I hope your garden won't be hurt too badly !

Being under the tree slows the hailstone in their fall and make the tarp stay intact I guess.

On way to assure the sleep place could be to put two tarp, one bigger above on smaller inside (same shape as actual) with some room between the two.
Or maybe put some foliage on the tarp to act as a pillow.
Just some thoughts not sure about the usage.