We have a mutant oakleaf hydrangea: this monster persistently grows 10-12 feet tall and just as wide. Unfortunately, it was planted smack-dab next to the central heat/air unit, so it's a constant battle between (wo)man and nature to keep the shrub in bounds.
I let it grow wild and woolly last year (I tell myself it was the stress of the season, and I didn't want to add to the plant's misery.) When our A/C unit needed some work in May of this year; the A/C guys took one look at the shrub and advised me I could either pay them to trim it, or pay for a second trip charge after I had it trimmed. Neither of those options sounded good, so I told them to hang tight for a few minutes. Then I and my loppers set in to giving it an emergency hair cut. When all was said and done, I hauled off nearly as much as I left growing. Can you say "hack job"? For certain, no one is going to hire me as an arborist or let me near their head with a pair of scissors!
But...the plant is a vigorous grower, so it's now busily filling in all the gaps I created a few months ago. Hopefully your mother's plant was of similar "sturdy" stock and responded with lots of new growth ;o)
Oh, how funny! Mom's plant must have been just as sturdy, because when I was back there a few years ago, that crazy bush was nearly as tall as the house and in full bloom. When Kay gave me these little starters a few weeks ago, I decided I would put them in the least likely place of destruction. The only thing they can overtake is my bedroom window, maybe.
I think yours will very likely fill in before you know it. I'll be sure to not call you when mine needs trimming.