The Dark Side of Decorative Tree Lighting
Edmonton has embraced year-round decorative tree lighting, and it’s hard not to love it! A 2015 Edmonton Journal article states that the City’s forestry department “installed lights on 1000 city-owned trees in six business revitalization zones.” Walking Whyte, Churchill, or Giovanni Caboto amongst twinkling giant elms is magical. But tree lighting has a dark side. Left unchecked, decorative tree lighting can cut into and even kill growing trees.
Death By Girdling
Beneath the bark of a tree lies a network of tissues that channel sugars, minerals, and water throughout the plant. When a cut or girdling object interrupts the flow of nutrients, the tree can weaken or die. The danger is that decorative lighting can’t expand as the tree grows.
The same article states, “the lights are secured to the trees with zip-ties, and as the tree grows, the zip ties will be loosened.” Unfortunately, this isn’t the case. While I’ve seen extension cords held to trees with ties, lights are secured by wrapping the tree’s trunk and branches. As such, the only way to loosen the lighting is to unwrap and rewrap the tree.
It’s a Matter of Time
But don’t trees grow super slow? Won’t it take years for decorative tree lighting to cause any damage? Since most of Edmonton’s light-wrapped trees are American elms, let’s look into the growth rate of elms. Fair warning: the following segment contains math.
How Fast Does an Elm Tree Grow?
According to Edmonton’s OpenTree data, the Whyte Ave elms between 104th and 105th Street have an average circumference of 51 inches. OpenTree doesn’t have their ages, but an Edmonton Journal article suggests some nearby elms were planted in the 40s. Let’s assume that the 104th and 105th Street elms are of a similar age.
Let’s start with the Elms on White Avenue and look at other examples I found online.
Whyte Avenue Elms
Average Tree Circumference = 51 inches
Estimate of Average Age = 72 years
Growth rate of Circumference = 0.71 inches per year
Let’s take a look at some historic elms from our Southern neighbours. These trees may or may not be representative of an elm growing in Edmonton.
The Treaty Elm, Philidelphia, PE
Circumference = 288 inches
Age = 280 years
Growth rate of Circumference = 1.03 inches per year
The Johnstown Elm, Johnstown, NY
Circumference = 196 inches
Age = 200 years
Growth rate of Circumference = 0.96 inches per year
Our math suggests elms can increase their circumference between 0.71 and 1.03 inches per year. That doesn’t seem like a lot, but each string of lights wraps around the tree 30 to 40 times. A string of lights would have to grow 1.5 and 3.5 feet every year to prevent girdling.
Considering that Edmonton has one of the largest Dutch Elm Disease-free American Elm populations, I consider every Elm a heritage tree.
Trees Grow Faster Thank String Lights
String lights cant grow or stretch 1 to 4 feet annually, so it’s a matter of time before they tighten, bite into the bark, and girdle the tree. To prevent girdling, decorative tree lights need to be removed and rewrapped regularly, or lights need to be run vertically up and down the tree – a technique called tracing.
Don’t get me wrong; decorative tree lighting is beautiful – especially during Edmonton’s long, dark winters. But if you’re not careful, a string of lights can go from loose to snug to deadly in a few short years.
Protecting Edmonton’s Trees
Edmonton’s elms are juveniles in the grand scheme of things and could live for another two centuries. It seems cruel to add slow strangulation to the list of ailments already threatening elm trees; Dutch Elm disease and Elm Scale. That said, Edmonton’s not the first municipality to use decorative tree lighting. So in the interest of preserving our lights AND our urban forest, let’s see what other cities are doing.
Municipal Decorative Tree Light Policies
The following decorative tree lighting guidelines are from the City of Portland’s Department of Parks and Recreations and Cincinnati’s Department of Urban Forestry:
- Non-seasonal lighting can not exceed three years.
- Lighting can not interfere with the routine pruning of trees.
- “The preferred method of installation is ‘draping’ or ‘tracing’. These methods have been found to be the least harmful to trees.”
- “The draping method may be used throughout the canopy” on branches one inch in diameter or larger.
- The “tracing” method involves running lights vertically and attaching them with an expanding tape such as nursery tape or poly-chain-lock.
- Cincinnati requires that lights are attached using the tracing method and fixed to the trees with eyelet screws rather than tape. “While it may be more time consuming to install the screw eyes and lights the first year, it is much faster to remove them and reinstall them the following years.”
- “All work on the lighting shall be performed while the trees are dormant.”
Reporting A Tree Girdling Tree to the City of Edmonton
Contact your municipality if you see a tree that has outgrown its lights. If you’re in Edmonton, call 311, and they’ll send someone to inspect the tree and lights.