The tree dressing at Coldrum and other Sacred Sites is quite common and has been going on for years. Tree Dressing and such are not specifically a Wiccan idea, as similar Well Dressing was very much a Christian practice (though possibly of Pagan origin). Many Pagan visitors to Coldrum continue this practice and as long as no plastics or metals are used then I don’t find it a problem.
The more serious vandalism such as the severe spray painting of the chamber a few years ago and the cracking of one of the outer stones by fire (which also caused a large chunk to fall off which subsequently dissappeared) is a far greater problem. This beautiful place our Ancestors made thousands of years ago is now being destroyed by idiots and if they continue it will all be gone, with nothing left for our decendants to see.