This goes back to what I said in my “Heat Tolerance” and my “Shade Tolerance” blog posts. This is where the age old conflict between turf and trees comes about. If you want some decent turf under your trees, you will probably have to trim or prune your trees. I’m sorry tree huggers. That’s just the way it is. That’s a fact, Jack.
Some of you know that I got my horticulture certification in January of this year (2014). So, here I go with my horticultural hat on.
Not only does trimming and pruning your trees help your turfgrass, it is healthy for your trees to keep them trimmed and pruned. Allowing the base of a tree or shrub trunk to remain damp, cool and shaded can bring about cankers, root rot or other problems…problems for the tree AND the surrounding turf. Letting your trees get too “full” may look wonderful and provide great, cooling shade. But it can also make the tree act as a sail when high winds occur. That’s when you start getting big branches breaking off and fall on your home, vehicles or other property. Leaving dead branches in your trees can affect the health and overall vigor of the tree. Dead branches could allow disease to spread throughout the tree, and, again, those dead branches can become projectiles or spears in a storm. Leaving trees in a forest to Mother Nature’s care is fine. Trees in a landscape need definitive care.
I’ve also said this before in this blog – you can NEVER underestimate the value of air flow. Air flow is absolutely critical to all plants. It is critical to trees, shrubs and turf. It keeps things cool and also helps things dry out. Pruning a little can also let the sun in to other plants that are below a canopy…..giving the smaller guys a chance for survival.
Some of you may be familiar with the term “dead-wooding” a tree. In order to maintain the over-all health of your trees, the specialized trimming procedure of dead-wooding is critically necessary. The term dead-wooding is referred to as the removal of dead, dying or diseased branches within the tree. Not only providing resistance against issues such as insect infestation, dead-wooding will also create a safe and sanitary environment for all of the trees in the vicinity and provide an aesthetically pleasing tree. Dead branches should be removed at the trunk, or actually at what is known as the “branch collar”. The cut should generally be as vertical as possible.
For some of the smaller branches (when the branches are less than, let’s say, two inches in diameter), one can hold on to and support the branch and make one single cut. For larger branches, one should use the “three point cut”. The first cut should be 12 to 18 inches from the limb's point of attachment. The pruning cut should be an undercut made 1∕2 way through the branch. This pruning cut is very important because it relieves weight from the branch collar and prevents accidental tearing of bark from the tree’s trunk when the limb is removed. The second pruning cut should be made on the outside of the first cut (i.e., farther from the trunk). Cut all the way through the limb from the top down, thus removing the weight of the branch. The final cut should be made next to the tree's trunk outside of the branch collar. Cut from the top down and cut all the way through the remaining branch stub. The branch collar should be left intact. DO NOT cut the branch flush with the tree's trunk. A proper cut avoids large wounds, and allows the tree's wound to close quickly.
There are also procedures called “thinning”, “crown thinning” or “crown reduction”. Crown thinning is a process by which the crown (the top, semi-circular area of the tree outlined by the circumference of all the leaves and branches) of the tree is reduced by removing branches – this will always be dictated as a percentage i.e. ‘25% thin to canopy’. By carrying this out, the “sail area” or canopy of the tree becomes thinner allowing wind and sunlight to pass through, thus solving two problems – more light to the area surrounding the tree and a reduction in branch failure due to strong winds. Thinning is removing small branches inside the tree and generally avoids leaving large wounds on the tree in question, leaving the tree less susceptible to bacterial diseases.
My big trees are pruned up to the first major “split” or “branch collar”. That’s at least 30 feet up in most cases. My shrubs are also pruned up…at least 3 to 6 inches. Some of my large shrubs or small to medium trees are pruned up a few feet. Again, allowing for sunlight, airflow and decreasing relative humidity.
As I mentioned earlier, arborists will say that trees and turf will compete for resources. I agree…if you do not intervene. As I’ve also said before, I fertilize both my trees and my turf. Arborists will say you have to have a bedded mulch area all the way out to the drip line of the tree. If I did that, I’d have no turf. My entire property would be one huge mulched bed for all my big trees. Sorry. I just can’t do that. So, I take special care of my large trees, just like I do for my shrub and turf areas. Although I have turf growing right up to the base or trunk of many of my large trees, I make sure my trees and turf do not compete for resources.
Let me say a few words about tree roots. In the fifth paragraph of my “Shade Tolerance” blog post, I talk about how far tree roots can extend. My point was to describe the rather large area under your lawn that can compete with your turf for resources. Here, I just want to give you some guidance on dealing with exposed large tree roots. In my yard, and in many yards in northern Virginia, Maryland and DC that have real big trees, there are exposed roots. These roots can be a real hazard to mowing. My neighbor hit a tree root once with his mower and it caused several thousand dollars in damage to his mower– more than the value of the mower! This is another reason to mow high. When I mow at 4 inches or higher, I can clear all the exposed roots.
Try to avoid cutting or pruning any roots for your large trees. You are risking structural damage to the tree, also risking an invitation to disease and either stressing the tree or you could kill it altogether. We know the rules of thumb regarding tree roots: (1)most roots are in the top foot of soil, (2) tree root systems can extend out 2–3 times the dripline, (3) roots can extend out about 1.5 times the height of the tree, and (4) more than 60 percent of the absorbing root system is beyond the dripline. We now know that trunk diameter is a more accurate means of computing root spread (for unobstructed trees). However, trunk diameter calculations may not be very accurate for conifers, palms and even some very mature trees.
If you really have to cut some roots, calculating the “Critical Root Radius” (CRR – Also called a PRZ – Protected Root Zone) of a tree is very helpful. The CRR (radius, not diameter, in feet) is calculated by measuring the diameter (not radius or circumference) of the trunk (in inches, not feet) at four and one-half feet from the ground and multiplying by 1.5. A tree with a five-inch diameter trunk (at the four and one-half foot level) will have a CRR of 7.5 feet. So, definitely do not cut roots within that radius of 7.5 feet. If you must violate theCRR/PRZ, the tree would require extensive treatment and monitoring for at least two years afterward. I would say cutting any roots at any distance should require additional treatment and monitoring.
Another rule of thumb is: If you remove or damage more than 25% of the root system of a tree, it will have devastating results. Never cut roots during the growing season or in the heat of the summer. Trees are already stressed at that time. Cut or remove roots in the fall or winter. And, finally, try to cut them sharply, or straight. Torn or jagged cuts will not regenerate new roots nearly as well as precise cuts.
Another consideration for determining the area to be mulched around a tree may not be out to the dripline. It is at least to the CRR/PRZ, if not further.
If the issue with the exposed roots is just mowing or tripping, things like that, then the best thing you can do is cover the exposed roots with soil. Covering them with no more than 2 inches of loose topsoil would be fine. You don’t want the soil too deep or to become too compacted. One of the reasons the roots are coming to the surface is for more oxygen. You do not want to impair oxygen access by burying the roots too deep.
I think that about covers tree care for now. Stay tuned for another topic with my horticulture hat on.