The feeder roots of a tree are fairly close to the surface....
It is generally not recommended that a tiller be used around them...

Some trees don't even like to have anything growing around their base...I think you can find this info on the net somewhere...

What kind of tree is it? What perennials are you planting?

This article may help a bit....

Here is a link that might be useful: Planting around trees...

As mentioned planting under trees can be a tricky business. Mature trees tend to pull up a lot of water and nutrients making it a tough environment for smaller plants. If the trees are evergreen the soil is generally higher in acid so you'd want to find acid loving shade plants. If the trees have lower branches there's not a lot of indirect light available under the tree.

Unless you want height you might consider some ground covers like lamium, vinca minor, lily of the valley or violets.

Hosta and columbine can tolerate shade and some drought and can give you some bloom time. If you go with columbine I suggest starting from seed as they have a taproot that can be fussy to move. This also saves you the chore of having to dig and preserves the soil structure under the tree. I tend to stick with the "vulgar" varieties as they tend to be pretty darn hardy. If you decide to plant hosta it's usually a lot easier to work with rather small divisions and let them grow into the space available. If you buy one in a gallon bucket you might divide it into 2 or 3 sections before planting. This does make for a smaller plant but you might find more long term success this way.

Whatever you decide to do you'll have to baby your plants for the first couple of years, at least. Water regularly since it's likely to be much drier under the trees than elsewhere in the yard.

Best of luck to you!

Is your tree still alive??

I would remove the decorative brick work and use the ground covers mentioned above. Movement of the tree roots will eventually cause the bricks to shift and look unsightly, anyway, unless your circle is well beyond the canopy line. Add mulch and be patient. Watch to see (long term) what plants do well and what don't.

I would cover the cleared area with wood chips and then make a new flower bed as far away from the tree as possible. Here is the easy way. Save up newspapers for a couple of months. Spread them at least 5-ply thick and cover with wood chips for the winter. Not a thick mulch of chips, just enough to prevent the paper from blowing away.
Next Spring, uncover the soil and VOILA! instant garden with no grass. Work in some compost or manure and plant.

Alternative plan, plant in pots. Sink them at least part way into the soil, as tree roots allow. This takes alot of watering, but looks nearly perfect. When it all freezes out, dump the soil and replace in the ground. If you need to, place rocks in pots to prevent blowing away during the winter.
In the long run, gardening under a tree is sort of a lose, lose situation. Unless you garden under a small and non-invasive type tree. Right now, I am doing great gardening under a River Birch. Eventually, the big bully (the tree) will win out over the little guys (the perennials) It is pretty futile to fight mother nature, but many of us continue to do it. Give it some thought. Pondy