Mistletoe on a tree

How Mistletoe Works

Mistletoe (Phoradendron flavescens or Viscum album) is a parasitic plant that grows on trees, particularly hardwood trees like oak and apple. A parasite is a plant or animal that needs another plant or animal to survive. As mistletoe grows on a tree and uses its roots to invade a tree’s bark, which allows mistletoe to absorb the tree’s nutrients. Sometimes, mistletoe can harm a tree and cause deformities in a tree’s branches, but usually it doesn’t kill its host. If the host dies, the mistletoe dies.

Mistletoe produces its own food by photosynthesis, and is able to live on its own, although it is mostly found in trees. It’s common for a mistletoe plant to grow on top of another mistletoe plant.

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Mistletoe is easy to spot in the winter because its leaves stay green all year long. In the United States, it grows in tropical and subtropical regions (from New Jersey to Florida). Mistletoe has pointy, green, leathery leaves, with waxy berries that are either red or white. The plant’s flowers can be a wide variety of colors, from bright red to yellow to green.

Ingesting mistletoe can cause severe stomach cramps and diarrhea, and in some cases can be fatal. If you have mistletoe in your house this holiday season, be sure that it is in a place where children and pets won’t be able to get to it.

Spreading the Seed

The red-and-white berries that grow on mistletoe are eaten by birds that eventually leave their droppings at their favorite hang-out spot — on a tree branch. The droppings contain seeds that sprout roots into the tree branch. The birds also help spread the seed by wiping their beaks on the tree bark to clean off the sticky seeds after they’ve eaten. The seeds are sticky because of the juice inside the berry. This stickiness helps the seeds stay in the tree rather than falling to the ground. Within six weeks, the mistletoe plant begins growing, although it takes five years to flower.

In the next section, we’ll look at the history behind mistletoe and find out what ancient Druids used it for.

12 Things to Know about Mistletoe

The white berries of mistletoe plants are poisonous to humans but valuable food to many other species.

Often used as a symbol of renewal because it stays green all winter, mistletoe is famed for its stolen-kisses power. But the plant also is important to wildlife, and it may have critical value for humans, too. Extracts from mistletoe—newly used in Europe to combat colon cancer, the second greatest cause of cancer death in Europe and the Americas—show signs of being more effective against cancer, and less toxic to humans, than standard chemotherapy.

Here are some mistletoe facts that may give you new respect for a plant that, until now, you might have considered as just an excuse to limber up your lips:

  • There are 1,300 mistletoe species worldwide. The continental United States and Canada are home to more than 30 species, and Hawaii harbors another six.
  • Globally, more than 20 mistletoe species are endangered.
  • All mistletoes grow as parasites on the branches of trees and shrubs. The genus name of North America’s oak mistletoe—by far the most common species in the eastern United States—is Phoradendron, Greek for “tree thief.”
  • Ancient Anglo-Saxons noticed that mistletoe often grows where birds leave droppings, which is how mistletoe got its name: In Anglo-Saxon, “mistel” means “dung” and “tan” means “twig,” hence, “dung-on-a-twig.”
  • Mistletoes produce white berries, each containing one sticky seed that can attach to birds and mammals for a ride to new growing sites. The ripe white berries of dwarf mistletoe, native to the western United States and Canada, also can explode, ejecting seeds at an initial average speed of 60 miles per hour and scattering them as far as 50 feet.
  • When a mistletoe seed lands on a suitable host, it sends out roots that penetrate the tree and draw on its nutrients and water. Mistletoes also can produce energy through photosynthesis in their green leaves.
  • As they mature, mistletoes grow into thick, often rounded masses of branches and stems until they look like baskets, sometimes called “witches’ brooms,” which can reach 5-feet wide and weigh 50 pounds.
  • Trees infested with mistletoe die early because of the parasitic growth, producing dead trees useful to nesting birds and mammals. A mistletoe-infested forest may produce three times more cavity-nesting birds than a forest lacking mistletoe.
  • A variety of birds nest directly in witches’ brooms, including house wrens, chickadees, mourning doves and pygmy nuthatches. Researchers found that 43 percent of spotted owl nests in one forest were associated with witches’ brooms and that 64 percent of all Cooper’s hawk nests in northeastern Oregon were in mistletoe. Several tree squirrel species also nest in witches’ brooms.

Mistletoe grows in tangled balls of stems that can be up to five feet across. They’re sometimes called witches’ brooms.

  • Three kinds of U.S. butterflies depend on mistletoe for survival: the great purple hairstreak, the thicket hairstreak and the Johnson’s hairstreak. These butterflies lay eggs on mistletoe, and their young eat the leaves. The adults of all three species feed on mistletoe nectar, as do some species of native bees.
  • The mistletoe’s white berries are toxic to humans but are favored during autumn and winter—when other foods are scarce—by mammals ranging from deer and elk to squirrels, chipmunks and porcupines. Many bird species, such as robins, chickadees, bluebirds, and mourning doves, also eat the berries.
  • The kissing custom may date to at least the 1500s in Europe. It was practiced in the early United States: Washington Irving referred to it in “Christmas Eve,” from his 1820 collection of essays and stories, The Sketch Book of Geoffrey Crayon, Gent. In Irving’s day, each time a couple kissed under a mistletoe sprig, they removed one of the white berries. When the berries were all gone, so was the sprig’s kissin’ power.

Happy Holidays!

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How to Manage Pests

Pests in Gardens and Landscapes

Mistletoe

Published 2/06

In this Guideline:


  • Identification
  • Life cycle and biology
  • Damage
  • Management
  • About Pest Notes
  • Publication
  • Glossary

Large-leaf mistletoes are evergreen as seen in this deciduous tree.

Dwarf mistletoe stems and seeds growing from a pine branch.

Mature plant of mistletoe, Phoradendron sp.

Foliage and fruit of mistletoe, Phoradendron sp.

Swollen limb or gall caused by mistletoe infesting ash trunk.

