Christmas Plants

Dec 12, 2017

And with this month’s Field Notes, Susan Knight of Trout Lake Station tells us about a few of her favorite holiday plants.

Although the leaves have fallen, and it is a bit gray, Christmas is a great season for plant lovers. We bring out cheerful plants to brighten the dark days of winter and remind us that spring and warm days will soon return. Three plants buoy us through the Winter Solstice and Christmas season: holly, mistletoe, and poinsettia.  Holly, with its shiny dark green leaves and red berries, is especially cheerful. Mistletoe, another evergreen plant with white berries, also brings joy this time of year. Poinsettias, with their red and green leaves, fit right in with our Christmas color scheme.

But what is the real story about these plants? Where do these plants come from, and can we find them growing in the northwoods?

Nothing says Christmas like Poinsettia. This is a lovely plant, but it is native to Mexico, and will never be found growing out of doors in the northwoods.  Although it might look like it has red flowers, the red parts are actually modified leaves, or bracts. Shorter days prompt the plant to produce the showy red bracts and to produce the tiny flowers tucked into the center of these red bracts. Because these short days coincide with the winter solstice and Christmas, Poinsettia turns red just in time for the holidays. Poinsettia is in the spurge family, or Euphorbiaceae, and its botanical name is Euphorbia pulcherrima. Wisconsin, especially southern Wisconsin is home to a number of other species of Euphorbia, but unfortunately, many of these are ecologically invasive and unwelcome.

Mistletoe is an evergreen plant with white berries, and so is especially attractive in winter.  There is a long tradition of hanging a sprig of mistletoe in your home. Couples meeting under the mistletoe would kiss, and with each kiss, would remove one of the white berries until all the berries were gone. However, the dirty truth is that mistletoe is a rather nasty plant. It is hemiparasitic, meaning that it is partially parasitic and while it does some of its own photosynthesis, it also steals some of the carbohydrates it needs from the tree it is parasitizing.  And, those pretty berries? They’re poisonous. When a berry lands on a host tree and germinates, special roots penetrate the tree and start siphoning off water and nutrients. In Europe, the mistletoe most commonly used as a Christmas decoration is Viscum album, and this species mostly parasitizes apple, basswood, hawthorn and poplar trees. Since these trees are all deciduous and lose their leaves each winter, the evergreen mistletoe is easy to spot once the leaves of the host plant have fallen.

We do not have the same mistletoe species in the US. Instead, we use American mistletoe or Phoradendron leucarpum, another hemi-parasitic plant, as a Christmas decoration. This mistletoe is native to Mexico and southern US and is in the same plant family as the European mistletoe.

Northern Wisconsin has its own home-grown mistletoe, Eastern dwarf mistletoe, Arceuthobium pusillum, and it is in the same plant family as the European and southern US mistletoes. Eastern dwarf mistletoe is most often found on black spruce, a common tree in our bogs. However, this mistletoe would make a lousy Christmas decoration, because it is inconspicuous, and it isn’t even green. The southern and European mistletoes have leaves that can photosynthesize for themselves to some extent. But our native Eastern dwarf mistletoe is a true parasite, and steals all the carbohydrates and nutrients it needs from its host tree.  To add to the misery of the host, Eastern dwarf mistletoe also tricks the host plant into producing a large cluster of stems and twigs – the better to provide nutrition for the parasite. This dense knot of stems is known as a witch’s broom and looks like a huge bird’s nest or a broom in the tree. Though not all witch’s brooms are due to mistletoe, Eastern dwarf mistletoe is unhealthy for the host tree, and may ultimately kill it. That’s even worse than getting coal in your stocking.

Holly trees, with their shiny green leaves and bright red berries are a cheerful sight at Christmas.  The most common holly seen at Christmas is Ilex aquifolium, a native of Europe, but commonly grown in the US. Unfortunately, it has become an aggressive invasive plant along the west coast, outcompeting and displacing native species. We have our own beautiful hollies in the northwoods such as mountain holly or Ilex mucronata, and winterberry, or Ilex verticillatum. Unfortunately, our hollies do not have evergreen leaves, and so are not used as Christmas decorations. However, the red berries may persist all winter, and are a cheery site in the wetlands where they grow.

So, when you see those pretty red and green and white plants that cheer us up mid-winter, remember they all grow naturally somewhere.  And even if we can’t find them right outside our door, it is wonderful to see them this time of year.

For Field Notes, this is Susan Knight of UW-Madison’s Trout Lake Station.