I know nothing about this town but I can say if you drive around the Philly Main Line area enough (granted a rather milder climate, but not that much milder - it isn't Sonoma Co. California!) you sometimes see really big rhodies lurching rather close to the roads, planted in people's front gardens.

I'm leaning towards offering to try it in front of our house with 3 plants. We do have a curb which reduces runoff from the road. Haven't located any Lady Eleanor Cathcart in the US yet though. Have checked with rodiesrus-W.W. Nurseries in PA and Rare Find Nurseries in NJ.

It's pretty doubtful that Lady Eleanor is available in the US. I'm also pretty skeptical that this variety is actually hardy to -10 even though it is so listed. R. arboreum is very tender - not at all surprising given its native range - perhaps +5 to +10. That combining it with r. maximum would produce a hybrid reliably able to withstand -10 doesn't seem at all likely. Other r. arboreum hybrids listed by Whitney Gardens and Singing Tree Nursery are barely hardy to 0.

If you want to try 3 plants, the readily available r. catawbiense based iron clads such as English Roseum, Roseum Elegance are tough, reliable and adapted to Eastern rather than British or West Coast conditions. If you have lots and lots of patience plus a genetic tendency toward a long life they will grow very large in time.

Agree with mainegrower, it is surprising that plant is listed as being so hardy. And I appreciate his more realistic voice to counterbalance mine!

That being said at least with the Coxes, I think they were trying to produce a truly well researched volume and were not coming up with willy-nilly figures for hardiness. I got to thinking, who could have suggested to them that Lady EC was especially hardy? David Leach came to mind so I checked the Holden Arboretum. They don't have a Lady EC, but they have a hybrid of it, 'Lady Longman', and even in that zn 6a/5b part of Ohio, the plant is 16' tall! http://rbg-web2.rbge.org.uk/cgi-bin/nph-readbtree.pl/dataset=holden/filename=holNAMES?ETI=Rhododendron+Lady&submit=Submit+Query

'Cynthia', of course, is the standard arborescent hybrid found in old PNW garden, and not imputed to be much hardier than 6b. My broad understanding is anywhere in NE Ohio went below -10F in 1994, though the plant might have had snow cover at that point.

I actually want this 'Lady Longman' now myself, because although the look of the flowers probably isn't my thing, it might be interesting to use for breeding.

Here's a 'Cynthia' next to a 'Alf Bowman', a variegated Pieris,...and a cat named Alle'.

Winter before last Cleveland (40 minutes from the Holden Arboretum) got to at least -11 because the Lake froze over, removing its modifying effect. I'm in Cleveland right now, and if I make it out to the Holden Arboretum this week, I will report back on how Lady Longman looks if I can find her.

I do agree that the best potential results will most likely come from one of the ironclads that akmainegrower referred to. They are readily available (I see them regularly at big box stores) and can get huge given time.

What a nice coincidence that you're there, NHBabs. Good luck finding it though, it's one of the largest arboretum in the country! But seriously there is a map reference when you look plants up there, so it should not be too bad.

Well NHBabs, at least let us know you got back to NH safely LOL.

I did the tree canopy tour and climbed the tower along with meandering along several trails, including through the rhodies, but didn't see many labels and was with non-plant nerds. It was fairly hot and sticky so looking for tags amongst the branches didn't happen. I'll try again in May.

It sounds like Rhododendron "Caractacus" is supposed to be tall and hardy. Maybe it is available in the US, one of the original ironclads. I have not found a retail nursery for that yet either.

http://homeguides.sfgate.com/rhododendron-size-70039.html

Sadly there isn't a lot of interest in such varieties these days. Many people prefer smaller, "bun like" rhododendrons.

OP might be gone but an interesting report on a BVT X arboreum in Toronto here: http://growingontheedge.net/viewtopic.php?f=2&t=7807&start=255

So, it was dying back in colder winters, but surviving overall...and had a very nice, exotic looking truss. Cross that with the hardiest R. maximum you can scrounge up from the mountains of New England or whatnot...and sure, I don't think a semi-arborescent rhododendron for 5b is out of the question.

The issue is as I said nobody has really tried to breed something like that - the goal after the 19th century in rhododendron breeding has almost always been low bushy plants; furthermore BVT, one of the hardiest species, wasn't really used by the pioneering first or 2nd generation of American hybridizers. It wasn't available to them.

A rhododendron in Olinda, Victoria, Australia.

Love it!

Thanks for keeping us updated.

Technically speaking, the best thing to 'counter' road salt, insofar as it is possible to do so, is calcium sulfate aka gypsum. That, and liquid rain!

Curious - do you get 100% reliable snow cover in winter, or are you too far from the lakes for that? If not I'm thinking something like a burlap screen or whatnot, might be desirable for their first couple years.

A little soil preparation conducive to a rhododendron's root system would help.

Yes, the ground is mostly covered through most of the winter. I think I do need a burlap screen on the road side for the winter, at least initially, because snow comes off the plows quite forcefully and the piles there are a foot or two deep some of the winter. Do you think they should be entirely surrounded by burlap, or is more sun more important?

For soil preparation I added some peat moss to the dirt beneath. Anything else I should have done?

no, protection from sun is more important!

Rhodies in winter hardly need much sun. For several years I kept small or tender ones in my south facing garage without supplemental light. (though of course, like most garages, it obviously does not have a picture window...it does have 2 standard sized windows on 2 sides, S & W. So probably a little more light than the average garage.)