Sunday Tree: Mistletoe
Mistletoes are parasitic shrubs that grow in the branches of trees
Many of us know mistletoe as a symbol of Christmas. We bring it into our homes to hang it in places where we might hope to get a kiss from a person dear to us. This is an ancient tradition dating back to the Druids. Plants that remain green in winter are symbols of the persistence of life, a promise that crops will return in spring. Mistletoe, though, is far more important than the brief time it spends in our homes.
Mistletoe is typically a small shrub. Why are we talking about it in our Sunday Tree story? Worldwide, mistletoes only grow in the branches of larger trees as parasites, getting all their nutrients from the host tree, or as hemi-parasites, obtaining water and mineral nutrients from the host tree, but capable of their own photosynthesis.
In the Kentucky Bluegrass where I live, a vast amount of mistletoe gradually becomes apparent as the tree leaves drop, their foliage remaining dark green and the plants appearing like bonsai tree scattered through the tree crown. In Kentucky and much of the southern US, our mistletoe is Phoradendron leucarpum in the sandalwood order, Santalales.
Up close, we see the small green leaves and branches. The plants are anchored in the xylem of the tree through a haustorium, an anchor that functions like a root system, drawing water and nutrients from the xylem of the tree. Mistletoes are flowering plants, and their fruit is a shining white berry.
The berry is important in understanding the plant. It is highly attractive to birds. The seeds inside the white fruit are covered with sticky mucilage. This allows the seeds to be eaten, then to be pooped out and stick to the branch that the bird is sitting on. Without that mucilage, the seeds would drop to the ground.
And that is where the name comes from: mistletoe means “poop-twig,” because the plants appear out of bird poop on twigs. The genus name, Phoradendron, means “tree thief” as early botanists recognized the parasitic nature of the plant.
Our mistletoe is just one of thousands of species of parasitic and hemiparasitic shrubs in many genera and several families in the Santalales throughout the forests of the world.
Some people regard mistletoe as a nuisance plant. It does shorten the lives of many trees, and causes breaking of branches and other damage. However, we know that mistletoes are critically important for wildlife, especially birds. Populations of many birds rise and fall depending on the abundance of mistletoes.
Mistletoe is widely used in folk medicine, especially the European mistletoe Viscum album, though it is toxic at high doses. Little scientific research has been done, but a recent phase 1 clinical trial at Johns Hopkins by Dr. Channing Paller showed that mistletoe extracts were reasonably safe and showed some efficacy in treatment of advanced cancer of several types.
I hope you will look for mistletoes as you go for walks. It is very abundant in many areas in the southern half of the US. Dwarf mistletoe, a fully parasitic plant, grows on conifers in northern forests as well, and we’ll discuss this interesting plant in a future story.
Do you see mistletoes where you live?
I haven't seen it here in Maine, but it was common when I lived in Kentucky. I didn't know trees as well then but I think in Louisville it was in our locust trees. Is that possible?
We have a lot of mistletoe where I live in Somerset, UK. This area was famous for its orchards and is still apple country, as our local cider-maker, Thatchers, now owns most of the orchards. But most of the old orchards have been lost, and with them, their mistletoe and the rest of their rare wildlife.