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© 2005 Tina McKeand
Tina McKeandUrban & Community Forestry Specialist
Treeshave been living on this
earth for 360 Million Years.
Mankindhas only been here for 2
million years.
Trees are Complex Organisms
Like people, trees are complex living organisms
And
Just like people, each tree is an individual
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A tree planted today in an urban location often has a life
expectancy of only 7 -10 years!?!*
*Alex Shigo, Modern Arboriculture, p vii, 1991
Making the Cut?
Things to consider . . .
Why we Prune?
What to Prune?
How to Prune?
Compartmentalization
Of
Decay
In
Trees
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TREE BIOLOGY 101
xylem
the vascular system
Like veins . . .transports water
and nutrients from the ground into the tree
Like arteries . . .transports sugar
from the leaves to growing points in the tree
phloembark
Like skin . . .
protects the vascular system from infection
Meristems
- Areas of cell division/growth
-Also found at tips of roots and shoots
Structure
The overall shape of the tree, its trunk, roots, and branches
And, the nature of its branchattachments
Structure is determined by . . .
Genetics
Environment
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Wild trees develop healthy structure by properly managing the limited resources in
theirNATURAL environment
Often . . .Commercially Grown Trees
Receive
&
Too Little Space for Growth
Too Many Resources
For many nursery grown trees . . , nutrition and water are not a problem,
but space and sunlight are at a premium!
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STRUCTUREHEADEDNATURAL
Photo Courtesy of Dennis Swartzell
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It’s not NATURALto Stake a Tree
A weak trunk and a heavy canopy requires the support of a nursery stake
The Cost . . . Not the Cure!
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Juvenile Branches
Promote healthy trunk taper
Aid in wind resistance
Provide protection from the sun
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JUVENILE BRANCHES ARE CRITICAL FOR DEVELOPMENT OF HEALTHY
TRUNK TAPER
REMOVING JUVENILE BRANCHES CONCENTRATES WIND IN THE
CANOPY AND PLACES STRESS ON THE DEVELOPING TRUNK
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JUVENILE BRANCHES HELP A TREE RESIST WIND DAMAGE
JUVENILE BRANCHES PROTECT IMMATURE BARK FROM SUNBURN
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Remove juvenile branches when they are bigger
than your thumb!
Juvenile Branches
“Rule of Thumb”
Why prune?
Nature vs. Nursery
Conformation to urban needs
Promote healthy structure
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What to prune?
Dead or Diseased limbs
Crossing & Co-dominant limbs
Poor branch attachments
Learn to spot hazardous structural defects when
the tree is young!
Co-dominants
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Pruning for Structure
BEWARE . . .
OF BRANCHATTACHMENTS
WITH<45O
ANGLE OF ATTACHMENT
0O
45O
90O
Sign of a healthy branch attachment
Branch Bark Ridge
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INCLUDED BARK
Co-dominants
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Included Bark & Co-dominantsLead to Catastrophic Branch Failure!
And, may lead to Hazardous Trees & Liability!
Promote Individual Character
Every species has uniquecharacteristics!
Each individual within a speciesis unique!
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Good Pruning is Like a Good Haircut . . . It shouldn’t be too
Obvious!
How to Prune
How Much to Prune
Where to make the cut
How to make the cut
How much to prune?
1-2 years = up to 50%Young trees = up to 30%Middle age trees = up to 25%Mature trees = > 15%
CAUTION!These are MAXIMUMS . . . Not GOALS!
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ALWAYS prune at Branch Unions
Branch CollarBranch Bark
Ridge
Proper Cut
Branch Collar
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Branch Collars
Branch Protection Zone
*Vascular intersections
occur at branch unions
Adventitious Buds
Triggered by CATASTOPHIC
INJURY
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Failure to Prune at Branch Unions results in many open wounds . . . And
rapid re-growth
Multi-Tip Dieback . . .from pruning with shears
3 Point CutMethod
Final cut – Just outside of the Branch Collar
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Using the 3 Point Cut method at a branch union results in healing callus
Failure to use the 3 Point Cut Methodmay result in hideous wounds
that are difficult to heal
Sooner is Better than Later
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TOPPING Results In . . .
Starvatio
n
Shock
Insects & Diseases
WeakBranch
AttachmentsRapidNew
Growth
Cost
Ugliness
DEATH
Crown Reduction
The sixth deadly sin of urban tree care is really about letting young trees be young trees. You might imagine that a tree fortunate enough to be planted in the right place, possessing good structure, planted proud, staked properly, and irrigated optimally wouldn’t have another care in the world. Unfortunately, it’s a harsh world out there in the big city, and as a rule, urban dwellers expect instant gratification. Young trees are often prematurely separated from their juvenile
branches in a misguided attempt to encourage a more mature growth habit. By definition, any young branch is a juvenile branch, but those most often targeted for removal are
the ones growing on the lower trunk beneath the establishing canopy line. It’s important to note however that trees of any age can and do develop juvenile branches as needed throughout their lives.
