The first thing that comes to mind is Pampas grass (Cortaderia selloana)
Pampas grass pictures

If it's not that it might be Giant reed (Arundo donax)
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Scott...send a picture! The two suggestions from treeguy don't seem to match up with your description, but a picture sure would help!

How high over the fence are they? Total height of 6' would lead to different guesses than total height of 15'.

I can't think of what would be blooming in August. Are you sure they aren't cat tails going to seed? Mine are right now and the description fits perfectly. LOL. So many people grow the dwarf catails and they're rather short, but the wild ones here do grow a good five feet easily.

I think cattails are what we in the UK call Bullrush,only ours don't get that tall!

Perhaps it is bugbane. The link below has decent pictures but doesn't really convey the tru height that these can reach (click on the picture for an enlargement). Mine grows stalks so tall that the full flower is over my 5' tall fence and the entire plant is about 3' in diameter (planted in fall of 2003). Also, google Black Cohosh, Cimicifuga, or Black Snakeroot for more pictures.

Here is a link that might be useful: bugbane

Indiangrass, perhaps?

I can't see the pic?

could be a perennial grass or cimicfuga, which blooms in late summer and gets 5 ft tall or more.

Time for a pic, Scott. You could have it answered pronto, I suspect.

I'll upload a picture tonight. It's definately not a tree, but some kind of decorative shrub. I know a picture is needed here for proper identification.

ilk5a - if you are talking about the picture I mentioned, it is on Wikipedia, which is what the link is to (sorry for the confusion)

aesculus parviflora (bottlebrush buckeye)?
they would have bloomed earlier in my area but maybe still flowering there.

Here is a link that might be useful: bottlebrush buckeye

Here are a couple pictures of it.

It has lost some of it's spectacular looks, but is still attractive in my eyes. It's been looking like this for almost a month now.

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Reminds me of Butterfly bush (Buddleia), but better.

Reminds me of Clethra alnifolia.

A white-flowered cultivar of Buddleja, for me.

Resin

It could also be an Itea, or sweetspire. I have all three, and would need a closer look at the leaves to tell the difference. My buddlea and also my clethra are both blooming now, I think my iteas bloomed last month. However, you are under a very different climate than I. Why don't you just knock on their door and ask? LOL. I do that a lot, and usually the owners are quite happy they have something somebody likes and is interested in.

When I looked up bugbane, I thought it looked similar and does seem to be talked about in Colorado. I agree it also could be butterfly bush. What I can't tell from my pictures is what the leaves look like and what the flowers look like close up.

I agree, I need to just pull over and ask them what it is ... the next time I see them outside. Hopefully they know what it is, I suspect they do, since its so unique (at least unique in my my neighborhood).

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It's hard to tell from the picture, but it appears to have opposite leaves. That rules out itea and clethra, so it may very well be a really nice buddleia. Mine never looked so good, however.

It does indeed say Buddleia but those flower heads are incredibly long,I'm used to them being about a half to a third as long! A fine specimen,reminds me of white fireworks going off!

Definitely not bugbane - the flower stalks are leafless and tower over the rest of the plant. Looks like a very nice butterfly bush to me.

It's a Buddleia (butterfly Bush), no doubt about it. Why not ask the owners if they know the cultivar?
(BTW, it's highly unlikely that Cimicifuga, Itea or Clethra would be thriving in Utah and the latter two would be done blooming by late August, as would the most common cultivars of the former.)