Tuesday, January 18, 2011

Friend or Fiend – What to Do About That Tree-Killing Ivy?

© 2011 by Tom King


You'd be surprised what people will fight about. While researching an article on ivy and live oak trees, I stumbled on a whole series of rather nasty exchanges between two points of view over ivy. One group says ivy is a natural part of the woodland eco-system and ought to be left alone. On this side are organizations like the Royal Horticultural Society, the University of Arkansas and Texas A&M University who clearly state that ivy is not parasitic and not a threat to the health of healthy trees.

On the other side are those who see a damaged or sick tree literally covered with ivy and make the intuitive leap that the ivy must have killed the tree. On this side are gardeners, the US Department of Agriculture (some of them) and even some authorities from the above mentioned Societies and Universities. It seems so obvious that ivy chokes trees to death that the idea is firmly entrenched despite evidence to the contrary. There are some folks who actually walk around with clippers and chain saws, cutting ivy off at the base of trees, thinking they are “saving the trees” from a noxious invader.

If you understand the nature of ivy, however, the so-called common sense view is shown to be wrong. Ivy is not a parasite. Ivy is symbiotic at best and benign at worst. Ancient trees like live oaks form almost complete and independent eco-systems, sheltering within their branches, not only animals, birds and insects, but also providing the food, moisture and shelter to sustain the fauna and flora that live upon the sturdy framework provided by the tree trunk, its branches and its leaves.

Trees have enormous root systems that draw moisture and nutrition from a huge volume of soil. The root system of even a very large ivy draws relatively miniscule amounts of water and nutrients from the soil at the base of the tree. All a tree must do to compensate is to extend its root system farther and deeper into the Earth.

Ivy do not burrow into the tree itself or into walls they climb or buildings they grow on. Ivies put out small suckers that attach themselves to the trunk and branches of the tree to hold the ivy in place. These anchors do not draw nutrients from the tree itself, but only act as a support for the growing vine. Most ivy supports itself entirely from these anchor points and do not twine themselves around the tree like strangler figs do in the jungles of Cambodia. Virginia creapers, English ivy and other ivy vines may grow large and cover the bark or the trees they grow on, but they don't “choke” the trees.

So why are sick trees often covered with ivy?

Good question. First understand that most trees naturally control the growth of ivy under the canopy. Ivy like deep shade. They tend to stay out of the canopy of a healthy tree because more sunlight trickles through at the higher reaches of the crown. The sunlight discourages ivy growth there. When ivy reaches the crown, it matures and begins to bloom and produce fruits. Animals and birds feed off the ivy's leaves and fruit and keep the ivy trimmed back. If trees become sick and decline, the animals, birds and bugs that have lived there all along may leave and be replaced by borers, pests and opportunistic critters that damage the tree further. The ivy, unchecked by the tree's animal population, grows further and further out the limbs.

If a tree is in decline, ivy can become a liability, contributing to the weight of weakened limbs and the “sail-effect” of extra leaves in the higher reaches of the canopy. The weight of ivy foliage can make the tree top heavy and more vulerable to storm winds.

The only time you really need to trim ivy vines from a tree is when you are treating the tree for disease. Ivy can hide fungus infestations, insect damage and rot. Removing the ivy allows you to reduce the weight of vegetation on weakened limbs and helps you find and treat damaged areas and bug infestation. Consult a tree expert before pruning problem limbs as too severe trimming can impede the tree's recovery process. Trimming ivy up the trunk and below the crown allows you to take the weight off compromised limbs while preserving the ivy's sheltering and forage benefits to the tree's animals, birds and beneficial insects at the lower levels of the tree.

While plants like English ivy may be invasive, especially in Northwest rain forests and may threaten to push out indigenous ground growing plants, there is no real evidence that English ivy are a threat to healthy trees.

I notice that the folks most interested in conducting a pogrom against tree-dwelling ivy tend to be gardeners and city-folk. I suspect it is ivy's penchant for covering over rotted and fallen trees and vegetation with a nice green blanket that most irritates the obsessive gardener. These guys seem most happy with a nice unlittered forest floor and trees with clean, unadorned branches. There are a lot of these neatnick foresters, especially in England and the damper parts of the United States. I'm not sure what a rainy climate has to do with it, but down here in sunny Texas, most of us seem less interested in running around with a machete, chopping down innocent ivy and grape vines for the dubious purpose of “saving the trees”.

Ivy is a natural part of the woodland ecology. Ivy grows on trees in the same way Spanish moss, ressurection fern, baby squirrels and birds do. There are some parasitic plants like mistletoe that can cause damage to trees. Healthy trees, though, seem to be able to resist or at least tolerate parasites. Ivy is not one of these parasites. If your tree is in good shape, in a good place and undamaged by you or by diseases transmitted from neighboring trees, then ivy serves only to support the tree's individual eco-system and to add a vibrant green cloak that accents the beauty of the tree.

The relationship of great trees to ivy was beautifully explained by the Eugene Fields in his sweet short story, “The Oak Tree and the Ivy”.  Fields believed trees don't live have to live forever. As they age and begin to decline, the lucky ones with a drape of ivy are gradually covered more and more thickly with vines and leaves and the story portrays this as a sweet thing.  As I write this blog, I'm sitting comfortably in my recliner, my hair gone white, with two blankets wrapped around my legs and feet. I would deeply resent it if someone snatched off my blanket because they thought I'd be healthier with cold feet! The truth is both people and trees naturally age and decline. You can't jerk off our blankets and hope we'll grow spontaneously younger.

God save us from busybodies, health gurus and horticulturalists who would rather we not age gracefully.


Sources:

Texas A&M: Live Oak Tree Problems and Solutions
UC Davis Cooperative Extension: Landscape Notes – Diagnosing Your Oak Tree
American Forests: Live Oak the Ultimate Southerner
Bartlett Tree Research Laboratories: Live Oak in Texas
Plant Conservation Alliance: English Ivy
Nature Net: Ivy on Trees – Kill It or Cherish It?
Royal Horticultural Society: Ivy on Trees and a Ground Cover Weed
US Department of Agriculture: Plant Guide – Virginia Creeper
University of Arkansas: Plant of the Week – English Ivy
The Oak Tree and the Ivy by Eugene Fields

2 comments:

  1. I am so glad I read your blog before I continued cutting any more Ivy from my trees. I love the look (and AM a gardener) but thought it was bad for trees. I'm going to take your word for it that Ivy is not a problem for trees.

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  2. Thanks for publishing this. In Sutro Forest in San Francisco, we've seen lots of tree trunks covered in ivy, but it doesn't get in the canopy. It's really valuable habitat for small birds.

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