A couple of years ago, I turned the sump pump area of my lawn into a dry creek. While doing so, I noticed quite a bit of blue (really dark grey) clay in the soil. I proceeded to plant in the area (dogwoods, water loving flowers, witch hazel, river birch) and everything accept one plant os growing well. The witch hazel, it doesn't seem to be getting any larger; though it also doesn't look like it's under any stress. To make things worse, all of the mulch is new and nice looking. I am wondering if there is a chance that there is too much clay around this particular tree and restricting it's growth? What can I add to help this tree get bigger?