Broadleaf mistletoe (Phoradendron macrophyllum) is an evergreen parasitic plant that grows on a number of landscape tree species in California. Hosts of broadleaf mistletoe include alder, Aristocrat flowering pear, ash, birch, box elder, cottonwood, locust, silver maple, walnut, and zelkova. Other species of broadleaf mistletoe in California include P. villosum, which infests only oaks, and Viscum album, which attacks alder, apple, black locust, cottonwood, and maple in Sonoma County only. Conifers are less often attacked by broadleaf mistletoes, but white fir (Abies concolor) is significantly infested in the southern Sierra Nevada mountain area. Dwarf mistletoes (Arceuthobium spp.) infest pines, firs, and other conifers in forests, and can be a problem in forest landscapes such as in the Sierra Nevada foothills.

IDENTIFICATION

Leafy mistletoes have green stems with thick leaves that are nearly oval in shape. Plants often develop a rounded form up to 2 feet or more in diameter. The small, sticky, whitish berries are produced from October to December. Evergreen clumps of mistletoe are readily observed on deciduous trees in winter when leaves are off the trees.

LIFE CYCLE AND BIOLOGY

Mistletoe plants are either female (produce berries) or male (produce only pollen). The berries of the female plant are small, sticky, and whitish; they are very attractive to birds such as cedar waxwings, robins, and others. The birds feed on and digest the pulp of the berries, excreting the living seeds that stick tightly to any branch on which they land. In most cases, the initial infestation occurs on larger or older trees because birds prefer to perch in the tops of taller trees. A heavy buildup of mistletoe often occurs within an infested tree because birds are attracted to the berries, and may spend a good deal of time feeding on them. In addition, seeds may fall from mistletoe plants in the upper part of the tree, creating new infestations on the lower branches. The rapidity with which mistletoe spreads is directly related to the proximity and severity of established infestations, and newly planted trees can be quickly infested if they are growing near old, heavily infested trees.

After the mistletoe seed germinates, it grows through the bark and into the tree’s water-conducting tissues, where rootlike structures called haustoria develop. The haustoria gradually extend up and down within the branch as the mistletoe grows. Initially, the parasitic plant grows slowly; it may take years before the plant blooms and produces seed. Broadleaf mistletoes have succulent stems that become woody at the base. Old, mature mistletoe plants may be several feet in diameter, and on some host species, large swollen areas develop on the infected branches where the mistletoe penetrates. If the visible portion of the mistletoe is removed, new plants often resprout from the haustoria.

Dwarf mistletoes are smaller plants than broadleaf mistletoes, with mature stems less than 6 to 8 inches long. Dwarf mistletoe shoots are nonwoody, segmented, and have small scalelike leaves. While broadleaf mistletoe seeds are dispersed by birds, dwarf mistletoe seeds are spread mostly by their forcible discharge from fruit, which can propel seeds horizontally into trees up to 30 to 40 feet away.

DAMAGE

Broadleaf mistletoe absorbs both water and mineral nutrients from its host trees. Healthy trees can tolerate a few mistletoe branch infections, but individual branches may be weakened or sometimes killed. Heavily infested trees may be reduced in vigor, stunted, or even killed, especially if they are stressed by other problems such as drought or disease.

MANAGEMENT

In newly developed areas or in older established areas where trees are being replaced, the ideal method of controlling or preventing mistletoe is to plant trees believed to be resistant or moderately resistant to mistletoe. Avoid trees like Modesto ash, known to be especially susceptible to mistletoe infestation. Where many new trees are being planted, control mistletoe in any surrounding infected trees to reduce the infection of new trees.

For treatment of existing trees it is important to remove mistletoe before it produces seed and spreads to other limbs or trees. Mechanical control through pruning is the most effective method for removal. Growth regulators provide a degree of temporary control but repeated applications are required. Severely infested trees should be removed and replaced with less susceptible species to protect surrounding trees.

Mechanical Control

The most effective way to control mistletoe and prevent its spread is to prune out infected branches, if possible, as soon as the parasite appears. Using thinning-type pruning cuts, remove infected branches at their point of origin or back to large lateral branches. Infected branches need to be cut at least one foot below the point of mistletoe attachment in order to completely remove embedded haustoria. Done properly, limb removal for mistletoe control can maintain or even improve tree structure. Severe heading (topping) is often used to remove heavy tree infestations; however, such pruning weakens a tree’s structure, and destroys its natural form. In some cases it is best to remove severely infested trees entirely because they are usually a source of mistletoe seed.

Mistletoes infecting a major branch or the trunk where it cannot be pruned may be controlled by cutting off the mistletoe flush with the limb or trunk. Then wrap the area with a few layers of wide, black polyethylene to exclude light. Use twine or tape to secure the plastic to the limb, but do not wrap it too tightly or the branch may be damaged. In some tree species callus tissue will form under the plastic, further weakening the limb. Broadleaf mistletoe requires light and will die within a couple of years without it. It may be necessary to repeat this treatment, especially if the wrapping becomes detached or if the mistletoe does not die.

Simply cutting the mistletoe out of an infested tree each winter, even without wrapping, is better than doing nothing at all. Even though the parasite will grow back, spread is reduced because broadleaf mistletoe must be several years old before it can bloom and produce seed.

Chemical Control

The plant growth regulator ethephon (Monterey Florel Brand) may be used as directed by the label to control mistletoe in dormant host trees. To be effective, the spray must thoroughly wet the mistletoe foliage. The ideal time to treat is in spring as temperatures begin to warm, but before the tree begins to grow new leaves. Daytime temperatures must be above 65 degrees Fahrenheit for good results. Spray only the individual mistletoe plants, not the entire tree. By treating when trees are dormant, the tree foliage will not get in the way of the treatment and the mistletoe is more visible than when leaves are on the tree. Spraying provides only temporary control, especially on well-established infestations, by causing some of the mistletoe plant to fall off. The mistletoe will soon regrow at the same point, requiring retreatment.

Resistant Species

Some tree species appear resistant to broadleaf mistletoe. Bradford flowering pear, Chinese pistache, crape myrtle, eucalyptus, ginkgo, golden rain tree, liquidambar, sycamore, and conifers such as redwood and cedar are rarely infested. These or other resistant species should be considered when planting in infested areas, or when replacing infested trees.

