it might hinge on how dark it is the rest of the day ...

in very bright shade.. high canopy.. etc.. things can bloom just fine ...

so i would think you must define how deep the shade is otherwise...

and perhaps.. which 6 hours of the day they get sun ... the last and first 2 hours of the day at much different than high noon.. as to sun intensity ....

its not really a function of simply the number of hours ...

ken

I can tell you that when we have had cool summers with much more overcast than normal our crape myrtles have not bloomed at all...so I think that sufficient sun is critical.

Sara

plantingman, get some very large mirrors for strong late afternoon sun, ask neighbor's permission... ;>)

hortster

Ken! You're smokin'! Good answer.

Ken, that is good information. I never thought of that way, which is why I come here and ask. Actually, the six hours of sun IS late afternoon sun, but still probably not enough for crape myrtles since they are in deep shade from sunrise until about 1:30 P.M. The only thing I could do is buy a cheap crape somewhere and try it to see what happens.
And, Hortster, my neighbors wouldn't mind the mirrors at all. They would probably want me to share them. lol

Thanks, Sara. You make a good point. When I purchase some spireas from a nursery, I may order an extra crape myrtle to try it in the space I am thinking of to see how well it does.

I've got hundreds under a shade tree for sale and they have bloomed. They do want full sun, but in my experience they can sometimes still bloom in some shade... I'm not recommending shade, just saying I've got some baby crapes blooming in the shade.

Clint
CrapeMyrtleGuy

Here is a link that might be useful: Crape Myrtle Guy