Phisherman wrote:

.... An established plant will take several applications
to kill it. RoundUp is probably safer than Weed-B-Gone. Wet as many
leaves as possible with the herbicide, repeat every two weeks. Be
careful not to get the solution on other plants...



Fall is the best time to RoundUp your PI. In the spring, the plant is
taking stored nutrients from the roots to form leaves, growing outward.
In the fall, the leaves are sending nutrients to the roots, growing
inward, so the herbicide will be transported to the roots more
efficiently then.

However, if you plan on several applications, you can do it at any time
during the growing season. It will just take more applications early in
the season than it will take later. I doubt that herbicide would be
effective in the winter, not to mention the difficulty in identifying
which vine is the PI and which is your perennial plant you want to keep.