I don't have specific experience with your question regarding tree root competition, so I'll let someone else take that one. The largest paniculata has got to be Hydrangea paniculata 'Grandiflora' or "PeeGee" hydrangea. It commonly is grown as a small tree/large shrub, and is one of the parents of many newer hybrid crosses.

In a word, yes! Paniculatas *are* that tough :) As far as root competition, both paniculatas and maples have some surface roots, and there is potential for them bumping up against each other there and deeper underground. If you start with a small size when you plant, paniculatas will usually find their way around their neighbors, and competition won't be much of an issue. As for the largest Paniculata around, I would look at 'Praecox'. The 100+ year-old parent specimen in the U.S. is still alive and well at the Arnold Arboretum, and is easily 15 feet tall by 18-20 feet wide. Its flowers are smaller though than many other paniculata cultivars, and 'Grandiflora' would certainly be a great choice for a large paniculata with showier flowers.

As long as you are not dealing with large trees with equally large and competitive root systems, you should be fine.

And "large" how? Largest plant? Largest flower? 'Pee Gee' typically is listed as being able to produce the largest inflorescences (18"x12"), but 'White Moth' may give it a run for its money. 'Mt Everest' is considered the largest overall shrub at 12' plus. The Swan ('Barbara') develops the largest individual sepals. 'Tardiva's claim to fame is that it extends the flowering period until very late into the season (as late as November in some areas).

Thanks for all the good info. I should have clarified the largest paniculata as in plant size.

The majority seem to have 10' listed as the mature height. Personally the largest one I've seen was a foot taller than me, so just over 7'. That was at the Chicago Botanic Gardens. Even the ones I see in mature landscapes are maybe 6'.

Bottomline I want one to block a view and need minimum 8'. If its 6' it doesn't do the job so I figure I get the largest one to be safe.

As for the root competition, I didn't know if it had a wimpy/sensative root system like an azaela (possibly a bad example for some). I knew it was a strong grower though.

This is pure conincidence, but I have one that was planted within the dripzone at the same time as a 10' freeman maple ('Autumn Blaze').

I want to plant a couple right inside the dripline of a 15 year old freeman maple (identity is still questionable). As well as just inside the dripline of a 7 year old freeman maple ('Celzam').

Angles of the sun will allow plenty of sun...maybe in 20 yrs sun could be a problem.

I need the late season bloom, 8' tall, tough, adaptable, partial sun...not alot out there...plus Hydrangeas are pretty much the greatest plant in the Mid-Midwest.

I just don't want to be naive about the root system...although I have clay soil so things don't tend to fight as much like in sandy soils.

Celzam is Celebration, right? I have that tree (along with Marmo, Autumn Blaze & Royal Red), and I've yet to see any tree being such a water hog that the plants around it suffer. Marmo is within a few feet of climbing roses. However, my maples are only about 7 years in the ground. I do not have any paniculatas near my maples (just by chance), but all of my hydrangeas are smashed in beds with other plants and none seem to suffer dehydration. =)

Paniculatas grow fast IMO.

Several time on this forum I displayed picture of 40 y/old PG in 20+' range. Though it's somewhat one of a kind 'king-size' plant, 12-15' tall PGs are not that a big rarity around here.
Re: Tardiva.
It attained 10' in my yard in a short 4 years.
Re: planting near maples.
Raise (4-6") soil level in a planting area and add a little bid every year for the next couple of years. Paniculatas have a strong root system and when established could compete with maples.