Integrated Pest Management in a Community

An effective mistletoe control program in a community requires a combination of methods and the cooperation of developers, homeowners, businesses, and public agencies. Property owners can substantially reduce mistletoe infestations in their own trees, but without community cooperation, infestations will recur. Public wooded areas, such as parks and stream banks adjacent to urban areas, can be a continual source of seed and, therefore, mistletoe infestation. For this reason, the planting of tree species not susceptible to mistletoe infestation should be a part of every city and park plan.

The most drastic and possibly the best control measure is to remove severely infested trees and replace them with less susceptible species. Economically, tree removal could be a practical approach for both public agencies and landowners, in addition to providing a source of firewood. To assist citizens in removing mistletoe from less severely infested trees on their property, some cities loan removal tools. In other cases, neighborhood residents may pool resources to hire a tree service to remove all mistletoe in their neighborhood.

WARNING ON THE USE OF PESTICIDES

COMPILED FROM

Perry, E. J. 1995. Broadleaf Mistletoe in Landscape Trees. Univ. Calif. Coop. Ext., Marin County, HortScript #14.

Torngren, T. S., E. J. Perry, and C. L. Elmore. 1980. Mistletoe Control in Shade Trees. Oakland: Univ. Calif. Agric. Nat. Res. Leaflet 2571.

PUBLICATION INFORMATION

Pest Notes: Mistletoe
UC ANR Publication 7437

Authors: E. J. Perry, UC Cooperative Extension, Stanislaus County, C. L. Elmore, CE Weed Science, UC Davis Emeritus
Produced by IPM Education and Publications, University of California Statewide IPM Program

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Colorado State University

Print this fact sheet

by R.D. Koski, W.R. Jacobi and C.E. Swift * (12/13)

Quick Facts…

  • Mistletoes are parasitic flowering plants that can infect and damage many tree species.
  • Dwarf mistletoes (Arceuthobium spp.) are leafless parasitic plants that infect several coniferous or evergreen tree species in many western states.
  • Ponderosa, lodgepole, limber, and pinyon pines and Douglas-fir are the most common trees affected by dwarf mistletoes in Colorado.
  • Juniper mistletoe (Phoradendron juniperinum) is another type of mistletoe that can infect several juniper species in many western states.

Figure 1: Lodgepole pine infected with dwarf mistletoe display witches’ brooms and dead branches.

Figure 2: Witches’ brooms – dense, multiple branches on lodgepole pine infected with dwarf mistletoe.

Dwarf mistletoes (Arceuthobium species) are leafless parasitic plants that infect several species of conifers in Colorado forests. Dwarf mistletoes produce root-like structures that grow in the living tissue just under the bark (phloem) and in the wood (xylem), where they extract both nutrients and water from their host plants. Germinating seeds of mistletoes produce specialized structures called holdfasts that allow newly emerged parasitic plants to penetrate the tissues of host plants, thus infecting the host plant.

There are five species of dwarf mistletoes that infect conifers in Colorado (Table 1). While some dwarf mistletoes are relatively host-specific and generally do not infect other tree species, other species infect a wide range of coniferous tree species (Table 1). In addition, juniper mistletoe (Phoradendron juniperinum) is also present in Colorado and is a different type of mistletoe that is less damaging compared to dwarf mistletoes, in that it primarily acquires only water from the host tree. Juniper mistletoe is found in western Colorado and can infect several juniper species (Juniperus species) in many western states (Table 1).

The dwarf mistletoes and juniper mistletoe are dioecious plants, meaning male flowers and female flowers are produced on separate plants. The flowers produced by these mistletoes are small and inconspicuous.

Symptoms and Signs

When viewed from a distance, coniferous trees infected with dwarf mistletoes may appear to have yellow foliage, reduced foliage, abnormally dense green and distorted foliage or witches’ brooms, and mortality of the upper portion of the affected tree (Figures 1, 2, and 3).

Closer examination of branches of affected trees will reveal the yellowish green, olive green, or reddish brown segmented shoots of the parasitic plant (Figures 4 and 5).

The first symptom of dwarf mistletoe infection is a slight swelling of the bark at the infection site. The parasite is identifiable when shoots protrude two to three years after infection. Dwarf mistletoe shoots are 0.7 to 15 cm (1/2 to 6 inches) long and 2-4mm in diameter (Figures 4 and 5). Douglas-fir dwarf mistletoe shoots are hard to see because they are only about 7 mm (1/2 inch) long. When shoots have fallen off, look for the remnants of basal cups on branches.

After initial infection, mistletoes can cause distorted branching or witches’ brooms in the host tree (Figure 2). When dwarf mistletoes infect occasional hosts – hosts other than the primary host – different and unique symptoms may occur. For example, lodgepole pine dwarf mistletoe causes very large and dense witches’ brooms when it infects ponderosa pine (Figure 7). Lodgepole pine dwarf mistletoe also induces on limber pine, large elongate galls with rarely any shoots present.

Juniper mistletoe plants are usually seen as large yellowish-green round masses of small branches in the crown of junipers (Figure 6). Juniper mistletoe does not induce the juniper to make witches’ brooms.

Table 1. Mistletoes of Colorado conifers.

