It is an arborescens...probably Annabelle. Blooms on new wood so it can be pruned at anytime, usually early Spring.....yg

Thanks, yellowgirl. That's just what I was hoping to hear, as I try to garden mostly with natives. I figured it probably bloomed on new wood, because it's got several old-looking, woody canes in the center (which are kind of bare and not very attractive) and lots of new growth around the edges, which is where the flowers are. I'll be sure to give it a good pruning next spring, and hopefully that will improve its shape.

I'm sorry, but it's not an arborescens, it is a paniculata.
Leaves are different and grow in a typical paniculata's whorles toward the tip.
The simplest test you could perform is to look at the main stems: if they are woody and branching, it's paniculata.
If new growth is green and unbranched it's arborescens.

Here is a link that might be useful: H. paniculata

Now I'm really confused. The leaves at the tip of the branches are whorled, but there is little branching of the stems (though the ones in the center are quite woody). New growth is greenish (I'm assuming you mean the stems) and unbranched (the only branching I see is where a branch has been broken off. The new growth in an area like this branches off in two directions.) I've attached another photo that shows the whole shrub. Maybe that will help for definite ID.

Here is a link that might be useful:

Humm... Is that all one shrub?? I don't think it is Annabelle either. The first picture made me think of an arborescens but not Annabelle - some other type. Perhaps a radiata. The bloom seems wrong for Annabelle - especially in the second shot.

The second shot seems more paniculata but I can't figure out what type. Is the picture recent? Are the leaves green underneath or silvery?

Regarding types of arborescens, here is a link that might help:

Here is a link that might be useful: Types of arborescens

Wow, what a difference a camera angle makes!!....I stand corrected. I had a funny feeling about the bloom in the first pic which is why I said "probably" Annabelle. I thought maybe Bounty, but it's not as common in the market. However, the long stems and leaves in the second picture, as well as the overall size of the plant, definitely looks like a paniculata. The blooms look so round as opposed to panicle shaped but the color is very limey like H.paniculata "Limelight" yet it doesn't appear to be as upright growing as Limelight. What a monster, it's gorgeous. It still blooms on new wood, so pruning requirements remain the same.....yg

I'm still standing on my opinion that this is a paniculata, especially after seeing the second picture, but...
in a month or so you'll be able to see another plant ID character...
a) arborescens flowers go thru this transformation:
green-white-GREEN-brown (if it's arborescens, indeed, they suppose to be already mostly green at this time of the year),
b) paniculata flowers go thru this sequence:
green-white-PINK-brown (if it's paniculata they suppose to be mostly white by now).

In any event, pruning timing remains the same, late winter or early spring. Both blooms on new wood.

Does anyone have any ideas for some shade structure maybe made from lattice work, I have hydrangeas that I am going to plant in the garden, they will have their own row, but I need to shade them with something attractive!

I'm leaning towards paniculata grandiflora due to the number of sterile flowers versus fertile.

The plot thickens. I had no idea this freebie shrub would spark such a interesting discussion! Silvergold, yes, it's all one shrub. It was just a sucker when I planted it, so there's no doubt about that.

The photo was taken yesterday. In looking at the photo again, it really makes the shrub look huge, but in actuality, it's probably about 5 ft. wide by about 4 or 4 1/2 ft. tall.

I don't know if I'd call the color of the underside of the leaves silver, Silvergold, but they definitely have a sort of dull greyish cast to them (I compared them to the closest nearby shrub, a viburnum prunifolium, whose leaves were definitely a true green on the underside).

While checking out the leaves, I noticed that at the base of some of the largest (and probably oldest) flowerheads, the individual flowers have a definite greenish cast to them, for whatever that's worth.

My real dilemma is this. This hydrangea, whatever the variety, is quickly outgrowing its current location between a PJM rhododenron and the viburnum prunifolium, and I am going to have to move it. (I knew next to nothing about how big it would get when I planted it, and it was such a little twig!) I have a new planting area which is totally shaded by a new deck and somewhat dry (I solicited ideas for a plant for this area in the native plants forum). My research led me to believe that an arborescens might do well here. Would a paniculata do equally well in that situation, or do I need to move it somewhere else if it's a paniculata? I'm really looking forward to having this shrub in a better spot and pruning it correctly as the blooms really are pretty, and I'd love to see more of them on the plant.

Another pieces of ID.

Paniculatas grow from the one single point no matter how wide it is, while arborescens have a suckering habit and will have a wide base where numerous branches coming up.

Paniculatas have a woody new growth (it might be green, but still woody), while arborescens have a fleshy, soft (perennial types) new growth.

Paniculatas will bloom very poorly in shade, while arborescens are not fussy in that respect.

Well, the reason I asked about the underside leaf color is that the flower reminded me a bit of that on Hydrangea arborescens radiata 'Samantha'. This has very distinct silver undersides on the leaves though.

Samantha:
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Also, check out my link above for other arborescens pictures to look at the foilage. As ego45 says, arborescens is a suckering plant too.

Paniculata can flower fine in shade. just depends on how bright the shade is.

Sure looks a lot like Limelight. Esp since you planted it as a twig a few years ago. Does it get much sun? They like a lot of sun.

Can you take a closeup profile of one flower? Try for a conical shaped one.

Here is a link that might be useful: Limelight Photos

Well, it's really looking like it's probably a paniculata. After reading ego45's description of the growth habit, I went outside for another look, because I was sure there were suckers coming up. When I lifted up the branches on one side, however, I found that all the growth was coming from one central trunk. What has happened is that where the branches have arched down and touched the ground, they have put out roots, which I had assumed were suckers.

In addition, I found one flowerhead where some of the flowers have started to turn pink.

I couldn't find a conical shaped flowerhead to photograph, squirrelheaven. All the blooms are round.

As far as its current site, it gets morning sun and afternoon shade and the soil is moist and well-drained.

You guys are certainly more knowledgeable on the subject of hydrangeas than I am, but if I had to guess, I'd say it's most likely a paniculata.

I'm still voting for pee gee then - paniculata grandiflora. I find mine isn't very conical. My limelight is much moe conical than my pee gee.

Paniculata Grandiflora:
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The flowerheads on mine look very similar to the ones in the picture of your paniculata grandiflora, silvergold. I looked at lots of pictures of Limelight, too, and something about the flowerheads just didn't look right when compared to mine. The individual sterile flowers kind of looked more delicate than the ones on my shrub.

Well, at least we had fun identifying it! Thanks for the challenge!

Check to see if the flowers are fragrant. Sometimes you have to be very close to determine that, othertimes, you can smell the fragrance from several feet away. If it is fragrant, then it is a paniculata.
Rb

If it didn't get much bigger, most likely H. Paniculata Little Lamb... If it is 8-10 ft... Probably H. P. Grandiflora.