Are the boxes set on dirt so that the roots can go into the soil with no landscape fabric under them?

No. The 4 x 4' box is 22" deep. The bottom is enclosed by 1/4" hardware cloth to keep out voles. It is set atop 3/4" gravel, which is over landscape fabric to keep weeds down.

I'm thinking of a panicular dwarf hydrangea such as Flare , Little Firelight, Vanilla Strawberry.

Without a general location (GA zone 7 is different than Cape Cod zone 7 or WA zone 7) panicled hydrangeas like those you listed will do the best in zone 7 with much sun of the various hydrangea species. Most of the other hydrangea species need afternoon shade or all day dappled shade since they naturally grow in the understory and are much needier of constant moisture.

I love panicled hydrangeas, and in my cold zone 5 they color up well. Fom what I have read they may not do that in zone 7 unless you have cool late summer and fall evenings and nights. In your case, I would ask around to see how they behave in your area. If they don’t color well and just go from white to tan without the late summer reds, consider a type where the fall pink tones aren’t a big part of the appeal like Limelight or Little Lime which are both lime green and white, or White Diamond that stays very white almost until frost here.

In the area where you have the box, I would remove the landscape cloth under it. That will allow the plants’ roots to grow into the soil under the boxes. The hardware cloth and gravel won’t be an issue for the roots, but the landscape cloth will interfere with movement of water and any deeper roots. And IME gardening in an area where voles are a constant issue, they don‘t seem to bother panicled hydrangeas at all. I have 5 of them in several gardens where other more succeptible plants have been killed while the hydrangeas have flourished.

shouldnt we be concerned about white flowers against the whole fence.. making it all too white??

^^ This. i moved Little Quickfire because i was silly enough to site it in front of a brick wall. As flowers aged they just disappeared against the brick. I’ve moved it and enjoy it much more now, for a good long time.

Hearing that tonight would be a final killing frost, I dutifully planted the 16 unplanted hostas, heuchies, and ferns I had in the backyard into raised cedar beds where they would NOT be staying permanently, just to protect them from the frost until I get a chance to assemble the newly arrived cedar raised beds and fill them with good topsoil. Hard work! And I had already done quite a bit of housework. So I rewarded myself by going to my area's premier nursery and acquiring a Vanilla Strawberry panicle hydrangea. It is pretty much the same as the smaller Flare (3 x 3') but gets like 7' tall by 4-5' wide. The cedar bed where it was intended is 4 x 4', so simply pruning it back each late winter/early spring should keep it within its allotted space. It can get 7' tall, as there is nothing above it to interfere. I'm hoping that birds will be attracted by the cover it will provide.

I don't understand why you aren't planting right in the ground. Soooo much more work to plant in and maintain those boxes.

Hydrangea should work OK. Oakleaf do best for me in part day sun. Most of them would get too big for the container. There are some dwarf ones, but the dwarf ones get a lot bigger than they're supposed to. I've found the dwarf ones are just slower growing. They end up just as big. At least that's been my experience.

Lacey, the soil is pure clay with lots of rocks. And, it's easier for me to use beds. I don't find it more work to plant in or maintain the boxes.