Thanks for all the responses. Just wanted to update in case anyone is wondering...My decision was made for me when I was able to get six 'Honey Maid' hollies that I had had my eye on for $4 each. We planted them as a short hedge in Site "A", because this site is more protected, and I thought would be more suitable for them, in addition to being more effective as a year-round screen because they are evergreen. The 'Limelights' went in Site "B". I think the fun is in seeing how they will do! According to my research, oaks are more deeply rooted than some other trees, so there is not as much concern about underplanting. In my neighborhood, which is unofficially named "Oakwood", you can see many old oaks with shrubs/trees planted underneath. I am very concientious about making sure things stay watered while being established, and we are going to put grass clippings and shredded oak leaves on as mulch on both areas this fall. In the spring we'll put hardwood mulch over these borders, to help hold in moisture. Thanks again for everyones input. Nora

You can leave them for winter interest or deadhead them. Either is fine. I normally let Mother Nature take care of spent blooms but she forgets about some of them sometimes and I end up cutting a few here and there in Spring. So, do as you like.

"I have been reading some forums that hydrangeas can cause issues with the tree's roots so I didn't know if anyone had experience with this." All kinds of claims that are not true are made on forums. Is there any corroboration of such claims by a viable source? It's not realistic in my mind that a couple of Hydrangeas are capable, or even have a hope of, killing or harming an oak tree. But as Babs says, it's pretty dark in there. Unless you remove some of the bottom branches and let light in, I don't think you're going to get a great display. A huge part of an older tree's magnificence is its trunk. Removing lower limbs exposes this great feature.

Pots of trailing annuals would be better than adding a raised bed which might smother the LL hydrangea roots and harm the LL. If there is room, you could also plant farther out to avoid he roots with a spreading annual such as wave petunias or Verbena Homestead purple (which might actually be a perennial for you) or a spreading lantana. Or consider a plant with bright foliage that is a perennial so that you don’t have to replant annually. Around me it might be Japanese foestbgrass (Hakonechloa) with gold or variegated foliage if there is part to full shade.