Common names Mistletoe species Common name of host Host plant species Description
Lodgepole pine
dwarf mistletoe
Arceuthobium americanum Lodgepole pine
ponderosa pine, whitebark pine, bristlecone pine, limber pine, mugo or mountain pine, Scots or Scotch pine, Engelmann spruce, blue spruce, subalpine fir
Primary Host: Pinus contorta var. latifolia
Occasional hosts: Pinus ponderosa var. scopulorum, Pinus albicaulis, Pinus aristata, Pinus flexilis, Pinus mugo, Pinus sylvestris, Picea engelmanii, Picea pungens, Abies lasiocarpa
Shoots are yellowish to olive green, mean shoot height 5-9 cm, with whorl-like branching; fruit is an ovoid berry, olive green and 3.5-4.5 mm long and 1.5-2.5 mm wide
Limber pine
dwarf mistletoe
Arceuthobium cyanocarpum Limber pine
whitebark pine, bristlecone pine, lodgepole pine, ponderosa pine, southwestern white pine
Primary Host: Pinus flexilis
Occasional hosts: Pinus albicaulis, Pinus aristata, Pinus contorta var. latifolia, Pinus ponderosa, Pinus strobiformis
Shoots are yellowish green, mean shoot height 3 cm, with fan-like branching; fruit is a bluish (cyan)-ovoid berry and 3.5 mm long and 2.0 mm wide
Pinyon
dwarf mistletoe
Arceuthobium divaricatum Pinyon pine Primary Host: Pinus edulis Shoots are olive green to brown, mean shoot height 8 cm, with fan-like branching; fruit is an ovoid berry, green/white and 3.5 mm long and 2.0 mm wide
Douglas-fir
dwarf mistletoe
Arceuthobium douglasii Douglas-fir
white fir, subalpine fir, Engelmann spruce, blue spruce
Primary Host: Pseudotsuga menzeisii
Occasional Hosts: Abies concolor, Abies lasiocarpa, Picea engelmannii, Picea pungens
Shoots are olive green, mean shoot height 2 cm, with fan-like branching, fruit is an ovoid berry, olive green/white, and 3.5-4.5 mm long and 1.5-2.0 mm wide
Southwest
dwarf mistletoe
Arceuthobium vaginatum subsp. cryptopodum Ponderosa pine Primary Host: Pinus ponderosa
Occasional Host: Pinus aristata, Pinus contorta var. latifolia, Pinus flexilis, Pinus strobiformis, Pinus sylvestris
Shoots are orange to reddish brown (sometimes very dark brown), mean shoot height 10 cm, with extensive branching; fruit is an ovoid berry, bi-colored, and 4.5-5.5 mm long and 2.0-3.0 mm wide
Juniper
mistletoe
Phoradendron juniperinum Rocky Mountain juniper, Utah juniper, oneseed juniper Primary Hosts: Juniperus scopulorum, Juniperus osteosperma, Juniperus monosperma Shoots are green to yellow green, leafless, mean shoot height 20-40 cm; plants globose, with extensive branching; fruit is an ovoid berry, pinkish-white, and 4 mm in diameter

Figure 3: Lodgepole pine with dead top and dense dwarf mistletoe-induced brooms on the lower stem.

Figure 4: Ponderosa pine dwarf mistletoe plants.

Figure 5: Lodgepole pine dwarf mistletoe plants. Note thin green-yellow shoots.

Figure 6: Juniper mistletoe. Note the globose growth in this juniper.

Figure 7: Dense dark green brooms on ponderosa pine infected with lodgepole pine dwarf mistletoe.

Damage to Host Trees

Dwarf mistletoe witches’ brooms extract nutrients from uninfected parts of the tree, gradually reducing host tree vigor and eventually causing premature death. Dwarf mistletoe infested trees decline and die from the top down as witches’ brooms on lower branches extract more nutrients and water (Figures 1 and 3). Death of the host tree occurs slowly in most cases and depends on the severity of infection and on the vigor and size of the tree.

A measure of dwarf mistletoe infection severity is based on a two-part rating system (Tables 2a and 2b). First, a tree’s crown is divided into thirds, and each third is rated. If there are no visible infections, that third of the crown is rated 0; if 1 to 50 percent of the branches are infected in that third, the rating is 1; and if more than 50 percent of the branches are infected, the rating is 2 (Table 2a). Add the ratings of each third to get a total dwarf mistletoe rating (Table 2b). Life expectancy information for trees afflicted with dwarf mistletoe is provided in Table 3.

Table 2a. Rating scale for dwarf mistletoe infection severity.

Percent of Tree Crown Section Infected with Dwarf Mistletoe1 Tree Crown Section Rating
0 0
1-50 1
51-100 2
1 Top third, middle third, and bottom third of tree.

Table 2b. Dwarf mistletoe rating system (DMR).

Rating1 Infection Severity
2-3 Light
4-5 Moderate
6 Heavy
1 Sum of rating of top third, middle third, and bottom third of tree.

Table 3: Approximate number of years needed to kill 50 percent of Ponderosa pine trees infected with dwarf mistletoe.1

Tree Diameter Infection Severity2
Light Moderate Heavy
4-9 inches 30 17 7
Greater than 9 inches 60 25 10
1Based on ponderosa pine in open, multi-aged stands.2Infection severity based on the following:light = a rating of 2 or 3; moderate = a rating of 4 or 5; heavy = a rating of 6.

Witches’ brooms develop over many years. Dwarf mistletoe witches’ brooms extract nutrients and water from uninfected parts of the tree, gradually reducing host tree vigor and eventually causing premature death. Junipers infested with juniper mistletoe will have clusters of the yellowish green parasitic plant growing amongst the juniper foliage (Figure 6).

Juniper mistletoe witches’ brooms extract primarily water from the host plant. On heavily infested trees, the parasite can cause death of portions of the tree during prolonged periods of below normal precipitation.

Spread

Dwarf mistletoes spread slowly from tree to tree. In closely spaced trees of about the same height, this spread is 0.3 to 0.6 m per year. The spread from large to small trees can extend 19 m (60 ft), but the average usually is less than 9 m.

The sticky seeds of Arceuthobium species are explosively discharged from the fruit at almost 60 miles per hour, adhering to any surface they strike. Seeds that adhere to young branches of susceptible trees germinate, and the mistletoe rootlet penetrates the bark. Dwarf mistletoe seeds generally are dispersed in August and early September. Birds and other animals can occasionally spread the seeds some distance to uninfected trees. Dwarf mistletoes have a relatively long life cycle between infection and seed production (six to eight years). The long life cycle allows for long-term disease management. Mistletoes are not common in nursery and ornamental plantings, but the parasites can be introduced into an area by planting trees unknowingly infected with mistletoe. Dwarf mistletoes can greatly impact the forest structure and appearance. Areas of expanding numbers of infected trees can be found in the forest where the center of the area or “donut hole” has no large trees left since they were infected years ago and died, and there might be some heavily infected understory trees in the center and a ring of heavily infected trees at the margin.

