Tree or standard hydrangeas have to be trained to grow like that. This is the reason they are expensive. The old plant might still make it and might do well. You should mulch it about 3-4" deep and up to the drip line. In your old plant's case, I would extend the mulch 6" from the stem minimum, up to 1'. Limelight blooms on new wood so you might still have blooms. However, it is imperative for the plant to develop a good root system instead during the first 1-2 years. Blooms, if you get them, should be considered icing on the cake.

I don't have a picture right now, but I'll take a look at it during daylight hours and see if I can take a cutting and start a new shrub from it. The tree has been so top-heavy lately that it's been basically falling over onto the ground. I thought it might be due to the extreme heat, but I think it may need staked to stay up. I really wanted a shrub when I bought this tree but was hoping possibly that I could keep taking off the top so it would grow down below. Thank you so much for your helpful information. What you say makes good sense to me.

If your tree is a paniculata hydrangea, it only requires mulch around the base, about 4 inches would be great. If it is a macrophilia, it requires winterizing. Check with "Heys" posts, he writes excellent articles on winterizing. Gary

Unfortunately, that width is probably going to limit you :-) The elevated nature of any above ground container exposes it to much colder temperatures than a plant would experience in the ground. Especially the roots, which are far more vulnerable to cold damage than the top growth with even very hardy plants. But the larger the soil mass around them, the greater the insulating factor, so with a large container in a moderately cold zone you can often get away with overwintering plants that would likely not survive in a smaller container. Pretty much any tree or shrub you may choose will easily produce roots that fill up that 10" width, resulting in a root system that will very likely come into contact with the planter walls and therefore be exposed to excessively cold air temperatures. And root damage (resulting in plant death) can occur at temperatures as mild as the mid 20's, depending on species. And you are looking at the possibility of below zero air temps in zone 6 :-(( You might be able to risk it with the selection of some hardy dwarf conifers but they tend to be expensive to use in what is essentially a hardiness experiment at best. I wouldn't recommend any flowering shrub or tree except perhaps something like alpine currant, as the root system will simply get too large too fast. Unless you are inclined to toss money away on this experiment, I'd stick to seasonal plants. You can always stick in cut willow stems or redtwig dogwood stems together with cut conifer greens and hollies if you want some winter interest.