Thank you. That's exactly the sort of info I was after--very helpful.

John Crichton wrote in
news:TbFed.448712$Fg5.17448@attbi_s53:

As long as we are talking about a fairly large tree, which if it is
covered in ivy it probably is, then you can spray roundup on the ivy
leaves without harming the tree. If it is at the base of the tree
where there are no limbs or tree leaves you should have no problem.
Roundup is absorbed in the green foliage and then travels to the plant
roots and kills the plant. I would try to keep it off the tree leaves
but it would take a tremendous amout of Roundup to harm a large oak,
assuming the trunk is many feet in diameter, not a young tree with a
6" diameter trunk. The ivy above the cut you made will eventually die
on its own. Since the weather is now so cool it can survive quite some
time cut off from the root system. Some Roundup on the green leafy
parts will hasten it's demise. One thing you need to know about
Roundup, it works best when the plant is in its active growing season
in the warmer months when it is actively exchanging water and
nutrients between the leaves and the root system. In the late fall
and winter this process can slow dramatically as the temperatures drop
and it can take a long time for the Roundup to take effect. Once you
get into freezing weather Roundup generally works poorly if at all.
In essence the tranport system in the plant has slowed to a crawl or
shutdown entirely and the leaves will not take up the material before
it gets washed off. Conversely, if you run into protracted hot dry
weather in the summer Roundup can loose it effectiveness since most
plants will try to shutdown and go dormant to ride out the dry spell.
Spring, early summer and early fall are the best times to use Roundup
type herbicides.