Ivy is generally not a loved plant, it is either tolerated or hated with a vengeance; in the United States its import or sale is banned.
Common Ivy (Hedera helix) will climb and spread over almost anything and can actually cause damage to buildings with its aerial roots digging into the mortar and brickwork and encroaching under roof tiles. Ivy will not kill trees or shrubs although they may suffer through having to share water and soil nutrients with this vigorous intruder. Often trees are so enveloped with Ivy it is difficult to see the host specie, and if the tree is dead the weight of the Ivy can cause it to collapse.
Ivy provides an evergreen screen and shelter for small birds and insects during the winter months, it flowers from late Summer through to late Autumn and is rich in nectar therefore an important food source for bees and other insects. The black fruits are also an important food source for birds.
From a landscaping and aesthetic point of view I feel it is quite acceptable as a ground cover plant which can suppress weed growth and survive in shady areas where other plants fail. The main problem is that if it is allowed to climb or spread without control it can appear unsightly as an intrusion into the general flora.
I would therefore advocate the removal of Ivy from shrubs, and particularly trees when established along the upper stems and lateral branches; severing at some 3.00M rather than at ground level is often more acceptable in order to provide a screen at low level.
In conclusion I would say that Ivy is not all bad, although it should be controlled and discouraged from encroaching over buildings and too far into shrubs and trees.
Anthony George, Tree Consultant, The Dulwich Estate