As we know, trees develop according to a genetic plan that has evolved over many millions of years, long before the introduction of more advanced beings that can’t even produce their own food! While we are only beginning to understand all of the possible benefits of juvenile branch placement, we do know that trees grow in a logical manner and it’s a sure bet that they are there for very good reasons. Thanks to Alex Shigo, Richard Harris and all of the other tireless researchers out there who are my personal superheroes, we understand at least some of these important functions.
Juvenile branches promote strong taper in a developing trunk. In order to withstand the environmental forces placed upon it, a tree must develop a strong trunk base that gradually tapers to the tips of the canopy. But if all of the food is being produced by leaves in the canopy, there might not be a lot left by the time the upper growing points take their share and the remaining photosynthates trickle down to the lower trunk. Young trees adjust for this by growing small branches along the trunk to provide a local “fast food” source to fuel the expanding trunk. Premature removal of these branches transfers energy towards increased canopy growth at the expense of trunk girth. Trees that are prematurely stripped of their juvenile branches often develop spindly, weak trunks that require chronic staking to provide necessary support.
Juvenile branches aid in wind resistance and reduce the likelihood of damage to the tree. In addition to promoting healthy trunk taper, which aids in flexibility, young branches on the lower trunk help by spreading the forces of wind load over a greater vertical surface. The premature removal of juvenile branches causes the canopy to act like a sail, concentrating the force of the wind in the upper canopy. This action concentrates the wind load at a more specific point on the developing tree trunk and increases the likelihood of trunk failure through stress fractures.
Juvenile branches protect young trees from sunburn. Young bark, like the skin on a baby is much more sensitive and susceptible to sunburn than mature bark. A fingernail can easily scratch through young bark and into the cambium layer. Try that with mature bark and you’ll need a quick trip to the manicurist! Juvenile branches protect the tender trunk tissues by diffusing the intensity of the sun’s harsh rays. The premature removal of juvenile branches often results in sunscald and peeling bark on the southwest side of the tree. As with all injuries to trees, the damage is permanent and results in a reduction of vascular flow and an inviting entry point for wood destroying insects and disease.
By Tina McKeand
Juvenile branches should be allowed to remain until they are at least as big (in diameter) as your “Thumb”. Tip back branches that interfere with traffic or view.
Juvenile branches can be found throughout a tree and on trees of any age. They may sprout to fuel the development of reaction wood for greater strength or in response to decay or disease. Preserving juvenile branches in the vicinity of larger pruning wounds can promote faster healing through greater wound wood development (compartmentalization). Wherever they occur, it is best to leave them on as long as practical so that they can perform their intended function.
Juvenile branches are often quite vigorous and may bravely grow out into parking lots, sidewalks, streets and other high volume traffic areas. This is unacceptable in an urban environment and they must be encouraged to conform to the constraints of the community while still performing their vital role. A juvenile branch that is growing in an undesirable direction can be tipped back to within 12 inches of the trunk. A reduction cut to a side shoot will direct the juvenile branch to grow in the direction of the selected side shoot. A heading or stub cut, while never recommended for a permanent branch, will promote an adventitious cluster of leaves. Either of these treatments will preserve the purpose of the branch while conforming to societal needs.
In a natural environment, trees shed juvenile branches when they are no longer needed. If you’ve ever crawled under a large canopy in the forest you may have noticed the dried up branch stubs still attached on the lower trunk. Trees naturally shed juvenile branches once their purpose has been fulfilled. Shade from the developing canopy eventually reduces sunlight to the small lower branches and renders them useless for photosynthesis. Once there is no longer a return on investment, the tree cuts off circulation and literally starves the non-productive member to death.
As mortals, we can’t possibly be expected to exercise the patience and restraint necessary to wait for juvenile branches to dry up and fall off, so I promote what I believe is a reasonable “rule of thumb”. When an undesirable branch reaches a diameter bigger than your “thumb”, it’s okay to remove it. Juvenile branches will continue to sprout as long as they are deemed necessary by the tree. Nature provides the answers if we are paying attention. A lot can be learned by touching and listening to trees. Ah, I hear them whispering now . . .I’m off!
Visit www.treesaregood.com for additional information on pruning a tree and other important topics.
© 2007 Tina McKeand. Originally published in Southwest Trees & Turf, June 2007.
[email protected] 928.246.7048
Tina McKeand Davey Resource Group 7627 Morro Rd., Atascadero, CA 93422
Sealing is not complete until the branch collar grows completely over the wound.
Arguably, trees are the most successful organisms on
earth. As a group, they hold bragging rights as the tallest (Coastal Redwoods), the most massive (Giant Sequoia), and the oldest (Bristlecone Pines) living beings on the planet. They produce their own energy and recycle available resources to molecular levels. As such phenomenal beings, they deserve our utmost respect and reverence. Unfortunately trees are often taken for granted, especially when it comes to our community
forests. Examples of improper pruning and topping abound in every community. While basic proper pruning is not terribly difficult to master, if the intent is to preserve the tree in
a healthy manner, it does require some understanding of tree physiology and biological response. Many trees are killed and permanently disfigured by well intentioned ignorance and the misunderstanding that everyone who works in landscape maintenance knows how to trim a tree. Armed with a basic appreciation of pruning response and recognition of harmful practices, many common errors can be avoided.
