When to Prune Deciduous Trees
Patrick Weicherding, Regional Extension Educator, Andover
Regional Center, MN
|
Pruning chart
Image credit:
Patrick
Weicherding |
A common question around this time of year is
"When is the best time to prune trees?" Mike Zins, former
Extension Horticulturist with the University of Minnesota
Extension Service, used to say "When your pruners are sharp." We
know this "tongue-in-cheek" adage can be applied any time of the
year for light pruning - just removing small, damaged, or
diseased branches - but more extensive pruning should be done in
late winter to early spring, in March or April once the threat
of severe cold weather has passed.
Many of you probably recognize that pruning in late winter to
early spring is the standard "dormant season" pruning
recommendation, but how many of you know "the science" behind
this standard?
Throughout a 1-year cycle, a tree has five major phenological
periods as illustrated in the accompanying illustration: The
tree's stored energy fluctuates as a result of what occurs
during these periods. During period one from February through
March, the tree is coming out of dormancy. Growth is beginning
but very little photosynthesis is taking place. At this point,
the tree is relying on reserve energy from the previous year.
In period two (April) bud break and leaf formation are the major
growth activities. Because photosynthesis is still minimal at
this time, the tree must rely on the energy from its energy
reserves. The tree begins to enter its lowest energy level
during the growing season. Insects and pathogens are also
becoming active at this time.
During period three from May through July, photosynthesis
increases dramatically. The new leaves begin to produce more
energy. Most of this energy is being used up in the process of
leaf formation in the early part of the period. All of the
tree's terminal and radial growth occurs within 6-8 weeks
following full leaf expansion.
In period four (August through September) the tree begins its
annual storage of energy for the following year. Much of the
seasonal growth is complete and the tree enters period five
(October through January). This is the dormant period when the
tree is at rest as it idles through winter.
From the above discussion it should be obvious that pruning
during the active part of the growing season (when the tree has
little or no energy reserve to expend on wound closure) has the
potential to change the growth of the tree. Removing living
foliage, by pruning, affects the tree's physiology and future
growth. The reduction in leaf area that results from pruning
will reduce the tree's overall photosynthetic capacity and may
reduce overall growth on the pruned section, or the entire tree.
Pruning during the growing season can therefore have a lasting
impact on the viability of the tree and can directly influence
it's longevity.
So, what's the best time during the dormant season to be pruning
trees? The late dormant season is best for most pruning on
deciduous trees. The advantages of late dormant season pruning
include:
ºFirst, wound closure is most rapid if it's done just prior to
when new shoots emerge. Pruning too early in the dormant season
allows the pruning wounds to crack and dry out, delaying wound
closure. (Most research on the timing of pruning indicates that
pruning cuts close more slowly in the fall than in any of the
other seasons.)
ºSecond, there are few insects and disease spores active to
infest pruning cuts. (This is the reason we don't recommend
pruning oaks and elms during the growing season - the risk of
Dutch elm disease and oak wilt gaining a foothold in fresh
pruning wounds is too great.) The chances of serious invasion of
pruning wounds by decay-causing organisms are at their height
during the fall coloration period.
ºThird, deciduous trees at this time of year have not yet
produced new leaves making it easier to make pruning decisions -
you can easily see the plant branch structure without leaves.
Some trees such as elm, maple, walnut and birch are "bleeders"
when pruned in late winter to early spring. This oozing sap is
annoying when it drips on cars and sidewalks. There is
scientific disagreement as to whether the oozing is harmful to
trees. Some researchers believe that the oozing might interfere
with the closure of pruning cuts while others argue that it is a
positive benefit in that it reduces the probability of
colonization of the pruning wound by decay organisms. If you are
concerned, prune these species in the winter but keep in mind
that this is not the best time of year to prune most trees.
Finally, what about the wound dressing question? Wound dressing
or pruning paint is not essential; recent research has shown
that pruning paint or wound dressing is not as advantageous as
previously thought. Dressings may actually harbor disease
organisms rather than exclude them. It has also been determined
that wound dressing slows the formation of callous tissue over
the wound. Wound dressings and pruning paints are strictly
cosmetic.
The only reason for painting a pruning wound is to aid in the
prevention of oak wilt during the growing season. Latex paint
can be used to discourage sap beetles from visiting the wounds.
Sap beetles are thought to be an overland vector of the oak wilt
fungus.
If you're interested, Bonnie Appleton and Susan French from the
Virginia Cooperative Extension Service have developed a nice
"Deciduous Tree Pruning Calendar" that lists the best (and
worst) times to prune specific trees. Temper their
recommendations with the fact Virginia's species diversity and
climate are not the same as ours in Minnesota. The Adobe Acrobat
pdf-file can be found at:
http://www.ext.vt.edu/pubs/nursery/430-460/430-460.pdf
Yard &
Garden Line News
Volume 6 Number 18
December
1, 2004
