My intern recently mentioned that there appear to be a lot of observations of a non-native rhododendron on iNaturalist, and she was right! There are more than 1,100 observations of R. ponticum in the eastern US. I can’t find any mention of this species naturalizing in the US (not included in EDDMapS, Weakley, or BONAP). Does anyone know of instances where R. ponricum is truly naturalizing in the US, or is this more likely a massive case of mistaken identify, for the most part? The common name “common rhododendron” probably isn’t helping, and if the computer vision system thinks this species is in the US then it is probably contributing to the problem. I’m guessing pretty much all of these that aren’t cultivated actually show a native species (R. maximum being the most likely). Thoughts? Perhaps these are widely planted and often observed in gardens?
This is a very popular small-size species in gardening; often runs away from gardens because it can withstand frosts up to - 24 degrees Celsius; of course it may be misidentified, but if it is in the gardens it will also be in the area …
A lot of them are probably correct identifications of cultivated plants. But it could also be an example of “runaway” misidentification where people chose the top auto-ID suggested species and others confirmed it using the auto-ID, which triggered the “seen nearby” aspect of the auto-ID and causing other nearby users to choose that species. This has been mitigated with recent changes to how computer vision suggestions are given, but it used to be a huge problem.
Before doing any Rhododendron id’s in the Eastern US, based on past experience I recommend taking a quick (or long, depending on season) pass through them in Identify and click “x” on the many obvious cultivated obs that will be in the mix. (You can always correct those cultivar id’s later, but it sorts them out for later.)
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I have spent quite a few hours over the last two days trying to clean up Rhododendrons in my area and noticed that most of these R. ponticum IDs in the eastern US are just plain wrong. Lots of azaleas, some native rhododendrons, a few mountain laurels, even stuff like roses, magnolias, spiraea and more. Many of them are obviously not Rhododendron ponticum, and most of them have the icon indicating they were CV suggestions.
I, too, have looked around to see if this species is even supposed to be here and can’t find any evidence for it outside of iNat observations. I know it’s an invasive in other places (e.g. UK, New Zealand) but apparently not in the eastern US. Neither BONAP nor the USDA PLANTS database offer a map for it, which I interpret as meaning it hasn’t naturalized here yet.
I’m not even sure it is common in cultivation either. I’ve pretended to be interested in buying one and failed to come up with straight species offers for sale. Most cultivated rhododendrons are hybrids, the cultivated ‘Ponticum look-alikes’ in the eastern US I suspect are probably mostly Catawbiense hybrids. The straight species is said to be less hardy and likely wouldn’t make it through winters in anything lower than zone 7, maybe 6 with some shelter/protection.
If anyone knows more about this or has a key to offer that includes both the American species and R. ponticum, that would be great and very appreciated.
Thanks, yes, I checked their website and found some good info on parentage for specific named hybrids etc. But it is very much geared towards garden enthusiasts looking to collect them all instead of identification of species, so no key that I could find. Weakley’s Flora of the Southeastern United States key doesn’t include R. ponticum, and the rhododendron key on Go Botany doesn’t have it either.
Here in NC, it doesn’t help that the NC State Extension service gets their info completely wrong. Their page on R. ponticum claims that it is the same as Hiryu/Kurume Azalea R. x obtusum, which is a Japanese hybrid. That’s of course complete nonsense and just misleading if someone checking through iNat suggestions is looking at that for confirmation.
I’m finding a good number of GBIF records for this that shouldn’t be for two reasons:
- obviously cultivated garden plants, and
- obviously misidentified.
The record holder so far is an observation in Minnesota (well outside the hardiness zone range for this species) with 5 confirming IDs following the initial misidentification and nobody marking it cultivated although it very obviously is a garden plant right outside a building. It’s a mess. So even if you don’t know how to sort rhododendrons from azaleas and such, you can still help with keeping garden plants from making it to GBIF by looking for evidence of being planted/cultivated and marking these accordingly.
