Mulberry Query: Big Decisions About Pruning Even Bigger Trees
Southwest Yard & Garden
By Marisa Thompson
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Question: I have a
very large mulberry tree that offers great shade on the west side of my house
in the summer. Last spring, I hired someone to cut back the limbs over the
house and carport where they were dragging on the roof and the electrical line.
There is an area of branches that are thick and will be hanging over the
driveway by mid-summer this year. I have talked to
a tree trimmer who is willing to do a minor trimming, but said that eventually
I will need to cut the tree back significantly, down by about half. He said it
will be healthy, leaf out, etc., and be better for the tree. He also said he
would not cut it back once it begins to bud out. Does a mulberry tree this size
need to be cut back by half now or in the future for its health or safety? I am
thinking I may just not prune this year until I know more and am sure of what
the tree needs.
- S. White, Doña Ana County
Answer: I’m so glad you’re asking this question. I too had a
huge non-fruiting mulberry in my yard when I lived in Las Cruces, and I
remember wondering the same thing. At the time, we opted to trim the few
branches that were touching the roof (and low enough to reach without a
ladder). While our decision was largely based on having a limited budget, it
turned out to be the right decision for the tree—and our precious shade.
I’m inclined to suggest waiting until the tree leafs
out this spring to focus on whether or not there is any branch dieback, and then
decide if the deadwood should be pruned out. Smaller dead branches within the
canopy may not become a problem. If larger limbs are dead, hiring a
professional tree pruner to remove them safely will be the best option. While
all pruning cuts injure trees, the larger the diameter of a cut limb, the more
energy is required to seal the wound.
Cutting the tree back by half sounds like a procedure
known as “topping.” In addition to being detrimental to tree health, this
practice has been proven to increase the chances of future limb breakage. As
noted in the 2001 International Society of Arboriculture (ISA) Arborists’ Certification Study Guide, severe
topping “causes branch dieback, decay, and sprout production from the cut ends,
resulting in a potentially hazardous situation once the sprouts become large
and heavy. Topping or heading back is not a recommended pruning practice.”
In the late, great Dr. Alex L. Shigo’s super-helpful book, Tree Pruning: A Worldwide PhotoGuide, topping is listed as one of seven major (and unfortunately common) pruning problems. “4. Topping and Tipping Large Trees. The practice causes serious injuries to large trees and leads to hazardous conditions. Do not plant trees under power lines. Or, plant only small-maturing species. Or, start pruning when the trees are small.” Scroll down down down for the other six pruning no-no’s and a link to Dr. Ed Gilman’s short and snappy article, “Do Not Top Trees.”
But hold on a minute. If we know topping is so bad
for trees, why is it still common practice? Well, there are several factors.
For one, changing normalized behavior is difficult. When we see topped trees in
our communities we tend to think that must be the way it should be. Also, large
tree pruning is HARD. This line of work requires skill and is physically difficult—at
times dangerous, and that’s why pruning services can be expensive. Selective,
careful pruning also takes more time. Furthermore, many of our urban trees were
topped decades ago, and corrective pruning (aka crown restoration), while
possible, usually requires multiple years of successive prunings.
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Some tree species are more sensitive to topping than
others. And an important distinction should also be made between proper care
for ornamental landscape trees versus commercial tree crops. As management
goals vary, so do management practices. For example, recommendations for pecan
production in New Mexico include mechanical hedging of mature trees on a
multi-year cycle to allow sunlight into the inner canopy to maximize yield
potential (both quantity and quality). Selective, careful thinning to open the
canopy of hundreds of pecan trees quickly becomes cost-prohibitive.
Speaking of orchard management, NMSU Extension Fruit
Specialist Dr. Shengrui Yao is hosting a
virtual Fruit Grower Workshop on Wed. March 2, 2022 from 9 am to 2 pm.
This is a free online event, but registration is required. Topics include
orchard maintenance, irrigation, planting, and growing grapes! Visit https://rioarribaextension.nmsu.edu
for more information.
For their added insights on this mulberry query, I shared the details with my predecessor and mentor Dr. Curtis Smith, and the person who inspired me to go back to school to study horticulture and pursue a career in the Cooperative Extension Service, Joran Viers. In addition to helping me with complex questions, in retirement, Dr. Smith continues to teach horticulture classes in New Mexico and Texas, and Viers is now a horticultural consultant and ISA Board Certified Master Arborist.
