(Editor's Note: This article was originally published on December 2, 2007. We hope you enjoy it as we count down to Christmas but please know that authors of previously published articles may not be able to respond to questions and comments.)
The association of the mistletoe with the oak tree was considered especially sacred in a number of pagan religions in antiquity, such as that of the Celtic Druids. The connection of the mistletoe with the winter solstice may have come from seeing the bright green leaves of the parasitic evergreen amid the bare braches of the deciduous trees on which it grew. With the Celtic revival of the Victorian years, the mistletoe become closely associated with Christmas, generating a great interest in this plant, particularly in Britain, where the mistletoe traditions have always been strongest.
There are hundreds of species of mistletoe, not all closely related. While they all belong to the order Santalales, worldwide there are several different families. The "true" mistletoe of myth is the sole European species, Viscum album. The name translates to "white sticky;" white for the color of its berries and sticky for their gummy coating, which attaches the seeds to the bark of the trees where they will take root. Mistletoe, like holly, is dioecious; only the female plants bear fruit, which ripens at about the time of the winter solstice.
In the wild, mistletoe is disseminated by birds that eat the seeds and either excrete them or rub off the sticky coating on tree branches. One European species, the mistle thrush, is named for its connection to the plant. While the etymology of "mistletoe" is uncertain, some people have claimed that it means "dung-on-twig", from the way the birds spread the seeds. In America, such birds as the silky flycatcher and cedar waxwing spread the seeds. The seed sprouts on the bark, and eventually the young plant's roots then invade the inner bark layers. Mistletoe is hemi-parasitic. Its leaves contain chlorophyll, but it absorbs moisture and nutrients from the host tree. This may harm the tree, but the European mistletoe generally does not kill its host.
While myth associates the mistletoe most closely with the oak, the European mistletoe can colonize hundreds of different species and is more commonly found on soft-barked trees such as apple. In Britain, the European mistletoe is cultivated on apple trees as a cash crop for the Christmas season, but the gradual loss of the apple orchards has recently caused the production of mistletoe to diminish.  Gardeners can attempt to cultivate their own mistletoe, but the failure rate is high, and the plant's growth is very slow and uncertain. The method is simple: take ripe berries which have turned completely white, and press them into the bark of a suitable tree in late winter.  As mistletoe prefers to grow in the upper branches of the tree, it is harvested commercially with a cherry-picker.
Matters are different in the United States, where Viscum album is classified as a parasitic and invasive plant; it is forbidden to import seeds of this species into the US, and in California, where it has been introduced, it is listed as a noxious weed. 
In America, there are many species of native mistletoes. Those most similar to the European variety belong to the genus Phoradendron, which means "tree thief," a reflection of its parasitic habit. Phoradendron species tend to be more specific in their preference for a tree host than the European mistletoe. Their berries can be white, pink or red. There seems to be some confusion about the various species and their common names; several different varieties are called "Christmas Mistletoe." Among the many other varieties, there are also American dwarf mistletoes of the genus Arceuthobium, which colonize conifers such as juniper and pine and is generally regarded by foresters as undesirable. While it is not illegal to attempt to cultivate the native American mistletoes, the practice is not encouraged. 
Aside from its value as a Christmas decoration, opinions of the American mistletoe are mixed. It is important as habitat and a food source for many birds. In some areas, the silky flycatcher is entirely dependent on the mistletoe berries. But while birds and some mammals eat them with no harm, both berries and leaves are toxic to humans. American mistletoes, and particularly the dwarf mistletoes, are believed to do more damage to the host tree than the European species, and they are widely regarded as invasive. There are probably more calls for its control than interest in its cultivation. This is undoubtedly an attitude that Frigg would appreciate.