Pink Diamond is a cultivar of Hydrangea paniculata and is a shrub rather than a tree. (Let's not restart the discussion on the difference between the two - I think most people understand what it means in common parlance). I have had a quick Google and in the first 5 hits the mature height was given as a max of 3m (ca 10 feet). Some sources give the mature height as 2.5m. I think you have been labouring under a misapprehension as to what this shrub will look like when fully grown. Yours sounds as if it is behaving exactly as it should.

Here is a link that might be useful: Pink Diamond

Might have been bought grown on a single stem, therefore "tree". Should be building height over time, although stems will tend to bend over.

Thanks to all for your comments. Yes...there obviously has been some misrepresentation from the vendor. When it was purchased, it was stated that the mature height was anywhere between 10'-15' in 6 to 7 years.

CeeView

there obviously has been some misrepresentation from the vendor.

===>>>

maybe.. maybe not.. if he is in a more temperate location.. as compared to CT.. then maybe it has greater potential down south ...

if your growing season is 5 months.. and his is 8 months.. you should be able to see that his trees can grow larger than your trees ...

ken

The vendor was Cottage Farms. This was not purchased as a shrub but as a tree. What I'm seeing that they're selling now is the Limelight and the Quickfire Hydrangea tree. These as well are supposed to get to a mature height of 8' - 10'.

Thanks,
CeeView

Tree form may be a better description - relating to a plant grown with a single stem - as these plants are considered shrubs rather than trees. I have a number of hydrangea paniculata that I've grown into a tree form with the tallest being about 7 foot tall in mid summer growth - but I prune mine back every spring so I'm keeping them short. If not pruned annually, I would think the stems would be very prone to breaking. My experience with Pink Diamond is that it's a very vigorous grower with annual stem growth at least 3 foot.

I see the same form with physocarpus opulifolius - Ninebark - being offered - especially Diablo.

Marshall