Rabbits were gnawing at the bark of my apple trees last winter, so I put spiral plastic tree guards around the trunks. I recently looked under the guards and was alarmed to see split, oozing bark and lots of ants. What went wrong?
The tree protectors you describe are helpful at keeping hungry critters away from tender-barked trees in winter. The white ones also help protect against frost cracking and sunscald by reflecting sunlight. Tree wraps made of burlap and paper are sometimes used for the same purposes.
But whatever style of tree guard you use, you should consider it a seasonal protection. Wait until the tree is dormant in fall to install it, and remove it as soon as the tree begins to leaf out in spring.
As you’ve discovered, these protectors trap heat and moisture around the trunk during the growing season, creating an environment that can attract insects, promote decay, or even foster fungal diseases such as canker. For similar reasons, it’s not wise to mound mulch against the trunk of a tree.
If gnawing animals are a year-round nuisance, fashion wide cylindrical guards from hardware cloth, which won’t trap moisture against the bark. Or use a stinky repellent, sprinkling or spraying it at the base of each tree.
Fortunately, many trees lose their appeal to rabbits as their bark becomes thicker and corkier. —Doug Hall