FireSmart Pruning – The Controversy of Pruning Spruce Trees
Courtesy of FireSmart Canada
It is no secret that we live in a beautiful community surrounding by forest and wildlife. It is what drew many of us to the area and it is why we call this piece of paradise home. Defined by FireSmart Canada as a zone where human development meets or intermingles with the natural vegetation and wildland, we certainly live within an area known as the Wildland-Urban Interface.
Understanding this fact can be key to preventing catastrophic consequences to wildfires as they encroach on populated areas. It is this concept that gave rise to the principles of FireSmart, a list of actions homeowners can take on their properties to prepare their homes to withstand a wildfire - protecting themselves, their neighbors, and the community.
FireSmart breaks its recommendations down into seven disciplines, one of them being vegetation management. This includes pruning up branches on trees so that all remaining branches are above 6 feet in height. This step is designed to eliminate ‘ladder fuels’, defined as vegetative fuel loads that allow ground and surface fires (think grass) from climbing up into the trees and creating a much larger and more difficult to manage fire. On the surface, this step is a simple task that any homeowner can undertake on their own to help build resilience to wildfire. However, understanding the effects of pruning the white spruce we have in such abundance on the tree’s health are critical, along with an understanding of the best way to help mitigate future issues to your trees.
White spruce, like most conifer trees, do not require pruning. Spruce trees naturally grow limbs right to the ground, and often look best left alone in this natural state. These low branches help to distribute the load of snow on the upper branches, act as dampening for the tree during periods of wind and help to shelter the soil beneath the tree from the sun to aid in moisture retention. In fact, it is often recommended NOT to prune your spruce trees!
So, if FireSmart says I should, how can I do that without hurting the tree? This is where it is important to find that thin line to tread between property and community protection and tree health.
The first step is to ensure your pruning cuts do not cause the tree undue harm or open avenues for insects and disease to enter the tree. This is best done by trimming the branch back to the trunk, just beyond the branch collar. Leaving stubs on your cut branches prevents the tree from sealing over the cut limb, taking away its ability to heal and increasing the chances of insect and disease penetration. How the branch is removed makes a difference, too. By simply trying to cut down through the branch in one shot, the weight can pull the branch down before the cut is finished and lead to the bark peeling off the tree, over the branch collar and on to the trunk. This again invites unwanted issues as the ‘injury’ to the tree is more extensive and difficult for it to repair. The best method to utilize involves making a cut with a pruning handsaw from the underside of the branch, up into the branch approximately 1/3 of its diameter. Then make a second cut slightly further from the main trunk of the tree on the top of the branch, cutting through until the branch falls from the tree. This removes the weight and prevents the bark from tearing out. The final step is to cut the remaining stub back at the bark collar to allow for appropriate healing.
The Proper Pruning Sequence
A Spruce Tree Healing it’s Wound After Pruning
Armed with information on HOW to prune, we need to know WHEN to prune. The short answer is NOW! Spruce trees are best pruned in late winter / early spring when the tree is still dormant, just before it awakens to begin a new growth cycle. This allows the tree to focus on new growth through its growing period and avoids shocking the tree by removing living tissue mid-cycle. Once you start to see new buds breaking on the spruce and emerging, the window is closed, and it is best to leave the tree be. It is possible to prune a spruce mid-summer once the new growth is well established, however best practice would be to wait until the following year to complete the task.
Next, we need to look at WHICH spruce trees you should prune up. When we look at pruning any tree, you want to avoid taking more than approximately one quarter of the foliage at any given time. Pruning too excessively can shock the tree as it tries to heal and seal off too many branch entries at once. If we look to prune a tree up 6 feet from the ground, we would be looking at roughly a 24-foot tree before we could complete the pruning effectively (6 feet is one quarter of 24 feet). That said, spruce trees typically have longer branches towards the base of the tree, so the math isn’t quite exact. Taking the bottom branches on a tree that is young and small also complicates the tree’s ability to hold snow load. As such, we would recommend letting your spruce growth and establish itself to a height of at least 40 feet or more before you look to prune up the bottom 6 feet.