Check out Dr. Curtis Smith's blog "Southwest GardenSmith" at https://www.southwestgardensmith.com and Joran Viers' website at https://www.jvhorticultural.com.
Smith and I spoke on the phone while carefully reviewing each of the submitted photos. It can be difficult to identify deadwood in the winter when all the leaves are gone—and even more so from a winter photo. We detected only a few smaller limbs that might be dead in the interior canopy. Overall, we agreed that the tree looks good may not need major pruning. Smith did note that some careful thinning could be beneficial, and the limbs over powerlines should be watched each year and may need attention if dieback becomes an issue.
Viers replied via email and remarked, “I took a look
at the photos. I doubt there is a lot of deadwood, and I don't see any need to
whack it back by half, for sure! There are a lot of small-diameter branches, and
even the big branches are smallish because there are so many branches in there.
Some thinning might be in order, but I balance that thought with thinking about
how all those intergrown branches help hold each other up and in place.
Definitely wait until mid-May or later to assess and then 1) prune deadwood
larger than a 2-inch diameter, 2) prune limbs causing clearance issues (house,
utility lines, etc.), and 3) do some light thinning in the outer half of the
canopy. If possible, don’t cut limbs larger than 3 inches in diameter.” He
added, “If the canopy size is adequate, leaving some of the dense growth inside
(less thinning) will distribute annual growth across more points, and consequently
each will grow less. It will also leave the tree with fewer wounds to have to
contend with, so less disease and decay potential.” As far as deciding whether
to prune this year or wait until next year, “The amount of deadwood might be
the determining factor, and even that is mostly from an aesthetic point. Mulberry
trees hold deadwood very strongly, and when it does finally break off, it more
often than not gets stuck on the way down, never making it to ground.”
Dr. Ed Gilman’s short and snappy article, “Do Not Top Trees” https://hort.ifas.ufl.edu/woody/do-not-top.shtml
From Dr. Alex L. Shigo’s book, Tree Pruning: A Worldwide Photo Guide, Pages 9-15
Pruning Problems
"Changes must be made to 7 tree practices that cause serious injury to trees. The practices all deal with pruning.
1. WRONG TREE, WRONG PLACE
The tree is later mutilated.
Know what size and shape of tree you want. Little trees grow to become big trees.
Know your planting site.
Select trees that will grow best on your site.
Get advice from professions.
2. FLUSH CUTS
A major tree defense system is destroyed.
Make pruning cuts as close as possible to the collar at the branch base.
Do not injure or remove the collar.
3. STUB CUTS
Stubs are food for organisms that start rot and cankers.
Do not leave living or dead stubs.
The swollen branch collar is not a stub.
4. TOPPING AND TIPPING LARGE TREES
The practice causes serious injuries to large trees and leads to hazardous conditions.Do not plant trees under power lines.Or, plant only small-maturing species.Or, start pruning when the trees are small.
5. OVER PRUNING
The tree framework is destroyed.The tree becomes a dangerous hazard.Roots are weakened. Root diseases start.Excessive sprouting starts.Boring insects infest the wood.
6. IMPROPER POLLARDING
Tree mutilation is often called pollarding. Proper pollarding is a fine tree practice. Start with a tree that lends itself to pollarding. Establish the desired framework when the tree is young and small. Remove sprouts every year. Do not injure the pollard heads.
7. WOUND DRESSINGS
Wound dressings do not stop rot.Do not apply dressing over rotted or infected wood.Some dressings stimulate rot.Some wound dressings stimulate the growth of woundwood, often incorrectly called callus. The woundwood may roll inward to form a "rams horn" and closure may be prevented."
.
For
more gardening information, visit the NMSU Extension Urban Horticulture page at
http://desertblooms.nmsu.edu/ and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/. Find your local Cooperative Extension Office at https://aces.nmsu.edu/county/.
Marisa Y. Thompson, Ph.D., is the Extension
Urban Horticulture Specialist in the Department of Extension Plant Sciences and
is based at the New Mexico State University Agricultural Science Center at Los
Lunas.
Edible New Mexico Magazine article crosslink -- GROWING YOUR FUTURE FAVORITE TREE
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