A live tree sparkling with lights and ornaments is the heart of many holiday celebrations.
Most of us expect that Christmas tree to have a particular look, which requires that it be a certain kind of tree. “We don’t decorate maples or willows,” said Julie Janoski, Plant Clinic manager at The Morton Arboretum in Lisle. “Our Christmas trees are nearly always pyramidal needled evergreen conifers.”
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Such a tree has a pyramid shape, like a green triangle. Its leaves must be long and slender, like needles, instead of flat and wide. It must be evergreen, keeping those leaves through the winter. And most evergreen trees reproduce through cones rather than flowers.
This set of expectations comes, like the tradition itself, from northern Europe. People there have long decorated their homes around the time of the winter solstice with evergreen plants as a reminder that life goes on through the darkest days of winter.
The custom of bringing an entire small evergreen tree into the home spread from Germany to Britain and the United States in the mid-19th century. Images from Germany fixed popular ideas of what a Christmas tree should look like, based on firs and spruces from northern European forests.
Evergreen trees in the great boreal forest that circles the globe high in the Northern Hemisphere tend to have a cone shape. The branches are staggered, with the longest at the bottom, tapering to the shortest, newest branches at the top. That allows needles on every branch to catch some of the low-angled winter sun. It also lets heavy snow slip off, sliding from branch to branch down the tree.
“Not every evergreen tree is cone-shaped,” Janoski said. For example, most species of pine are more irregular in their form, especially as they grow older. Pine trees grown on Christmas tree farms are heavily pruned to make them pyramidal, but a white pine planted in a Chicago yard will gradually become more open and less regular.
Needled conifers’ leaves are long and skinny to protect them. Most plants’ leaf cells die in the cold because ice crystals form that puncture cell walls, and sharp winds dry out tissues. That’s why most trees don’t even try to keep their vulnerable leaves through a cold winter.
The slender leaves of needled evergreens can survive because their surface area has been reduced to a minimum, exposing as few cells as possible to cold temperatures. They have a waxy coating that protects the leaf tissue and reduces water loss.
In both the house and in the forest, water is critical to a Christmas tree. “If it dries out, it will become a fire hazard,” Janoski said. Make sure the tree’s trunk is freshly cut when you place it in the stand so it can take up water. Check the tree stand’s reservoir daily and keep it filled. If the tree’s needles become dry and brittle, discard them immediately.
If you’re looking for an evergreen to bring the North Woods look to your yard, choose a species carefully.
“Most evergreen trees look like little Christmas trees when they’re in a pot at the nursery, but not all of them will keep that shape as they grow,” she said.
Many fir, spruce and pine species not cut down young to adorn someone’s holiday will get huge, as much as 50 to 80 feet tall and 30 to 40 feet wide. Some species that are popular as Christmas trees will struggle in Chicago-area soils or are vulnerable to pests and disease.
“Do plenty of research about your site and the appropriate species before you plant an evergreen,” Janoski said. The Plant Clinic can help you decide on the right kind of tree.
“A Christmas tree is cut down when it’s only a few years old and lasts in your house for just a few weeks,” she said. “The same species planted outdoors will live for decades, and if it’s a bad fit, it will be an ongoing problem. It’s worth investing the effort to get it right so you choose a tree that will brighten your winters for a long time.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.