A: Well, the answer to this one isn’t simple or perfect, but your best course of action, coming up:
1. Take down your stone tree ring and remove that soil. This tree was planted at normal grade, then an extensive ring wall was added, along with nearly a foot of soil atop the root ball. Adding excess soil above grade can damage the root system and compact the soil, which encourages more suckering than usual. Fully expose the original root flare.
2. Prune – remove as many of the suckers as you can a bit under what’s become your new grade after removing the excess soil over the root flare. Try to leave the fewest cut surfaces that you can that will still remove the suckers, even if you need to cut back a little further along a surface root. Minor root pruning this time of year won’t be a problem, but for something like this, you might want to wait until December. Avoid cutting large roots.
3. Once it’s reached this extent of suckering, you’re likely to never completely get rid of it. It will definitely help the tree’s health overall to remove that excess soil and should give you fewer suckers to deal with, though.
4. Don’t use herbicides to attempt to stop the suckers. It doesn’t help much, and can do more harm to the mother tree they’re attached to than you want.
5. Finally, skilled arborists use specialized tools that can make this process easier and less damaging to the tree. You may well consider calling in one who has experience in this sort of soil removal to do the work correctly for you because of the extreme case involved. It’ll be easier to control the tree’s suckering later if you have an expert do the initial work.