English ivy, Hedera helix, is probably the nastiest and most aggressively invasive exotic species that I work with. It climbs high into redwood trees and overwhelms understory plants (that live below the trees). It invades many of the landscapes, and worst of all, it climbs building where it ruins paint and causes rot. It grows faster than we can keep up with it.
English ivy is actually a nice ground cover plant for refined landscapes. I grew it at my home in town. Contrary to popular belief, it does not root into and parasitize the trees that it climbs. Actually, it rarely overwhelms and shades out large trees. It prefers to keep them alive for support, from which it disperses its seed. However, it does promote decay in the trunks that it climbs, particularly where it retains moisture at ground level. Native trees are not accustomed to that.
We try to remove as much English ivy at work as possible, which includes removing it from trees and buildings. So far, with a few exceptions of small bits of ivy that broke off high in the trees that it climbed, I have been able to remove all ivy from the trees and walls that I have worked on.
Others were not so fortunate. When quick and efficient removal of ivy from the bases of as many mature trees as possible is the priority, ivy is more often severed down low, and left to die on the trunks of the infested trees. It looks shabby to say the least, and takes many years to deteriorate and fall away. In the picture above, dead ivy that was severed within the past few years is already being replaced by new ivy, which will also need to be severed.
The same technique happens with ivy on buildings. It gets cut at the foundations, but left on the walls to shrivel and turn brown. The dead ivy in the picture below was reaching upstairs eaves when it was severed, and remains there a few years later.
What annoys me so much about this technique is that it does not take much extra effort or time to tug quite most of the lower ivy from trees and walls. For most situations, all ivy can be dislodged, although tiny aerial roots remain. It is much easier to dislodge while fresh than after it is dried and crispy.