But if it's something you must do... what kind of tree? Any surface roots to contend with? What kind of mass floral display - annuals (more bang for your buck bloom-wise) or a succession of perennials (which generally bloom once a season)? Watering conditions; sun/shade conditions? How wide a ring do you envision? Is this tree taking pride of place in the middle of your lawn? Does anything grow under it now - like grass up to the trunk and would a ring really enhance its appearance?

I don't know - trees just aren't that little black dress screaming for a strand of pearls.

It is best to do it when planting the tree, so the roots will not be disturbed when the tree (and roots) are larger. Make sure to leave room for the tree to expand.

It is really best not to plant annuals, and disturb the roots every year.

What to plant is a personal choice. since you are in zone 5 like me, I like the look of masses of day lillies around a tree and really like layering day lillies and hosta I am now in the process of planting 6 trees with them.

If your trees are already established..be very careful. When a tree or its roots is injured it usually takes a few years to see the results of the damage. You could loose the tree. I would not plant around a tree that has shallow roots.

Good luck

Which you have to mow around in a tight circle.

Trees are best under-planted with suitable shrubs, preferably evergreen, with annuals, bulbs and herbaceous perennials being displayed elsewhere in the scene.