The Dark Side of Decorative Tree Lighting
Edmonton has embraced year-round decorative tree lighting, and it’s hard not to love it! An Edmonton Journal article from 2015 (Tiny white lights to adorn city tree year-round) states that the City’s forestry department “installed lights on 1000 city-owned trees in six business revitalization zones: Alberta Avenue, Beverly, Downtown, North Edge (107th Avenue), Old Strathcona, and 124th Street).” Walking Whyte, Churchill, or Giovanni Caboto amongst twinkling giant elms is magical, but it also has a potential dark side. If 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 this flow of nutrients is interrupted by a cut or object wrapped tightly around the truck – a process called girdling – the tree can weaken or die. The danger of decorative lighting is that it can’t expand as the tree grows.
The same Edmonton Journal article goes on to state the “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. Though I’ve seen zip ties used to secure extension cords running vertically up the trees, the decorative lights are secured by continuously wrapping the tree’s trunk and branches. As such, the only way to loosen the lights would be to remove and reinstall them.
It’s a Matter of Time
But don’t trees grow super slow? Won’t it take years for decorative tree lighting to causes any damage? Let’s take a closer look – since most of Edmonton’s light wrapped trees are American elms, I thought I’d look into their rate of growth. Fair warning, the following segment contains math.
According to the City’s OpenTree data, (and some help from pi), the elms between 104th and 105th Street on Whyte have an average circumference of 51 inches. Though OpenTree doesn’t say their age, an Edmonton Journal article about the removal of diseased elms (between 99th street to 96th street) claims that they were planted sometime in the 40s. Let’s assume that the 104/105 elms are of a similar age.
Whyte Avenue Elms
Average Circumference = 4 feet 3 inches (51″)
Estimate of Age = 72 years
Growth rate of Circumference = 0.71″ per year
Since my Edmonton data is spotty, let’s turn to some old elms from our Southern neighbours. These trees may or may not be representative of an elm growing in Edmonton.
The Treaty Elm – Philadelphia, PE
Circumference = 24 feet (288″)
Age = 280 years
Growth rate of Circumference = 1.03″ per year
The Johnstown Elm – Johnstown, NY
Circumference = 16 feet (196″)
Age = 200 years
Growth rate of Circumference = 0.96″ per year
The math shows us that an elm can increase its circumference somewhere between 0.71 and 1.03-inches per year – which at first glance doesn’t seem like a lot. But consider that each strand of light wraps around the circumference of the tree 30 to 40 times! To prevent strangulation, a string of lights would have to increase its length by 30 to 40 times the annual growth of the tree’s circumference. That’s between 1.5 and 3.5 feet per year!
Though most trees can handle a few years covered in decorative lighting, lights can’t accommodate 1 to 4 feet of annual growth it’s a matter of time before they tighten, bite into the bark, and interfere with the flow of sap. The only way to prevent this is to remove and rewrap the tree at regular intervals or to run the lights vertically – a technique called tracing.
Don’t get me wrong, I love Edmonton’s decorative tree lighting – it seriously adds something to the city – especially during long, dark winters. But I also love trees, and the math doesn’t lie – a string of lights can go from loose to snug, to deadly in a few short years.
Protecting Edmonton’s Trees
In the grand scheme of things, Edmonton’s elms are juveniles and could live for another two centuries. Considering that elms are already under threat from Dutch Elm disease and Elm Scale, it seems cruel to add strangulation into the mix. That being said, Edmonton’s not the fist municipality to use decorative lighting on trees. So in the interest of preserving our lights AND our urban forest, let’s see what other cities are doing.
The following decorative tree lighting guidelines are hand-picked 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 (Update)
If it sounds like I’m being tough on the City of Edmonton I must apologise – the work they do it beyond exceptional as demonstrated by the fact that they’re caring for and maintaining an inventory of over 267000+ urban trees, 7400 hectares of River Valley, city-wide naturalization, and running Roots for Trees and numerous other community beautification projects! When it comes to nature and urban forestry, you’d be hard-pressed to find a city as ambitious as Edmonton. They wrapped 1000 trees in stunning decorative lighting! 100 trees! How cool is that?! Seriously! But there are many more of us then there are of them and we can help! So if you see a tree that has outgrown its lights, contact the City by calling 311 and they’ll send someone to check it out.