Professors at University of Oxford have undertaken a study, Ivy on Walls, to discover whether ivy acts bioprotectively or biodeterioratively with stone walls. Read about the study here.
I look forward to the results of this study because I am of the belief that ivy doesn't cause deterioration, it merely takes advantage of a deteriorating wall or sick tree. Its rootlets are not actual roots in the sense that they do not penetrate a surface in search of moisture or nutrients.
According to the above referenced book, Ivies, published in 1992, ivy is and always has been unfairly maligned. An experiment was conducted by Winchester College in the U.K., beginning in 1890, where half of the trees in an oak woodland were stripped of ivy every ten years and half were left covered with ivy growth. When the trees were felled in 1942, there was no difference in height, girth or average cubic content.
Ivy not only thrives where nothing else will, it provides shelter for birds and butterflies and food with some of the last nectar and berries going into winter. Ivy was the inspiration for poets and artists; it has a history of use as a medicinal herb and is the symbol of everlasting life.
In Delaware, it has been defined as an invasive, nuisance species so now people are ripping it out and putting in mulch which is so much better. Garden stores are advised against selling it and instead are requested to sell indigenous plants. I cannot argue that it is not indigenous to the U.S. but I'd much rather look at ivy in a dark corner of a shaded yard than a pile of mud or mulch.
While I was looking through the book for information, I found a four-leaf clover tucked in its pages. :-) I'll keep you posted on the study.