Whenever container culture has ever been involved the first thing I look for when non-climbing woody plants can't hold themselves up is bound roots. Are those a possibility in this instance?

Is it breakage or falling over (at the roots). If the later, I'm with Embothrium. If the former, I've noticed A. pseudosieboldianum tends to form weak V-crotches.

As has already been said in other ways, grafting is not responsible for hybrid vigor.

I think this definition is a fair (even if somewhat incomplete) one:
hybrid vigor - increase in growth, size, or yield in hybrids over those of the parents

Quercus x schuettei is a good example where you can get the qualities of swamp oak (some fall color) but yet is can grow on dry alkaline soils. Where the straight species could struggle.

Sorry for the delayed response and thank you for your replies. The trees in question were container grown, but the roots in all three cases were not congested. I've dealt with plenty of root bound stock, but not with these. They've rooted in nicely since planting and I've not experienced tipping of the plant at the roots. The soil was not amended and no fertilizer applied.

Simply put, it appears to be excess canopy growth, which is too heavy for the stem to support. I suppose this would fall under the "structural" umbrella.

BTW, I did a search of the web. A book on Forest Genetics, which almost made my head explode, refers to four hypotheses to explain this phenomenon: dominance, overdominance, additive, and hybrid habitat. I believe Sara's explanation would be overdominance.

Hybrid Vigor refers to how SOME hybrids between closely related species inherit the best traits of both species. An example is mules, which are smart as donkeys but able to carry a person as easily as a horse, or sugar-and-butter corn.

This has nothing to do with what you described. Sometimes the rootstock just doesn't match that well with the desired plant, and one grows too much.

I don't think I have ever heard anyone use the term to describe "inherit(ing) the best traits of both (parents)".

Heterosis, hybrid vigor, or outbreeding enhancement, is the improved or increased function of any biological quality in a hybrid offspring. The adjective derived from heterosis is heterotic. (Wikipedia. I only quote from the best.) No need to go to the mat on this one. Read about it.

Taken as a whole, the wikipedia article is probably not a bad source for helping to understand the phenomenon. One could take a few sentences out of context though and misunderstand what it's all about. Just because a trait found in a hybrid offspring is "better" (endless possibilities can be imagined) than that found in either parent does not mean it's a case of hybrid vigor (even in it's broadest meaning).

One can take anything out of context and misunderstand what it's all about.

Just means improved growth from having an assortment of genes....look at ligers for example....bigger than both tigers and lions

"Just means improved growth from having an assortment of genes....look at ligers for example....bigger than both tigers and lions"

Yes, but at the trade-off of infertility. That's the never ending evolutionary genetic see-saw that's going on with outbreeding versus inbreeding.* The notion of a species as a group of organisms capable of producing fertile offspring is one of the absolute keys of biology. So even though outcrossing can produce somatically "better" individuals, if those individuals don't reproduce more effective in habitat, their 'betterness' is a moot point. (and it's funny how these biological truisms almost certainly apply even more vastly than most people give them credit for...but I don't want to touch anything that could become a political hot button...)

This is one thing that makes the rhododendron genus so impressive to me...that within in sections of the genus, plants from around the world can cross and produce garden superior (i.e., somatically 'better') individuals. My success rate with RSF plants is about 1/10, my success rate with hybrids is about 9/10. Is this the fault of the RSF? NO, their plants are some of the best grown mail order plants you can buy in this country. It's a fault of 'the genes' they are selling only being adapted to a mountainside in China at 10000 ft...and not my summer steambath. (Seattle/Tacoma is a little more to their liking, obviously, which is why they are based there) But put those hybrids back into where the parent species came from, they would not be superior in that setting.

* - but ligers are a bad example in comparison to plants, for a lot of reasons.

And to clarify something, it's not like a plant like Rothschild's 'Leo' was hybridized to withstand hot summer muggy climates. The point I'm making is just that having hybrid vigor, period, means such hybrids are more tolerant than their parent species would otherwise be. Even if neither parent species actually came from a muggy climate like R. hyperythrum.

Just because a trait found in a hybrid offspring is "better" (endless possibilities can be imagined) than that found in either parent does not mean it's a case of hybrid vigor (even in it's broadest meaning).

Is there term for this otherwise?

This is sort of a how many angels on the head of a pin argument in my opinion.

As I tried to explain, the effect of heterosis is possibly only an improvement in a certain context.

"One can take anything out of context and misunderstand what it's all about."

Now that's one more insightful analysis!

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David, if the angels' offspring experienced hybrid vigor, less of them might be able to fit on the pin.

And that's what it's all about!

I've never known 'hybrid vigour' to relate to plants. Animals, yes.

But in plants, hybrids are often talked about. I don't know what they say, apart from telling us how great they are, when maybe the real reason for their brainwashing the masses is to force people to buy plants/bulbs etc because these things won't set seed, I don't know. I'm not keen on hybrids in plants.

As for the maple who can't hold himself upright, I'd inspect its root system. It's probably twisted around in the most objectionable fashion, having been constricted in its youth in too small a pot.

Isn't a mule an example of a bi-generic hybrid vigor?

"I've never known 'hybrid vigour' to relate to plants. Animals, yes."

Well, you just haven't had enough conversations with plant breeders in that case. It is absolutely a concept and term used by them.

If you cross the american quaking aspen with the european quaking aspen, the resulting tree will grow faster than both its parents. The same will happen if you cross the caucasian wingnut with the chinese wingnut. That is hybrid vigor, isn't it?

Not if they fall down and die after two years. Don't confuse speed with vigor.

Actually, come to think of it, I think technically that that trait (speed) is a result of hybrid vigor, but it may not result in a desirable organism (the plant).

I could swear I've heard it relating to plants, but in fruit/nut/acorn production and growth also. But that's without going back and reading on Mossy oak natives website pertaining to their hybrid oaks. I may remember wrong, I haven't looked for a while, but they have "rapid mast" seedlings that are oak hybrids so that is seed production, and I think some grow a bit faster too.

Here's a paper that is somewhat readable even for this non-technical tree nerd. Species hybridization is a very interesting subject; it says so at the start of Chapter 3.

I will say that my Compton's oaks ( Q. Lyrata and Q. Virginiana) should be making acorns by year 8 in my yard according to Mossy oak natives nursery website. So there is some benefit of the hybrid of those 2 oaks. I have a year or 2 before I get acorns I guess. Year 8 is not far off, I need to check my trees.

How come Leyland Cypress hasn't been mentioned? That's most obvious as far as I know. A sterile bi-generic cross more vigorous than both it's parents.

Sorry Mikey man, I haven't seen that one in Pa.

I have several Compton's oaks, and I must admit that not all of them grow super fast. It may be that some of them are in a more fertile area. All of them do about 2 ft a year AT LEAST which is good, but, some do much better. Maybe those who did best are in a more fertile area? I really don't know the reason some do better. Mike, sorry for the Mikey man stuff, I'd been drinking..........

I don't mind. People call me all sorts of names, mostly good. ;-)

I've been known to have a beer once in awhile. :-)

Pine trees have been hybridized for forestry timber production and hybrid vigor is one of the traits they want. I can't find the link but I recall seeing a patula hybrid that was 7 or 8 feet tall after two years It wasn't thin or spindly either. It had a heavy thick trunk