Question
Is it safe for the pine tree to make a planter on top of the ground around the base of a pine tree to plant a bed of hostas?

Answer
Hi Deb;
As long as you leave enough room between the walls of the planter, and the trunk of the tree to grow.
You need an inside wall to keep the soil off the trunk of the tree, and room to allow the tre to grow in dirth.
I would suggest bringing the wall out at least 6 inches all around the tree, that should allow for plenty years of growth, unless the tree is just a sappling, then I would allow 10 inches to a foot.
Built the orter wall and fill with landscaper's mix ( sometimes called planter's mix), with some of your existing soil. If you have good, loose draining soil, then a 1 to 1 mixture should be ok, but if your soil is not that good,m then 2 parts mix and 1 part soil.
If you don't have any place you can take soil from, you can buy a few bags of topsoil and mix with the planter's or landscaper's mix.
The depth of the soil on top of the feeder and outlying roots doesn't matter, as long as you water enough to soak down so the roots will get water too. If your planter is less that 3 ft. across, between the walls, and the tree is a pretty well established one, then it should get enough water from the feeder roots.
The feeder roots extend as far out as the tips of the furthest branches.
If soil is put directly up against the trunk, it will rot the trunk, and kill the tree. As long as the trunk has room to expand, it will be fine.
Charlotte