The fruits and seeds of Phoradendron juniperinum are spread by birds and consequently can be spread a great distance in a single season. A number of bird species feed on the juniper mistletoe fruits and disperse the seeds by excreting or regurgitating them. Seeds are deposited on the top side of branches of juniper hosts. Germinating seeds produce a holdfast that penetrates the host plant.

Management

Mistletoes cause a gradual decline of plant health and do not quickly cause serious injury, thus long-term management options are feasible. However, mistletoe-infected trees may become sufficiently stressed as to attract mountain pine bark beetle (MPBB), Ips bark beetles, and twig beetles that may breed and kill parts (twig and Ips) of or the whole tree (MPBB, Ips). Mistletoe management options include branch pruning, tree removal, and planting resistant tree species.

Management Options:

1. Pruning and removing trees is the best management measure available to reduce or eliminate dwarf mistletoe infestations in ornamental trees or urban forests.

2. Plant resistant trees under infected trees to replace trees when infected ones are removed.

3. Use ethephon sprays in high-value areas where planting with the same species under infected trees is the only option.

Branch Pruning and Tree Removal

Pruning out the witches’ brooms and removing infected trees is the best management measure available to reduce or eliminate dwarf mistletoe infestations in stands of high-value trees. First, remove severely infected trees (trees rated 5 and 6) or those with only a few live branches. Trees with high, unreachable mistletoe infections will continue to shower seeds on nearby trees if not cut down. However, it is not necessary to completely eradicate the mistletoe, as this may require removal of all trees. Pruning infected branches and removal of a few heavily infected trees can keep a green forest on the property.

Pruning off the lower and the largest witches’ brooms from lightly to moderately infected trees (trees rated 1 to 4) can improve the health and allow these trees to survive for decades. When removing a witches’ broom, prune the entire branch at the branch collar near the trunk. Examine trees every two or three years, and remove any newly infected branches. When pruning, keep 30 percent to 40 percent of the branches on the tree (from the top down), even if that means leaving some infected branches. Mistletoe shoots die as soon as the tree branch is cut, so no special disposal of pruning debris is needed. Trunk infections are not as detrimental as branch infections, so their removal is not necessary. If space allows, create 18 m buffer zones between infected trees and healthy trees by cutting or by planting resistant trees. Contact a professional forester, the Colorado State Forest Service, or other professionals to obtain help in these decisions.

Plant Resistant Tree Species

Planting resistant or non-host tree species in areas with infected trees will ensure that trees will be in the area even after the infected trees are removed (Table 4).

Table 4. Recommended species to replant in mistletoe affected areas.

Mistletoe-infected Species Recommended Species
ponderosa pine white fir (Abies concolor), blue spruce (Picea pungens), bristlecone pine (Pinus aristata), pinon pine (Pinus edulis), limber pine (Pinus flexilis), Douglas-fir (Pseudotsuga menziesii), and/or Rocky Mountain juniper (Juniperus scopulorum)
lodgepole pine subalpine fir (Abies lasiocarpa), Engelmann Spruce (Picea engelmannii), bristlecone pine, limber pine and/or Douglas-fir
Douglas-fir Engelmann spruce, lodgepole pine, and/or ponderosa pine
juniper Ponderosa pine and/or pinyon pine

NOTE: Scotch or Scots pine (Pinus sylvestris) is susceptible to both southwest and lodgepole pine dwarf mistletoes.

Deciduous trees and shrubs, such as birch, peashrub, ash, aspen, cottonwoods, and Gambel oak also can be planted in affected areas because dwarf and juniper mistletoe do not attack these plants. Keep in mind that site conditions and moisture availability will determine what trees and shrubs can be planted in a particular area.

Chemical Sprays

Ethephon (2-chloroethanephosphonic acid) is a growth regulating chemical that can be used to remove mistletoe shoots and reduce seed production. Ethephon is usually only used in high value areas where young trees should be protected until infected overstory trees are removed. This treatment does not kill the entire mistletoe plant, just the shoot. Retreatment is necessary until infected trees are removed, mistletoe infections are pruned from the tree, or new non-host trees are planted.

Additional Information

Colorado Master Gardener Garden Notes (available from the Colorado Master Gardener Program website, https://cmg.extension.colostate.edu/volunteer-information/cmg-gardennotes-class-handouts/

Fact Sheets (available from the CSU Extension website)

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If the seeds do hit their host tree, they produce root-like structures (sinkers) that develop into the phloem of the bark. The small discrete dwarf mistletoe is fed by the sinkers which leech water and nutrients from the tree which slowly kills the host tree. The crown of the tree typically dies out first as the lower infected branches need more and more water and nutrients to support the increasing number of dwarf mistletoe plants.

After years of silent infection, witches brooms will begin deforming the branching structure of the tree. The witches’ brooms are made up of clusters of dead twigs, creating a fuel ladder into the tree’s canopy. Dwarf mistletoe also weakens the trees immune system. Various insects including the ravenous mountain pine beetles can detect these weakened trees and will select them over healthy vigorous trees.

The long-term cost effective solution to controlling dwarf mistletoe is to aggressively remove infected trees and to create buffer zones greater than 60 feet wide when non-infected trees are threatened. Professional foresters throughout the United States are finding that the simplest, most cost effective method of removing dwarf mistletoe is to grind the infected trees in place.