Just a follow-up on this: I tried again, pretending to be a gardener/rhododendron collector, to find this species for sale for garden use in the US. The only nursery I’ve come up with that offers it is located in Oregon and doesn’t even have a picture for it (“image coming soon”). The price is 2x as high as other species rhododendrons. I seriously doubt the casual gardener would go to the trouble of searching for and mail-ordering from a specialty nursery and paying big bucks just to get their hands on a R. ponticum. As mentioned before, the straight species isn’t even hardy enough for much of the US. It should only grow without problems in the green area on this map:
Conversely, looking at rhododendrons and azaleas easily sourced from big box stores and nurseries, I find that they are pretty much exclusively hybrid cultivars. Here’s a list I came up with of hybrids/cultivars likely to be found in gardens:
Pink-lavender-purple flowers:
- ‘Roseum’ varieties - Catawbiense hybrids (large shrubs, English garden/park type)
(English Roseum, Roseum Elegans, Purpureum Elegans, Roseum Superbum) - ‘Boursault’ varieties - Catawbiense hybrids (even found obs with this name tag)
- ‘Grandiflorum’ - Catawbiense hybrid (also used as parent for other hybrids)
- ‘PJM’ varieties - Minus/Carolinianum hybrids (smaller than the others, very popular in smaller gardens, several color options)
White flowers:
- ‘Chionoides’ - Ponticum hybrid (commonly offered for sale)
- ‘Album’ - Catawbiense hybrid/selection (rare, haven’t seen this offered for sale)
- ‘Dandy Man Color Wheel’ (white with pink/red rim) - Asian hybrid
Red flowers:
- ‘Nova Zembla’ - ‘Grandiflorum’ hybrid
- ‘Skookum’ - Asian hybrid
Azaleas for sale all seem to be hybrids, too, and there are hundreds of varieties for sale, most of them Asian (e.g. Japanese azaleas). This brought to light another candidate for scrutiny: Rhododendron indicum. Like R. ponticum, this azalea species has quite a few iNat observations in North America (over 7,000 total, with over 1,600 “verifiable”). However, there is no indication outside of iNat that it should be here at all. It’s not on EDDMapS, nor on BONAP. The ones I can find for sale that look like it appear to be all hybrids, not the straight species.
Speaking of hybrids, iNat does have a taxon called Rhododendron x hybridum, which I assume is there because it is listed on POWO where it says it is a published name for R. maximum × R. viscosum. However, it appears it is being used on iNat as a catch-all for all sorts of hybrid rhododendron and azalea cultivars rather than that specific hybrid. I’ve left a flag asking for clarification on the proper use of this taxon for iNat purposes and would appreciate some input. So far, I’ve just been bumping things back to genus with a note that it’s a hybrid cultivar.
For some reasons the CV loves to identify sometimes cultivated rhododendrons in the US as alpine roses (Rhododendron ferrugineum and Rhododendron hirsutum).
Well, there’s no appropriate CV suggestion available for all the cultivated hybrids - I’m not sure yet the Rhododendron x hybridum label applies and even if it does it is not included in CV at all, not even listed as pending. So consequently, it looks for similar species instead. Since the vast majority of the cultivated ones are hybrids though, any species suggestions are going to be wrong.
Of course there is - genus Rhododendron.
Well, that’s the appropriate ID but it seems a lot of the casual users wanting to know what grows in their gardens go with the top species suggestion. Here’s an example what iNat suggests on one of these planted rhodo hybrids in my area:
There shouldn’t be any ponticum or indicum - at least catawbiense gets close but it’s a hybrid cultivar of that. So someone has to go through and bump them all back to genus with a note that it’s a hybrid and cultivated.
Just another fun example… Clearly wild, clearly R. maximum, slam-dunk ID for anyone with just a little bit of local plant knowledge so that should really be the only suggestion here. Yet, the top CV suggestion is R. ponticum. I’m not sure where it’s pulling the ‘nearby’ avocado and photinia suggestions from, probably planted in gardens but not marked cultivated.
Just out of curiosity, I revisited this exact same observation to check out suggestions after the recent CV update and me cleaning out all RG ponticums from the Southern Appalachians. R. ponticum dropped way down! Avocado is still there. I checked the avocado observations in the closer vicinity and they were all potted plants, some even obviously kept indoors, and some of them had made it to GBIF as well despite being potted/house plants. The others now at least fit the definition of seen nearby in the wild, even though I’m scratching my head over some of them. Fraser magnolia and sassafras - really? I didn’t think these could be so easily confused with rhododendron.
I think I see part of the problem: Rhododendron ponticum has the common name “Common Rhododendron.” This would cause people to assume that it must be common, and therefore the most likely choice.
According to Wikipedia, another common name is Pontic rhododendron – not, I assume, referring to having dentures, but to the Black Sea coast of Turkey. I wonder if making that the default name would help to reduce misidentifications.
Pontic usually refers to the whole region that was the kingdom, so most of Black sea coast.
Agree, the name is likely what pushes people to choose it.
This situation might be a good target for the IdentiFriday folks or to add to the CV cleanup thread if it isn’t already there.