I've never had reason to study the rate of increase, but I can tell you I got them at the same time and they are running a close race for increase. The height is determined more by location in relation to the sun, because they all get the same nutrients and same soil quality. They don't all get the same sun, though. Some have taken up residence beneath a smoke tree that grew like a gigantic giraffe this year in all the rain. They look to be a little shorter than the others and are shorter than they were in past years -- shade from the smoke tree is my guess.
