As many as twenty-three species of mistletoe in the genus Phoradendron, of the Santalaceae (Sandalwood) Family, have been identified in the U.S. (though some authorities lump them together into fewer species). Only one mistletoe of the genus is known to occur in Arkansas: Phoradendron leucarpum (in the strictest sense…a few others are sometimes lumped with it as subspecies). Phoradendron leucarpum ranges from Mexico into the U.S. to Arizona and Kansas, eastward to Illinois, New York and southward. In Arkansas, it occurs statewide. The genus name is derived from the Greek for “tree thief.” The word “mistletoe,” from Anglo-Saxon, relates to “twigs” and “birds.” Common names for the Arkansas species include oak mistletoe, eastern mistletoe, or just simply “mistletoe,” which is used herein. This species has previously been placed in the Loranthaceae or Viscaceae Families, an artifact of the evolving understanding of mistletoe taxonomy. Similarly, other scientific names have also been applied to this species, including Phoradendron serotinum. It should be noted that mistletoe is not the same as “witches’ broom,” which is actually a collection of deformed tree branches and twigs caused by fungi, viruses, pests, or bacteria.
Mistletoe is a perennial, evergreen shrub which depends on deciduous trees for its niche and birds to “plant” its seeds. Mistletoe, a hemiparasite, has its own chlorophyll to produce sugars, but relies on host trees for water, minerals, and other nutrients. Mistletoe favors mature, open-crowned host trees in open spaces or at edges of wooded areas. The plants tend to be found high up in trees or at the crown perimeter where sunlight is stronger and the hosts’ branches are of appropriate size–that is, branches not too thin and bark not too thick.
Photo 1: Mistletoe plants, often remaining hidden while host trees bear leaves, become starkly obvious in winter.
With stems of a single individual radiating in all directions from a single portion of a host’s branch, mature mistletoe shrubs may appear ball-like. The stems are bright green and break easily at swollen nodes from which leaves and side stems grow. A stem may be two or more feet long. Oblong to obovate leaves are bright green, thick, simple, opposite, and entire. Leaves range in size up to 1.5” long and 1” wide. The leaves and stems have a thick, waxy outer layer and are noticeably hairy when young. They are evergreen, with leaves persisting for years.
Mistletoe shrubs are either female or male (the species is dioecious). Inconspicuous, greenish-yellow squat flowers (less than 1/8” wide) are crowded on short axillary spikes in the fall. Flowers have a three-lobed calyx. Female and male flowers look similar, but with close examination, female flowers can be seen to have a style and stigma over an inferior ovary. Female shrubs have fewer flowers per spike as compared to male shrubs. Pollen is transported by wasps and bees.
Photo 2: A female mistletoe shrub with developing flower spikes.
Fruit development, in late fall into winter, first shows as green berries that become glassy bright white at maturity. The fruit, numerous and crowded, are round and about 1/8” in diameter. The smooth skin of the fruit easily separates from an inner mucous-like, very sticky glob (viscin) with one embedded, white, flattened seed. A tangle of fine hairs is embedded in the viscin.
Photo 3: A mature female mistletoe shrub showing ball-like structure and white fruit.
Photo 4: Close-up of mistletoe stem with fruit. Note that some fruit has already gone (yellow arrow) while several flowers persist (red arrow).
When birds eat (or attempt to eat) the fruit, the viscin may stick to their beaks and feet, causing the birds to scrape their beaks or feet on nearby branches to rid themselves of the sticky object. The viscin, with embedded strings, then dries to the branch in such a way to essentially glue the embedded seed to the host tree. A germinating seed produces a “radicle” (initial root sprout) which forms a “holdfast” that anchors the seed to the branch. Once anchored, the seed produces haustoria (specialized parasitic roots), which, all things favorable, penetrate the branch’s epidermis and grow into the xylem* and phloem* layers of the bark. Once water and nutrient flow is established from the host tree, the new mistletoe plant develops stems and leaves. With time, the haustoria extend laterally farther along the branch; however, stems continue to grow only from near the anchoring site.
Mistletoe takes all its water and minerals from the host’s xylem, as well as any sugars it cannot produce through its own photosynthesis from the phloem. Unless a host tree is significantly “overgrown” with mistletoe or during periods of extreme stress, such as severe droughts, the host tree is not usually significantly harmed except that the portion of the branch beyond a mistletoe shrub may become less vigorous and even die. If the stems of a mistletoe are broken off the host branch, the shrub can regrow from the parasitic roots within the branch. Mistletoe is the only hemiparasite that occurs on trees in Arkansas.
Some people see mistletoe as an “infestation” to be rid of, but mistletoe is an important part of the ecosystem. The fruit is an important food source for birds and and other animals, especially during dry years when mistletoe has all the water it needs from its host to bear fruit. The shrub provides shelter to birds and possible nest sites. Mistletoe is also the only plant on which caterpillars of the Great Purple Hairstreak (Atlides halesus) feed. Be aware, though, that mistletoe can be toxic to people and should not be consumed.
* Xylem is the transport tissue of vascular plants that moves water and soluble minerals from roots throughout the plant. Phloem is the alternate transport tissue that moves sugars produced in the leaves to the roots and developing fruit.
Article and photographs by ANPS member Sid Vogelpohl