I don't think you want anything behind them. Generally the tallest plants in a garden, in front of the house, will go up to the windows, not above. These go up to the windows, and should remain the tallest plants you have. In fact, if they get taller next year, trim them back to be at the height of the window sill, and keep doing so each year. They will fill in sideways, but shouldn't be taller.

You have lovely windows, great brick that goes well with your roof. These things should show, and not be covered. *IF* you felt you needed something else, the only thing I might consider is a small trellis to the side of the window nearest the porch (opposite side, to the left when facing) with a climbing floral plant like clematis if that will grow in your planting zone.

HI -- I would Not put anything behind them . I would just lay down some landscape cloth and then cover with some fine gravel or a bark mulch . Make sure to have those plants pruned well in the spring to keep them below the windowsill levels .

I love clematis. I thought maybe I should put something there because a large hole was created when we removed the bushes. You can see it more when standing on the porch looking down. I am hoping the hydrangeas fill in even more, but not sure if they will fill in from the back. They have grown since the other bushes were removed which was early summer. Part of the issue is the middle hydrangea is not as tall as the two on either side.

Spring pruning? I usually prune in the fall, but not Spring. Good to know. I was a little shy pruning too because I heard some varieties of hydrangea are not supposed to be pruned. (They won't come back the next year if you do). I am not sure how that works though because they always get those dead brown stalks.

HI -- needlesing --- better to be safe than sorry -- check with your local garden center or horticulture experts to see what time is best for pruning in your area .