Pruning Young Trees
Proper pruning is essential in
developing a tree with a strong structure and desirable form. Trees that
receive the appropriate pruning measures while they are young will
require little corrective pruning when they mature.
Keep these few simple principles in mind before pruning
a tree:
- Each cut has the potential to change the growth of the tree.
Always have a purpose in mind before making a cut.
- Proper technique is essential. Poor pruning can cause damage that
lasts for the life of the tree. Learn where and how to make the cuts
before picking up the pruning shears.
- Trees do not heal the way people do. When a tree is wounded, it
must grow over and compartmentalize the wound. As a result, the wound
is contained within the tree forever.
- Small cuts do less damage to the tree than large cuts. For that
reason, proper pruning (training) of young trees is critical. Waiting
to prune a tree until it is mature can create the need for large cuts
that the tree cannot easily close.
Making The Cut
Where you make a pruning cut is critical to a tree’s
response in growth and wound closure. Make pruning cuts just outside the
branch collar. Because the branch collar contains trunk or parent branch
tissues, the tree will be damaged unnecessarily if you remove or damage
it. In fact, if the cut is large, the tree may suffer permanent internal
decay from an improper pruning cut.
If a permanent branch is to be shortened, cut it back to
a lateral branch or bud. Internodal cuts, or cuts made between buds or
branches, may lead to stem decay, sprout production, and misdirected
growth.
Pruning Tools
When pruning trees, it is important to have the right
tool for the job. For small trees, most of the cuts can be made with
hand pruning shears (secateurs). The scissor-type, or bypass blade hand
pruners, are preferred over the anvil type. They make cleaner, more
accurate cuts. Cuts larger than one-half inch in diameter should be made
with lopping shears or a pruning saw.
Never use hedge shears to prune a tree. Whatever tool
you use, make sure it is kept clean and sharp.
Establishing a Strong Scaffold Structure
A good structure of primary scaffold branches should be
established while the tree is young. The scaffold branches provide the
framework of the mature tree. Properly trained young trees will develop
a strong structure that requires less corrective pruning as they mature.
The goal in training young trees is to establish a
strong trunk with sturdy, well-spaced branches. The strength of the
branch structure depends on the relative sizes of the branches, the
branch angles, and the spacing of the limbs. Naturally, those factors
vary with the growth habit of the tree. Pin oaks and sweetgums, for
example, have a conical shape with a central leader. Elms and live oaks
are often wide-spreading without a central leader. Other trees, such as
lindens and Bradford pears, are densely branched. Good pruning
techniques remove structurally weak branches while maintaining the
natural form of the tree.
Trunk Development
For most young trees, maintain a single dominant leader.
Do not prune back the tip of this leader. Do not allow secondary
branches to outgrow the leader. Sometimes a tree will develop double
leaders known as co-dominant stems. Co-dominant stems can lead to
structural weaknesses, so it is best to remove one of the stems while
the tree is young.
The lateral branches contribute to the development of a
sturdy well-tapered trunk. It is important to leave some of these
lateral branches in place, even though they may be pruned out later.
These branches, known as temporary branches, also help protect the trunk
from sun and mechanical injury. Temporary branches should be kept short
enough not to be an obstruction or compete with selected permanent
branches.
Permanent Branch Selection
Nursery trees often have low branches that may make the
tree appear well-proportioned when young, but low branches are seldom
appropriate for large-growing trees in an urban environment. How a young
tree is trained depends on its primary function in the landscape. For
example, street trees must be pruned so that they allow at least 16 feet
of clearance for traffic. Most landscape trees require only about 8 feet
of clearance.
The height of the lowest permanent branch is determined
by the tree’s intended function and location in the landscape. Trees
that are used to screen an unsightly view or provide a wind break may be
allowed to branch low to the ground. Most large-growing trees in the
landscape must eventually be pruned to allow head clearance.
The spacing of branches, both vertically and radically,
in the tree is very important. Branches selected as permanent scaffold
branches must be well-spaced along the trunk. Maintain radial balance
with branches growing outward in each direction.
A good rule of thumb for the vertical spacing of
permanent branches is to maintain a distance equal to 3 percent of the
tree’s eventual height. Thus, a tree that will be 50 feet tall should
have permanent scaffold branches spaced about 18 inches apart along the
trunk. Avoid allowing two scaffold branches to arise one above the other
on the same side of the tree.
Some trees have a tendency to develop branches with
narrow angles of attachment and tight crotches. As the tree grows, bark
can become enclosed deep within the crotch between the branch and the
trunk. Such growth is called included bark. Included bark weakens the
attachment of the branch to the trunk and can lead to branch failure
when the tree matures. You should prune branches with weak attachments
while they are young.
Avoid over thinning the interior of the tree. The leaves
of each branch must manufacture enough food to keep that branch alive
and growing. In addition, each branch must contribute food to grow and
feed the trunk and roots. Removal of too many leaves can “starve” the
tree, reduce growth, and make the tree unhealthy. A good rule of thumb
is to maintain at least half the foliage on branches arising in the
lower two-thirds of the tree.
Newly Planted Trees
Pruning of newly planted trees should be limited to
corrective pruning. Remove torn or broken branches, and save other
pruning measures for the second or third year.
The belief that trees should be pruned when planted to
compensate for root loss is misguided. Trees need their leaves and shoot
tips to provide food and the substances that stimulate new root
production. Unpruned trees establish faster with a stronger root system
than trees pruned at the time of planting.
Wound Dressings
Wound dressings were once thought to accelerate wound
closure, protect against insects and diseases, and reduce decay.
However, research has shown that dressings do not reduce
decay or speed closure and rarely prevent insect or disease
infestations. Most experts recommend that wound dressing not be used. If
a dressing must be used for cosmetic purposes, use a thin coating of a
material that is not toxic to the plant.
Source credit
International Society of Arboriculture
http://www.isa-arbor.com
