Thursday, January 17, 2019

Not a pretty picture

I didn't know this.

Hoomak Lake sign: Dwarf mistletoe

Legend: Dwarf mistletoe are perennial flowering parasitic plants. They live on and injure their host, in this case coniferous trees. In BC., dwarf mistletoes can cause extensive damage in our forests, resulting in the loss of valuable wood. On the coast western hemlock and mountain hemlock are the principal tree species affectd. Some forests containing large volumes of hemlock are seriously damaged.
Notice masses of thickened branches in the trees in front of you. They are called "witches brooms", and are the result of dwarf mistletoe in western hemlock trees. The mistletoe parasite will eventually kill the host tree.

This was written 20 years ago. The poor tree in front of me as I read the sign is long dead.

Legend, continued: The dwarf mistletoe plant looks like a leafless, segmented, woody stem. It is a greenish-yellow colour and it "roots" in the host tree, absorbing nutrients from its host. It grows in a small mass of shoots just a few centimetres long. New plants are formed when a single sticky seed is projected from an exploding mature fruit and lands on live tree bark. Removing the infected trees is the optimal way of eliminating dwarf mistletoe.

And here's that infected tree.

Not a pretty picture.

I have seen trees like this before, and wondered briefly what chewed them up, out here in the forest where no machinery wanders. I never looked too closely; just shuddered and went on my way. Now, I paid more attention, and came home to study up on it.

Look at the photo above: you can see the short stalks of the mistletoe. They sprout directly from the tree bark, and almost, but not quite look like part of the tree. Also note the fat branches. The root system interferes with the normal growth of the branches. These thick branches are weakened, and easily broken (in a wind storm, for example).

Fat branch and "brooms".

The balls of deformed branches plus a tangle of mistletoe are called brooms. (Not my idea of a witches' broom, which is designed for speed. Not these.) They don't appear until the tree has been infected for several years; at first, the mistletoe lies hidden beneath the bark.

The mistletoe damages the tree in other ways; it absorbes its nutrients, primarily carbohydrates, from the tree sap. And it can somehow extract water from the tree xylem even in times of extreme drought. The tree above the mistletoe infestation is starved and thirsty, and dies off.

I have looked at these and said "How ugly!" I have thought they spoiled the forest. I cringed at the sight.

I was wrong. I've been too stuck on my limited human ideas of beauty. These witches' brooms are valuable wildlife habitat. My "Wildlife and Trees" guide mentions fishers, martens, flying squirrels, black-backed woodpeckers, marbled murrelets (endangered), and spotted owls (ditto) as species which roost or nest in the shelter of these knots.

Dwarf mistletoes may contribute in various ways to biodiversity - by creating openings in the forest following tree death, by providing nesting sites in the 'brooms' and by providing food for a range of vertebrates and invertebrates. There can therefore be some conflict between the requirements of forest exploitation, and environmental concerns. (Plantwise Knowledge Bank)

E-Flora BC has several good photos of dwarf mistletoe on live hemlock. Here's one. Next time I see one of these trees, I'll stop and examine it closely, to get a good look at the mistletoe.


  1. Interesting.Some mistletoes do little or no damage to trees' growth. Here , in Australia, we have different types. (Most people think of the "Christmas car" variety!) When we had goats, I didn't know we also had mistletoe, high in a eucalypt.But when the goats pee'd bright pink -!!!A quick check with a breeder, some books from the library and I relaxed. Apparently, it does no har, even to milking goats.

    1. I used to have goats. Pink goat milk? Pink goat cheese! I wonder: did the milk taste different?

    2. Mine were Angora so I didn't milk them, but I'm told that occasionally eating mistletoe(at least this kind) does no harm.Seems to affect only urine.And I think most dairy goats would be on farm pasture.We were on rough land.Their favourite browse was a listed weed!

    3. It would be. Goats prefer weeds to pasture grass. Mine kept the weeping willow trees pruned up as far as they could reach, and cleared land that I planned to use later for farming. When I penned them in the pasture, (because they broke through the fence into my vegetable garden) they sulked. One went so far as to starve herself until I let her out into the weeds again.


