Cherry Explosion's dark pink lacecap blooms may be a good choice in alkaline soil. It also seems to produce a lot of bloomage from new growth in northern locations, which is good when winter zaps all the stems down here in the South.
CE's blooms may also turn lavender in acidic soils so what you see in the picture may not be what you will get... Because Huntsville is near the Limestone Valleys and the Appalachian Plateau, it is hard to tell if the soil there -in that bed- is acidic or alkaline (unless it is tested first) and so it is also hard to guess what color the blooms eventually will be. But I sure like the shade of pink in the picture! I hope it produces something close to that for you.
But consider that, if you remove the soil on the top 3" to get rid of weeds, it becomes hard to tell how that will eventually affect the color of the blooms within a given shrub
. Typically, the roots absorb aluminum (or at least they try to) based on soil pH and this makes all/most of the blooms within one shrub turn the same shade of pink or lavender. But if the top 3" have a certain level of soil pH/aluminum AND the rest of the soil has a different level, bloom colors thru a given bush might be slightly different since some roots uptake more or less aluminum depending on whether they are in the top 3" or not. Not forever. Temporarily at least. Eventually, those 3" will turn the same soil pH as the soil is below those 3" and then the bloom color would match more within each bush. I have never removed/replaced soil and planted hydrangeas there so, I am guessing there might be a little color change of some blooms within the bush, if any. The majority of the hydrangea fibruous small roots are in the top 4" and these absorb a lot of the water plus minerals. So do not panic if there are a few bloom differences within a shrub.
I also figure that, if the flower bed gets morning sun and afternoon/evening shade, CE's Fall foliage will be a nice wine/burgundy-ish color. That is just what CE does. Just make sure any afternoon sun does not make the leaves brown out early.
Since the growing season in AL will be slightly longer than in the northern states, I am not sure if the plant will stay corralled at 3' by 3'; it may get to 4' wide by late Fall instead. But really, who knows... Ha! That is always hard to tell/guess unless you can find another person who is already growing it in the same city/neighborhood.
By the way, smooth hydrangea (H. arborescens, a.k.a. Annabelle-like hydrangeas) and oakleaf hydrangea (H. quercifolia) are native to Alabama. That includes some of the Annabelle cousins with pink blooms.
One thought/question... if CE gets 3' wide (or more), will it cover the walkway in the picture? If it does, will that be ok? I am not sure of the width of the bed there... Should it exceed its bounds, pruning (around July-August more or less) may be required to keep it in check. It will develop invisible flower buds on old wood by "then" (by July-August more or less). Maybe prune it by the end of June or by early July, just to make sure next year's early Spring flower buds are not pruned off?
Mushroom compost is fine with most plants, including hydrangeas. Regular organic compost and composted manure are fine too. But I am not sure I would apply mushroom compost or fertilizers now to the shrubs... in the middle of the summer I mean.
https://questions.gardeningknowhow.com/ ... ts/page/6/
If the plants already contain those round fertilizer pellets that wholesalers add as fertilizers to the pots, I would not fertilize the potted plants with compost until Spring 2018, maybe just spread the compost in the new soil around the hydrangeas, not on the hydrangeas...