From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156719-173235
Question: Is it harmful to a tree to have ivy climbiing it? Many of
our neighbors have ivy growing up their trees, and it looks nice, but
my mom thinks the ivy will kill our tree. Will it?
Answer: Ivy can cause all kinds of problems when it is allowed to
climb up tree trunks. It can hold too much moisture against the bark,
leading to rot. It can cause splitting of the outer bark of the tree
due to the shear force of the growing vines. If it's allow to climb up
very far, it will become heavy enough to cause stress to the tree and
it can act as a sail, catching wind and pulling a tree over. Add this
to the potential of strangulation and girdling, and the facts indicate
it is wise to remove ivy from the tree. At a minimum, cut enough ivy
away to expose the flare of the trunk of the tree where it meets the
ground. It would be even better to remove the ivy within several feet
of the trunk. After removing the roots and cutting the vines at ground
level, pull what you can from the tree. This might mean that you'll
have to cut the intertwining vines and pull them off in pieces.
Anything you can't reach to remove (including the disc-like pads),
will eventually weather away. Don't worry too much about removing the
pads - they won't produce new plants. And, unless a piece of an ivy
vine has gotten a foothold in a crevice or other moisture holding spot
on the tree, the vine should die off after being cut at ground level.