Western dwarf mistletoe on a western hemlock tree: not the sort of mistletoe you'd like to kiss under.

The Dark Side of Mistletoe

By Marlene Siemens,

Whistler Naturalists

At this time of year when naturalists and non-naturalists alike are decking the halls, osculating minds turn to mistletoe. As with many Christian traditions, the use of mistletoe began in pagan times. Two hundred years before the birth of Christ, the Druids used it to celebrate the coming of winter. They believed it had special healing powers, everything from curing sterility to counteracting poison. Scandinavians associated mistletoe with their goddess of love, and hence, the custom of kissing under mistletoe is believed to have arisen. Because of its pagan origins, early church leaders banned mistletoe and suggested holly instead, for Christmas greenery.

British Columbia is home to many species of mistletoe but, though many of us may have inadvertently kissed beneath it as it grew in the forest canopy overhead, the B.C. varieties are not the typical Christmas adornment. The mistletoe sold commercially in North America is most likely Phoradendron flavescens , naturally occurring in the south-eastern part of the U.S. Nevertheless, our own dwarf mistletoe ( Arceuthobium spp.), a parasite found on conifers, displays its own signs of romance. To wit, its explosive method of seed dispersal (use your imagination here); and anthers that in some species are capable of opening and closing in response to environmental changes (sounds a bit like kissing, doesn’t it?).

The earliest symptom of Arceuthobium infection is the development of a swelling at the point of penetration of the host by the dwarf mistletoe (enough already). The mistletoe then develops an extensive haustorial (modified root) absorptive system producing "sinkers" which penetrate the host xylem for carbohydrates, water and minerals. Growth is usually slow, but infections often lead to the production of profusely branched dense masses of distorted branches called witches’ brooms. This is a very advanced stage and can result in the death of the tree.

So if tempted to kiss under the mistletoe this Christmas, beware that kiss doesn’t lead down a road littered with witches’ brooms.

January 6 — Monthly Bird Walk . Join the first bird walk of 2001, which meets at the base of Lorimer Road at 8 a.m. Contact Michael Thompson (932-5010) for details.