The general rule with trees (and I expect with shrubs) is to NEVER change the soil level around the base of the tree. If you want to make a raised bed, you are supposed to make a well around the tree base to manitain the soil next to the tree at its original level. You can use stone or some other barrier strip to hold in your soil. Essentially you end up with a donut around the tree. I'm sorry that I'm not explaining this very well. If you make a field stone wall of 5-6" height about 3-6 feet out from the lilac, then you will have to make a another 6" stone wall about 1 foot out from the tree (encircling the lilac). The raised flower bed will then be the area between the two stone walls. Does this make sense? Lily of the valley will spread underground indefinitely, so it might very well try to grow under or over the stone wall. Can you put landscape fabric or some other barrier next to the stone wall to prevent the lily of the valley from trying to grow through the wall?

Thanks ontnative!

I decided not to do the raised garden as I do not want to make it big enough to make the inside barrier and I don't want to harm the lilac.

Ken