In The Two Towers, Fangorn describes Gandalf as the wizard with the greatest love of trees. Why would he not think of Radagast in this way?
Radagast was certainly a lover of nature and had been a servant of Yavanna—the fashioner of the first trees—in Aman. However, in Middle-Earth, he seems to have been less obviously concerned with the plants of the land than with the creatures; he may have been the closest the beasts and birds and fish of any of the Istari.
He also had a house at Rhosgobel in Rhovanian, which he probably had built. Placed just beyond the eaves of Mirkwood, his house was probably (like Beorn's steading, not so very far away) built of wood—that is, of felled trees. Gandalf, in contrast, never made a fixed abode, so he probably never had had the need to engage in woodcutting and carpentry. This may explain why Treebeard preferred Gandalf to Radagast.