Mistletoe on trees: what extension and forest agencies say
Mistletoe is a parasitic plant that grows in tree crowns. Agencies in the United States and a British botany journal describe two very different groups, and they do not agree that it does the same harm everywhere.
Checked against the sources at the bottom of this page on October 8, 2026. Rules, fees and pay change: the source has the last word.
This page describes what the named sources say about mistletoe and where to read it. It does not say whether any tree with mistletoe should be pruned or left alone, and it gives no removal method. The sources it reads are written for California, Oregon, South Carolina, North American conifer forests and Britain, so what applies in another place depends on the species there. For a general plain-words page on how trees take up water and food, see how a tree works.
Two groups with different habits
The University of California statewide IPM program (UC IPM), in a Pest Note revised in March 2022, says all mistletoes in California belong to the sandalwood family and are either broadleaf mistletoes or dwarf mistletoes. Broadleaf mistletoes have green stems and thick, nearly oval leaves. They often grow into rounded, shrub-like clusters that can reach 2 feet or more across, and they stay evergreen, so they show in the bare crowns of deciduous trees in winter. UC IPM calls them hemiparasites: they make their own food by photosynthesis but depend on the host for water and nutrients.
Dwarf mistletoes are smaller. UC IPM says their mature stems are not woody and are less than 6 to 8 inches long, and that they grow only on members of the pine family. It calls them true parasites that take virtually all their nutrients from the host. Oregon State University Extension (publication EM 9679, published November 2021, reviewed 2026) gives the same split for Oregon forests: dwarf mistletoes grow only on conifers and are generally species-specific, while true or leafy mistletoes are green, carry some chlorophyll and spread with the help of birds.
How each source describes the harm
| Source | Broadleaf or true mistletoe | Dwarf mistletoe |
|---|---|---|
| UC IPM (California, 2022) | Branches may swell where the mistletoe attaches; branch parts beyond it may wither. Healthy trees are often able to tolerate a few, at the cost of weakened or sometimes killed branches. Heavy infestations may reduce vigor. | Slower growth, often stunting. Swelling at the infection site and witches' brooms. UC IPM lists premature mortality, especially where a tree is stressed by drought or disease. |
| Oregon State Extension (EM 9679) | Effects on host trees are described as typically negligible. Trees with numerous infections can show growth loss, branch dieback and drought stress. | Described as able to cause significant health issues: growth loss, distortion, topkill, mortality and a tendency to be attacked by bark beetles. |
| USDA Forest Service (RMRS-GTR-98, 2002) | For mistletoes of North American conifers, the abstract lists deformation of the infected stem, growth loss, greater susceptibility to other disease agents or insects, and reduced longevity. It says the effects in heavily infested stands and recreation areas may be beneficial or detrimental depending on management objectives. | |
UC IPM adds that witches' brooms are not proof of dwarf mistletoe, because genetic mutations, mites, viruses and fungi can also cause them, and that the plant itself has to be found to confirm it.
Hosts and spread
UC IPM lists broadleaf hosts that include alder, flowering pear, ash, birch, box elder, cottonwood, locust, silver maple, walnut and zelkova. It says birds eat the sticky whitish berries and pass the seeds, so new infestations often start on tall trees where birds perch. It says dwarf mistletoe seeds are thrown by the fruit itself, and that a new host tree needs to be within about 40 feet of an infested one. Clemson Cooperative Extension in South Carolina describes American or oak mistletoe on pecan, oak, elm, hackberry, maple and wild cherry, and says its white berries contain chemicals poisonous to people and domestic animals.
In Britain the species is a different one. A paper in British and Irish Botany (John Box, volume 1, issue 1, 2019) reports that mistletoe on oak is rare there: thirteen confirmed mistletoe-oaks, in and around Herefordshire, with a population of between ten and twenty trees that has looked stable over time.
Where the sources differ
Both the Oregon State publication and the UC IPM note separate light from heavy infestations, and both separate dwarf from broadleaf types. The Forest Service document is about conifer forests, where management goals decide whether the effects count as a problem. None of the sources here gives a single answer for every tree. Each of them has its own management section, linked below, and the agencies name different approaches.
For the pests and diseases that regulators track, see the pests and diseases guide. Where a source's management advice involves pesticides, the guide on who may apply pesticides covers licensing.
Sources
- UC IPM, Pest Notes: Mistletoes (publication 7437), revised March 2022, read October 8, 2026
- Oregon State University Extension, Mistletoe's effects on trees vary by type (EM 9679), reviewed 2026, read October 8, 2026
- USDA Forest Service, Mistletoes of North American Conifers (RMRS-GTR-98), September 2002, abstract, read October 8, 2026
- Clemson Cooperative Extension Upstate District, What to know about mistletoe, read October 8, 2026
- British and Irish Botany, Oaks parasitised by mistletoe in Britain, 2019, read October 8, 2026