Don’t Prune Them Young - Let Them Grow and Establish Themselves!
All of that said, we do not recommend pruning any of your spruce limbs up this year. We’ve mentioned the spruce budworm a few times, and its no secret they’ve caused some considerable stress and defoliation over the past few years of this infestation. The spruce trees are stressed to the point they are far more susceptible to insect and disease infiltration. Creating ANY avenue for that to occur when the tree is under stress may cause detrimental effects to your tree. Our advice would be to hold off on FireSmart pruning for the infestation to be brought under control.
But what about dead wood? If you have dead branches on your spruce that have lost their bark entirely, it is safe to prune these off back to the trunk. By removing the dead wood, you are allowing the tree opportunities to focus on new and more vibrant growth.
The next thing we need to talk about is soil protection. Removing the bottom branches of a spruce can have a detrimental impact to the retention of moisture in the soil beneath it. A simple solution to this is the addition of a mulch layer of adequate depth around the tree, typically 3-4” in depth. This mulch layer should extend just beyond the drip line of the tree, essentially the imaginary line drawn from the longest branch straight down to the ground around the tree’s circumference. Take care to ensure the mulch is pulled back ever so slightly from the trunk of the tree as well. Piling mulch against the trunk can introduce mold and decay to the trunk of the tree. Mulch volcanoes can severely impact your tree! You should be able to see the root flare of the tree as it enters the soil. This mulch layer helps retain the moisture in the soil and acts as a natural weed barrier if deep enough. Landscape fabric should NOT be placed under your mulch bed! The fabric can block the transmittance of moisture and nutrients and typically does little to stop weed growth, as the weeds will simply grow on top of the fabric layer. Mulch also helps avoid excessive soil compaction under the tree, which can suffocate the roots of your tree and lead to early decay.
Pull Your Mulch Back and Let the Tree Breath
Mulch can be defined as any material used to cover the soil surface, and not all mulches are created equal. FireSmart will suggest any wood mulch applied within your gardens can lead to a source of ignition for windblown embers and serve to enhance your fire risk. While that is true, there are some mulches that are far better than others in terms of combustibility and fire spread. The University of Nevada’s Cooperative Extension published some research into the combustibility of various landscape mulches, looking specifically at store-bought cedar and bark mulches, pine needles, “Tahoe Chips”, and the rubber mulches you can find at hardware and garden centers. The rubber mulch was by far the worst, being both the most combustible and leading to the highest rate of fire spread. Pine needles and various bark or cedar mulches purchased from garden centers also did not do well, with higher flame heights and rates of spread than “Tahoe Chips”.
So, what are Tahoe Chips? Simply put, Tahoe Chips are wood chips processed directly from a tree service chipper. For the purpose of the study, these chips came from tree chipping operations in the Tahoe Basin, hence the name Tahoe Chips. The chips produced by tree service shippers are less combustible and lead to a lower rate of fire spread than store-purchased mulches. That said, they are still wood, and CAN be ignited by windblown embers during a wildfire event. As such, our recommendation is to use wood chips as mulch under your trees more than 1.5 meters (5 feet) from the house, and only in isolated beds that do not provide a path for fire to move within the 1.5-meter non-combustible zone defined by FireSmart.
If you’d like to read the study yourself, you can find it at the following web address:
https://naes.agnt.unr.edu/PMS/Pubs/1510_2011_95.pdf
Pruning up your tree branches to a height of 6 feet can certainly help eliminate ladder fuels that can compound the effects of a fire within the community. Understanding the effects of pruning your spruce trees and how to mitigate them and armed with a knowledge of how and when to prune, this simple task can be undertaken by any homeowner to help protect your property and the community from the catastrophic effects of wildfire.
Learn more about FireSmart by visiting https://www.firesmartcanada.ca/
Written by Sean Sterna - The Rocky Mountain Arborist & Used with Permission
