Get educated and read on!
|Crape Myrtles -- photo from Southern Living|
Saw your (old) blog post just now on crape murder and thought I would add my two cents.
Believe me, I am with you 100% on strongly advocating against topping trees, and for all the reason you and the ISA list. Interestingly, though, most of those don't really apply to crape myrtles. Actually, the only two I would say factor into it are ugliness and cost over time. Rapid growth does happen (very rapid) but the resultant branches and branch attachments are not overly very weak (plus their relatively small stature means that wind-throw does not come into play).
No doubt, you have observed countless crapes that have been topped for many years in a row, sometimes hideously.
|We've all seen this horrid sight before (Source)|
However, in all the ones I've seen treated in this manner, none have been actually "murdered," in the sense of tree death. No shock and no starvation, either. (Perhaps, more precisely, there's no evidence of shock or starvation, given the strong re-growth.)
As far as insects and disease go, that seems to be more a result of cultivar selection. The major insect issue - aphids - is not exacerbated by hard pruning. Nor is the major disease issue - powdery mildew. Interestingly, the crapes in one of your BBG images are 'Carolina Beauty', one of the most aphid- and mildew-susceptible cultivars going. The presence of the disease, worse in dry years, seems of little relevance to the overall vigor of the trees. At worst it renders the fall color moot (leaves abcise prematurely).
It seems to me that crapes have an unbelievable capacity to form the chemical walls that Alex Shigo identified, which prevent the spread of decay. The most important wall is Wall 4, which prevents decay from moving down into the center of the branch or trunk (into the heartwood). That wall is also, usually, the slowest to form, and pruning angle and branch collar seem to have a lot to do with formation. But, seemingly, not for crapes. Also, it seems that physical wound closure on the crapes happens more slowly, but when you look at the repeated wounding/pruning, there's just no rot, in any direction. So it's either rapid chemical wall formation or inherent resistance on the part of the crapes to the resident wood-decaying organisms, and the latter explanation seems ridiculous to me.
Garden writers in the south have spent a great deal of energy and column-space in efforts to dissuade people from this hard pruning. Southern Living ranted about it regularly and I think Steve Bender even coined "crape murder" in an early article. Obviously, all that writing has had little impact on the practice. Most people don't think it's as ugly as you and I do, and the argument that topping them will kill them doesn't hold any water at all. I think if it had, most people would have stopped doing it.
After being here - and working with and observing crape myrtles for almost eleven years now (we did not really have them up north) - I think I can add as to why people do it.
1.) Landscapers that don't have tons of work in Jan-Feb recommend it (and introduce the customers to it).
2.) Size control (which you mentioned) and a resultant "neatly trimmed" appearance (which some people really do like).
3.) "Everybody does it - must be right".
And, I think, the most important one:
4.) To get really huge flower panicles.
I believe that is the kicker. Flower panicles on unpruned crapes tend to be rather small. Flower panicles on hard-pruned crapes can be simply gargantuan. Larger panicles will flower longer (more individual florets over time) and many people love huge flowers and almost all gardeners want longer flowering times on their plants.
Personally, I have taken a softer approach on this concept than when I first got here. I still don't advocate it, and I still think that, in the main, it's almost always hideous. But I do readily admit the huge flower thing and I tell people that if "crape murder" (for whatever reason they do it) gives them pleasure - go ahead and do it. They're not hurting the plant.
However, I always follow up that discussion with a strong admonition that just because it doesn't harm crapes, it does not mean you should do the same thing to other trees. It will certainly do all the bad things you listed. Personally, I think that's the biggest risk with crape murder...people think it's okay on this tree...must be okay on all of them.
|In natural un-topped form, in winter|