Outdig the holes...to the place you think the roots will max out at, and you want to replace it with a sand substance. Just make sure the sand is not fine. You are going to have an interesting thing happen. The clay is slow draining...so your material (sand) that you fill the hole with will let the water pass, but when it hits the clay it will stop. This will cause a little ponding so you will have to be careful with not have having to much where the plant is.