Things growing on trees: ivy, mistletoe, lichen and moss

This page describes what the named sources say. It does not diagnose a tree, say whether any tree is healthy or failing, or tell anyone whether to remove or leave a growth. Sources are agency and university publications, and each page gives where they differ.

Compared at a glance

Growth	What the sources say it is	What they say about harm
Ivy	An evergreen climber that attaches to bark with aerial roots (Royal Horticultural Society). The sources lay out different positions for Britain and North America. See the page on ivy on trees.	The Royal Horticultural Society says it is not directly harmful to trees. Rutgers Extension says invasive vines such as English ivy can kill healthy, mature trees. The New York Botanical Garden library says it is generally not damaging in Europe but can overwhelm a tree in the United States.
Mistletoe	A parasitic plant in the crown. Broadleaf (true) mistletoes are hemiparasites that make some of their own food. Dwarf mistletoes grow on conifers and are true parasites (UC IPM, Oregon State). See the page on mistletoe.	Differs by type and amount. Oregon State calls effects of true mistletoe typically negligible and dwarf mistletoe a cause of significant health issues. UC IPM says healthy trees often tolerate a few.
Lichen	A fungus living with an alga or cyanobacteria, in flat, leafy, crusty or hair-like forms (Florida IFAS). See the page on lichen on bark.	Florida, Clemson and Washington State say lichens do not cause harm, and that heavy growth is seen on declining trees.
Moss and Spanish moss	True mosses on bark, and Spanish and ball moss, which are bromeliad epiphytes rather than mosses (Florida IFAS). See the page on moss and Spanish moss.	Washington State says mosses are not parasitic. Clemson says Spanish moss causes little to no detrimental effect. Florida says concerns are significant only for trees already in decline.
Slime molds	Fungus-like organisms that live on soil, mulch and rotting logs and feed on decaying matter (Clemson).	Clemson says they do not feed on green plants and do not pose a direct threat to garden plants.

The parasite and the passengers

Florida IFAS Extension separates harmless epiphytes from mistletoe, which it calls a plant parasite: epiphytes attach to a plant but get minerals from water running over leaves and branches. Clemson Cooperative Extension says a less vigorous plant tends to be more open, so more sunlight reaches the bark and lichen growth rises, and Washington State University says mosses and lichens are often associated with declining trees but do not cause decline. Mistletoe is different because it connects to the water-carrying tissue of its host, as UC IPM describes.

Bats and nesting birds that use thick growth on trees are covered in the guide on wildlife and the law. For growths with an insect or mite cause, see tree galls. The mechanics of a tree's water and food tissues are in how a tree works.

Things growing on trees: ivy, mistletoe, lichen and moss

Compared at a glance

The parasite and the passengers

Related pages

On the tree, one page each

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

Lichen on tree bark: what extension services say it is

Mistletoe on trees: what extension and forest agencies say

Moss on trees, and Spanish moss: what extension services say

Sources