Ivy on trees: what UK and US sources say, and the studies behind them

This page lays out the positions the named sources take on ivy (Hedera helix, also called English or common ivy) and the evidence they give. It does not say whether ivy on any tree should be cut or left, and it gives no removal method. It does not diagnose a tree or say whether any tree is safe or unsafe.

The positions side by side

Source	Where it speaks for	What it says about ivy and the tree
Royal Horticultural Society	United Kingdom, where ivy is native	Says ivy is not directly harmful to trees. It says the aerial roots do not enter the bark and serve only to hold on, and that a mature, healthy tree can bear some competition from ivy for water and nutrients at the ground and for light in the crown.
New York Botanical Garden, Mertz Library	Europe and the United States	Says ivy is generally not damaging to trees in Europe, where it is native. In the United States it says the pests and diseases that control its growth are lacking, so ivy can eventually overwhelm a tree. It says ivy is not parasitic and its roots do not take nutrients or moisture from the tree.
Fareham Borough Council (England)	United Kingdom	Says the jury is still out on whether ivy kills trees. It reports that some botanists see ivy taking only water through its clinging roots without harm, and that others point to weight and shade once it reaches the top of a tree.
Rutgers Cooperative Extension, New Jersey	New Jersey woodlands, where it treats ivy as invasive	Says invasive vines such as English ivy can kill healthy, mature trees, that ivy climbing a trunk smothers the canopy and prevents photosynthesis, and that its added weight will often cause limbs or the whole tree to collapse.
USDA Forest Service, Fire Effects Information System	North America	Reviews the literature (Waggy, 2010). It reports that impacts may be less in the northeastern United States and Canada, and lists the studies and the anecdotes it draws on. See the next section.

The studies the US Forest Service page points to

The Forest Service review (Melissa Waggy, 2010) separates what was measured from what was speculated. It says that in general English ivy is considered to affect ecological communities by displacing native ground plants and weakening or killing host trees, and that NatureServe ranks its ecological impact as medium, with the impact on community structure the greatest concern.

Claim in the review	What it rests on, as the review gives it
Host trees may fall in storms	The review says trees hosting ivy may be susceptible to windfall, especially if they are weak or carry several ivy stems. It cites the British Isles biological flora in the Journal of Ecology (Metcalfe, 2005) for the weak-tree point and a study of river forests (Schnitzler and Heuze, Forest Ecology and Management, 2006) for the several-stems point. It adds that one author speculated that water or ice on the evergreen leaves may add storm damage.
American elms may be especially affected	In a riparian forest in Washington, DC (a National Park Service monograph, Thomas, 1980), 13% of fallen American elms had supported ivy, whereas only 9% of all the other fallen tree species had.
Ivy may hold back host growth	The review reports an Oklahoma study suggesting that ivy may hold back the growth of both the crown and the roots of host trees, maples in particular.
Ivy kills supporting branches	The review labels this anecdotal: as ivy climbs, it covers supporting branches and blocks sunlight, and the host tree may eventually die from steady weakening.
Ivy does not move to a new host	In French river forests, ivy did not spread to another host tree after the original tree was uprooted, but survived another 17 years by developing adventitious roots (Schnitzler, 1995).

The review also says several other effects it lists, such as soil nitrogen and water quality, have mostly not been well documented, and it names ivy as a host of the bacterium behind bacterial leaf scorch, a pathogen that harms native trees including elms, oaks and maples.

When the sources say ivy matters more

The Royal Horticultural Society names four cases where it says removal might be desirable or necessary: ornamental trees with decorative bark, young or newly planted trees, trees with sparse canopies such as ash and larch, and old, weak or previously damaged trees, where it says ivy growth can hide problems such as cavities or signs of decay and add weight to an ailing canopy in wind. It says ivy often grows into trees that are already in decline. The New York Botanical Garden library separates trunk ivy, where it says removing it or letting it grow is largely a matter of personal preference, from ivy in the upper canopy, which it says can affect stability in high winds, crowd out light and compete for water and nutrients.

What the sources say about wildlife

The Royal Horticultural Society calls ivy of enormous value to wildlife: dense evergreen leaves give year-round hiding, roosting and hibernating places, it is a preferred nesting plant for dunnocks, flycatchers, wrens and robins, and its flowers feed pollinators and its berries feed birds. Fareham Borough Council adds that the ivy mining bee depends on ivy flowers. The US Forest Service review reports the opposite for North America: several invasive species publications call English ivy of little value to native wildlife, and in the Pacific Northwest, sites dominated by ivy had lower diversity of mammals, birds and amphibians than uninvaded sites. Rutgers says deer rarely eat it. Roosts and nests in thick growth, and the rules that go with them, are in the guide on wildlife and the law.

Where this leaves the argument

The sources do not disagree about the plant so much as about the place. The British sources describe a native plant, the US sources describe an introduced plant in forests where, the New York Botanical Garden library says, the pests and diseases that control it are lacking, and the Forest Service review labels the claim that ivy kills supporting branches as anecdotal. Other things that grow on trees are on the things growing on trees page, and the one parasite among them has its own page. Where a source's management section involves herbicides, the guide on who may apply pesticides covers licensing.