For more information contact, Jim Wahl, Wahl Marketing Communications, (513) 259-3809 or [email protected]

Dwarf mistletoe

Dwarf mistletoe, any plant that is a member of the genus Arceuthobium (family Viscaceae), which contains about 8 to 15 species of small-flowered plants that are parasitic on coniferous trees. The species are distributed primarily throughout the Northern Hemisphere, though a few tropical species are present in the Caribbean, Mediterranean, and Southeast Asian areas.

dwarf mistletoeDwarf mistletoe (Arceuthobium minutissimum) growing on a pine tree.S.Kenaley

The common dwarf mistletoe, A. minutissimum, is one of the smallest plants having specialized water-conducting tissues. Its flowering stems extend less than 3 mm (about 1/8 inch) from its host plant. The fruits of most Arceuthobium species are about 4 mm long, and each contains a bullet-shaped seed covered with a sticky substance. Pressure that builds up inside the maturing fruit causes the thick skin to rupture, shooting the seed away from the plant at a high velocity. As the sticky seed covering dries, it attaches the seed to the surface on which it landed, usually the branch of a nearby tree. Dwarf mistletoes spread in this way throughout a forest without being transported by wind or animals. These parasites cause economic damage to many species of ornamental and timber trees.

Kiss Mistletoe Goodbye This Season For Better Tree Health

Trees infested with the sap-sucking parasite would like to kiss the Christmas novelty goodbye.

And that may become easier — even without holiday harvests — due to new research at the Texas Agricultural Experiment Station.

“Mistletoe is unsightly and adversely affects the health of trees,” said Dr. Todd Watson, Experiment Station urban forest researcher.

Watson completed the first year of a two-year study aimed at eliminating the pest from urban landscapes and found promising results with at least one new treatment — a plant hormone.

The problem with mistletoe is that it stays with the tree until the tree dies. Spread by birds who eat mistletoe, the parasitic plant grows from seed deposited in bird feces on tree limbs. Watson said mistletoe left unchecked can cause die-back of tree limbs and occasionally the death of the tree, especially in drought conditions.

“Mistletoe grows into the wood of the tree, drawing water and minerals out,” he said. “Mistletoe is a plant, so it makes its own nutrients from photosynthesis, but it is the tree’s water that it pulls from and that weakens the tree and causes stress.”

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Yet that’s oft overlooked. For hundreds of years, mistletoe has been associated with various cultures in countries around the world as a plant symbolic either of peace or of romance. Its yuletide custom of suggesting a kiss underneath suspended mistletoe apparently is linked to English tradition.

But arborists have a decided lack of love for this parasite, stemming from the fact that mistletoe is especially hard to kill without harming the tree in the process, Watson explained.

“One can repeatedly cut off mistletoe to prevent it from making seed and therefore spreading, but that is very labor intensive,” he noted. “Or one can prune the infected branch, but that is only affective if there are a small number of branches with mistletoe.

“Some have covered affected limbs with black plastic to kill mistletoe by cutting it off from the sun,” Watson added, “but that is as unsightly as the mistletoe and very labor intensive. A chemical currently labeled for use against mistletoe has not been very effective in totally eliminating mistletoe in one application when used at the recommended label rates.”

To find a more reasonable, effective way to eliminate mistletoe, Watson tested eight different treatments in elm trees on the Texas A&M University campus. There was a controlled, untreated, group of trees; a group where mistletoe was pruned out; a group where entire branches were pruned; a group treated with the labeled chemical; a group in which the mistletoe infestations were covered with dark caulking; a group sprayed with glyphosate; a group treated with 2-4D; and a group treated with a specific plant hormone.

Watson said the trials were looking for at least 90 percent control in the 25 mistletoe plants treated in each group to be considered successful. The plant hormone yielded better than 90 percent control, he said.

The trial will be continued for another year and additional twists on the tests, such as developing formulations to improve the effectiveness of the plan hormones will be implemented, Watson said. If successful, the research said, the method likely will be patented for use on mistletoe throughout the United States.

An arrow of death sent by the gods? A plague on entire forests? A little investigation reveals that mistletoe has quite a sordid story indeed—so where did all this business about Christmas and kissing come from? Here are nine things you need to know about mistletoe.

What sort of plant is it?

Mistletoe is definitely not your typical shrub—it’s a parasite that attacks living trees. Technically, mistletoes—there are over 1,000 species found throughout the world to which botanists ascribe the name—are actually hemi-parasites. This means they obtain a portion of their energy through photosynthesis, and the rest is extracted from other plants. Mistletoe species have evolved to plant themselves on hosts ranging from pine trees to cacti, but the species most commonly associated with European-based mistletoe mythologies (like kissing beneath it on Christmas) are typically found on large deciduous trees, like oaks.

Does mistletoe kill its hosts?

It can, eventually. The plant sends its tiny roots into the bark’s cambium layer, where it siphons off water and nutrients, slowly weakening the tree. A mature tree can withstand a small amount of mistletoe with no problem, but if it spreads profusely the tree will eventually die, one limb at a time, as the life is literally sucked out of it. However, mistletoe doesn’t take out whole forests like some diseases—just a tree here and there. Ecologists actually view mistletoe as an important part of a healthy ecosystem, as the berries are a major food source for birds, who also find the dense foliage useful for nesting—and the dead trees become purchase for raptors.

How does it get up in trees to begin with?

Mistletoe reproduces by seeds, just like any other plant, but has evolved special adaptations to keep its seeds from falling to the ground, where they would be unable to sprout and develop into a mature plant. If you squeeze open the whitish semi-translucent berries—by the way, don’t eat the fruit, as some species are poisonous—you’ll find that the seeds are incredibly sticky. They are covered with a glue-like substance called viscin, so they stick to whatever they fall on. They mostly fall on branches high up in trees because the berries are a favorite wintertime snack for birds, who then excrete the seeds where they roost.

Has mistletoe always been associated with wintertime rituals?

No, but it was revered by a variety of ancient cultures. One of the most famous legends concerns the Norse god Baldur, who was considered invincible until an unknown assailant finally killed a him with an arrow made from mistletoe. Separately, in an ancient Celtic ceremony, Druids would sacrifice two white bulls, then climb an oak tree to fetch some mistletoe to make an elixir that was said to cure infertility. This is believed to be the origin of mistletoe’s association with love and romance.

How did the Christmas connection come about?

Historians are fuzzy on the matter, but it seems that mistletoe’s association with fertility and ritual and wintertime slowly morphed into the modern Christmas tradition. It makes sense that mistletoe, with its evergreen foliage and attractive red berries, would be brought indoors as decoration during the barren winter months, just as people do with fir boughs and holly branches.