Because a tree is a dynamic, living organism, every pruning cut results in a chain reaction within the tree, including response to the injury, changes in energy distribution and flow, and a reduced ability to produce food.
Trees do not heal in response to injury in the same way as animals. Instead of replacing bruised and damaged cells, trees compartmentalize, or seal their injuries by building barriers and diverting circulation around the damaged area. These compartments become permanent dead areas inside of the tree. Internal compartmentalization begins immediately in response to injury. Chemical walls form laterally, horizontally and radially to prevent the damage or disease from progressing and invading healthy tissue within the tree. Since tree bark serves as an outer layer of protection, much like our skin, compartmentalization is not complete until new tree rings grow over and completely cover the wound. Until this process is complete, the tree is vulnerable to invasion by insects and decay organisms. Painting the wound does not facilitate this process, since these treatments are no substitute for new growth rings and in some cases actually retard the natural sealing process.
An important goal in proper pruning is to reduce the compartmentalization response. One way to minimize and speed up the sealing process is to minimize the size of wounds by removing undesirable branches when they are relatively small (preferably less than 4” diameter). This is why, with the exception of mitigating a hazardous situation, structural pruning should be performed when trees are young and actively growing. Additionally, it is important to always prune to a branch union, being careful to cut just outside of the collar. The collar of a branch is actually the outer rings of the parent branch or trunk folded back around the base of the branch (like a towel draped around your neck). Once the branch is removed, the collar will grow over the wound, completing the sealing process. The branch union is also where natural intersections of the vascular system occur, and pruning to these intersections makes it easier for the tree to “detour” the vascular energy that once flowed into the missing limb. Failing to preserve the branch collar by “flush cutting” or allowing branches to rip off the tree substantially increases the area of compartmentalization and severely inhibits sealing.
By Tina McKeand
Topping Kills Trees!
Every pruning cut alters the flow and distribution of energy within a tree, and understanding the basics is necessary for anticipating and controlling pruning response. In an un-pruned branch, the terminal bud (outermost tip) performs like a crew leader, using hormones to direct growth and keep the lateral branches in check. Removing the terminal bud releases growth energy to the remaining lateral branches and results in a leadership void. Pruning to the union of a lateral branch transfers the leadership role and redirects growth into the selected lateral. Removing an entire branch transfers growth energy into the remaining branches. The central leader (main stem) should not be cut or tipped back as this action will permanently alter the natural shape and structure of the tree.
Pruning removes leaves. Since leaves are responsible for producing energy, removing them has an impact on the tree’s ability to grow, protect itself from disease and pests, and even to fuel the compartmentalization necessary to recover from the pruning event. The amount of leaf canopy that can be safely removed depends on many factors, including the age of a tree, species, general health, and season. A general rule of thumb is to avoid removing more than 25% of the living canopy. This is a maximum number, not a goal to be reached, and proper pruning practices seek to minimize canopy removal as much as possible while still correcting the structural issues.
Topping trees is never an acceptable practice. Topping results in many open wounds with no consideration to pruning at branch unions or collars. The cut stubs begin to compartmentalize, but they cannot completely seal since they are not at a branch union and are far removed from a collar. This leaves the stubs vulnerable to attack by insects and decay organisms. Unfortunately, topping a tree stimulates the growth of adventitious sprouts (multiple leafy shoots, especially on stubs), leading to the misconception that the tree responded favorably to the treatment. In fact, adventitious growth is an emergency response undoubtedly developed by nature to respond to catastrophic injuries caused by natural phenomena (e.g., hurricane, lightning strike, etc.). Stimulation of adventitious growth is a sign that the tree has suffered severe damage and is trying mightily to replace its food source. Adventitious branches grow quickly but are poorly attached just underneath the bark (of the dying stubs), leading to hazardous conditions as the branches become larger and heavier.
The objective of proper pruning should be the removal of crossed and competing branches as well as dead and diseased wood and the correction of other major structural defects. Since urban trees are destined to be part of a community, they should be trained at a young age to conform to the standards set by the community. As with children, a properly trained tree is more likely to become a positive, contributing member. Trees must be guided to co-exist in harmony with other components and branches cannot be allowed to block stop signs or interfere with the normal flow of vehicular and pedestrian traffic.
Always keep in mind that pruning creates wounds, leaving the tree vulnerable to pathogen invasion. The power to shape growth and divert energy must be used wisely and sparingly to promote healthy structure while still preserving the natural grace and character of both the individual and the species. In order to preserve the value and dignity of a tree, most pruning should be performed by properly trained personnel, ideally, someone certified as an arborist or a tree worker.
For additional information and illustrations for pruning a tree or for assistance contacting a Certified Arborist, visit www.treesaregood.com
© 2007 Tina McKeand. Originally published in Southwest Trees & Turf, July/August 2007.
[email protected] 928.246.7048
Tina McKeand Davey Resource Group 7627 Morro Rd., Atascadero, CA 93422