It is believed that by the 18th century, kissing under the mistletoe at Christmas time was a fairly widespread tradition, though the first clear historical reference comes from 1820 when Washington Irving, author of Sleepy Hollow and The Headless Horseman, wrote of the plant: “the mistletoe, with its white berries, hung up, to the imminent peril of all the pretty housemaids.”

Is it good for anything besides kissing and bird food?

Quite a bit, actually, especially in the realm of health. Historically, mistletoe has been used to treat not just infertility, but epilepsy, hypertension, arthritis and many other ailments. In modern times, it has gained a reputation as an anti-cancer herb, and while numerous studies have been conducted to look into this claim, there is little in the way of conclusive evidence regarding its efficacy. Still, pharmaceutical preparations of mistletoe are available in Switzerland, the Netherlands and the UK. In the U.S., mistletoe extract is sold by herbal suppliers, but the FDA has not approved it as a cancer treatment.

Where did the name come from?

The English word for the plant is derived from a defunct Anglo-Saxon dialect. Apparently, having noticed that mistletoe often sprouts from bird droppings on tree branches, the words for dung—“mistel”—and twig—“tan”— were conjoined, and the mashup “misteltan” evolved over time into “mistletoe.”

Can you farm it?

You would think this would be a losing proposition given that the plant eventually destroys its host, but there actually are people who farm mistletoe commercially. More often, though, the mistletoe you see at a florist in the winter is wild harvested. If you have access to wooded land, you can “plant” mistletoe seeds for your own picking come Christmas.

How do you grow it?

For best results, harvest seeds from mistletoe in the early spring, when the fruit is fully ripe. The tree harboring the mistletoe you collect seeds from should be the same species as the one on which you will grow your own. Cut a slit into a tender piece of bark as high in the canopy as you can safely reach, and deposit the seeds there, spacing them a few inches apart. The more seeds you plant the better, as the germination rate is low. There is no need for water or fertilizer. For protection from birds, tie a strip of burlap over the seeds. Have patience, as it takes at least five years for the plants to mature and produce berries.

More stories from Modern Farmer:

  • The Strange, Horrifying History of Cherry Research Farm in North Carolina

Mistletoe grows in many parts of Europe and North America. In Central Texas, you are most likely to see this parasitic plant or shrub on an oak tree or cedar elm. Mistletoe is parasitic because it draws the host’s carbohydrates into itself. It can contribute to water and food loss and cause tree death in extreme cases, especially during drought conditions.

What does Mistletoe do to a tree?

Mistletoe steals nutrients and water from its host tree. The little, shrub-like plant sends out roots called haustoria into the cambium of a tree and takes over the tree’s carbohydrate and moisture sources. Overall, this won’t harm a tree unless it has a lot of mistletoe plants on it. Each plant can only steal so much from the host tree. Of course, it can reduce the production of a tree since some of its resources are impacted. This parasitic plant can also cause problems for a tree that is stressed due to soil compaction or drought. Even the little bit of moisture and nutrients it steals can make a big difference when a tree is already under stress.

Is mistletoe easy to get rid of?

It is generally considered to be easy to get rid of mistletoe. However, the plant can be stubborn and it may spring back. Simply cutting the twigs and leaves will not kill the mistletoe from your tree. You need to actively kill the roots and, with it, the entire plant.

What’s a non-chemical way to remove mistletoe?

The best – and probably the safest – way to get rid of mistletoe is without using chemicals of any sort. You simply prune it out, or hire a reputable tree service to do it. Hopefully, they are experts at mistletoe removal. If they have a certified arborist on staff that would be best. A certified arborist knows best how to remove large pieces of wood without adversely affecting the tree’s health. If you do the pruning yourself, you will want to remove the infested material back to the branch collar.

To kill the growth permanently, you must cut back the leaves and stems to the wood and then wrap the area with wide, black polyethylene to block light and prevent it from re-sprouting. Consistently cutting out the growth will not do the trick. It just doesn’t kill the plant. It will, however, prevent it from flowering and creating seeds that will spread the mistletoe to other parts of the tree or other trees in your yard.

What about using chemicals to get rid of mistletoe?

If you want to control mistletoe by using chemicals, you really should seek professional help. It’s kind of a last resort treatment. You should only attempt chemicals when nothing else has worked or other methods are just not practical. Spring spraying of the growth regulator ethephon has been shown to have some effect on eradicating mistletoe. The leaves of the mistletoe must be completely wet for this treatment to work. The process needs to be done before the host tree has leafed out.

Truthfully, most trees are able to withstand these parasitic infestations. Removal is not absolutely necessary. You can promote good tree health by adequately watering it and fertilizing it in spring. If you do decide to remove it, please seek out a reputable service provider to help you with the process.

Mistletoe Control Info: How To Get Rid Of Mistletoe Plants

Mistletoe grows wild in many parts of Europe and North America. It is a parasitic plant that draws the host tree’s carbohydrates into itself. This activity can reduce the health of the particular branch to which the mistletoe is attached and minimize fruit yield. Orchard owners know how to get rid of mistletoe in order to increase crop production.

Controlling mistletoe plants is extremely important in areas like northern California where the plant is a pest and colonizes production orchards.

Mistletoe in Trees

Mistletoe in trees steals nutrients and water from the host tree. The little shrub-like plant sends out root type organs, called haustoria, into the cambium of the tree and pirates the tree’s carbohydrate and moisture sources. Overall, this doesn’t harm the tree a great deal unless there are many mistletoe plants on it. However, it can reduce the production of the tree since some of its resources are impacted.

Orchard situations are especially sensitive to the

presence of the parasite. It is easy to kill mistletoe growth, but the roots can be persistent and the plant may simply spring back. Simply cutting back the twigs and leaves will not kill mistletoe. You need to actively kill the roots and, therefore, the entire plant.

Non-chemical Mistletoe Control

A non-toxic way to remove mistletoe is to simply prune it out. In order to prevent harm to the tree, you may want to use the services of a certified arborist. They know best how to remove large pieces of wood without adversely affecting the tree’s health. If you do the pruning yourself, remove infested material back to the branch collar.

To kill mistletoe growth permanently, cut back the leaves and stems to the wood and then wrap the area with wide black polyethylene to block light and prevent it from re-sprouting. Consistently cutting out the growth will not kill the plant but will prevent it from flowering and fruiting, creating seeds that will spread the mistletoe.

How to Get Rid of Mistletoe with Chemicals

Controlling mistletoe with chemicals should be done by a professional and only in instances where other methods are not practical. Spring spraying of the growth regulator ethephon has been shown to have some effect.

The leaves of the mistletoe must be completely wet and the process needs to be done before the host tree has leafed out. Temperatures should be around 65 F. (18 C.). This is really more of a bandage on a boo-boo. Only some of the mistletoe will fall off, but the plant will slowly grow more.

Trees are able to withstand most mistletoe infestations, so removal is not absolutely necessary. Promote good health in the tree by giving it plenty of supplemental water and fertilizing in spring.

What’s Growing On: Managing mistletoe in trees

You may have seen mistletoe hung in doorways over the past few weeks. It is a traditional holiday decoration, but when it’s growing on trees in the landscape, this parasitic plant may not seem quite as charming.

There are two types of mistletoe: broadleaf and dwarf.

Broadleaf mistletoe (Phoradendron macrophyllum) is an evergreen parasitic plant that grows on a number of landscape tree species in California. Hosts of broadleaf mistletoe include alder, Aristocrat flowering pear, ash, birch, box elder, cottonwood, locust, silver maple, walnut and zelkova. Other species of broadleaf mistletoe in California include P. villosum, which infests only oaks, and Viscum album, which attacks alder, apple, black locust and cottonwood.

Dwarf mistletoes (Arceuthobium spp.) infest pines, firs and other conifers in forests, and can be a problem in forest landscapes such as in the Sierra Nevada foothills.

Leafy mistletoes have green stems with thick leaves that are nearly oval in shape. Plants often develop a rounded form up to 2 feet or more in diameter. The small, sticky, whitish berries are produced from October to December.

Mistletoe plants are either female (produce berries) or male (produce only pollen). The berries of the female plant are attractive to birds such as cedar waxwings, robins and others. The birds feed on and digest the pulp of the berries, excreting the living seeds that stick tightly to any branch on which they land. In most cases, the initial infestation occurs on larger or older trees because birds prefer to perch in the tops of taller trees.

After the mistletoe seed germinates, it grows through the bark and into the tree’s water-conducting tissues, where rootlike structures called haustoria develop. The haustoria gradually extend up and down within the branch as the mistletoe grows. Initially, the parasitic plant grows slowly; it may take years before the plant blooms and produces seeds. Broadleaf mistletoes have succulent stems that become woody at the base. Old, mature mistletoe plants may be several feet in diameter, and on some host species, large swollen areas develop on the infected branches where the mistletoe penetrates. If the visible portion of the mistletoe is removed, new plants often resprout from the haustoria.

Dwarf mistletoes are smaller plants than broadleaf mistletoes, with mature stems less than 6 to 8 inches long. Dwarf mistletoe shoots are nonwoody, segmented and have small scalelike leaves. While broadleaf mistletoe seeds are dispersed by birds, dwarf mistletoe seeds are spread mostly by their forcible discharge from fruit, which can propel seeds horizontally into trees up to 30 to 40 feet away.

Broadleaf mistletoe absorbs both water and mineral nutrients from its host trees. Mistletoes also can produce energy through photosynthesis in their green leaves. Healthy trees can tolerate a few mistletoe branch infections, but individual branches may be weakened or sometimes killed. Heavily infested trees may be reduced in vigor, stunted or even killed, especially if they are stressed by other problems such as drought or disease.

In newly developed areas or in older established areas where trees are being replaced, the ideal method of controlling or preventing mistletoe is to plant trees believed to be resistant or moderately resistant to mistletoe. Avoid trees like Modesto ash, known to be especially susceptible to mistletoe infestation. Some tree species appear resistant to broadleaf mistletoe. Bradford flowering pear, Chinese pistache, crape myrtle, eucalyptus, ginkgo, golden rain tree, liquidambar, sycamore and conifers such as redwood and cedar rarely are infested. These or other resistant species should be considered when planting in infested areas, or when replacing infested trees.

For treatment of existing trees it is important to remove mistletoe before it produces seed and spreads to other limbs or trees. Mechanical control through pruning is the most effective method for removal. Growth regulators provide a degree of temporary control but repeated applications are required. Severely infested trees should be removed and replaced with less susceptible species to protect surrounding trees.

The most effective way to control mistletoe and prevent its spread is to prune out infected branches, if possible, as soon as the parasite appears. Using thinning-type pruning cuts, remove infected branches at their point of origin or back to large lateral branches. Infected branches need to be cut at least one foot below the point of mistletoe attachment in order to completely remove embedded haustoria. It is best to call an arborist if mistletoe is infesting your trees and you are unable to reach it to prune it.

Mistletoes infecting a major branch or the trunk where it cannot be pruned may be controlled by cutting off the mistletoe flush with the limb or trunk. Then wrap the area with a few layers of wide, black polyethylene to exclude light. Use twine or tape to secure the plastic to the limb, but do not wrap it too tightly or the branch may be damaged. Broadleaf mistletoe requires light and will die within a couple of years without it. It may be necessary to repeat this treatment, especially if the wrapping becomes detached or if the mistletoe does not die.

Simply cutting the mistletoe out of an infested tree each winter, even without wrapping, is better than doing nothing at all. Even though the parasite will grow back, spread is reduced because broadleaf mistletoe must be several years old before it can bloom and produce seed.

For a list of certified arborists in our area or for more information related to mistletoe, you can contact the UC Master Gardeners at (209) 953-6112. More information can be found on our website: sjmastergardeners.ucanr.edu/CONTACT_US.

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