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Old Oaks
and How to Care for Them
When to prune oaks in San
Antonio, Austin and the adjacent Hill Country has been contentious for
several decades with the inexorable spread of Oak Wilt. Now, with warmer
fall temperatures and the winter of 2015-2016 having had no hard freezes,
the timing of fall pruning is being debated. While an El Niño gave
us the warm winter last year, it is not a constant feature of our climate.
Mild fall temperatures continuing
to the winter Solstice is becoming the norm for us. Those with vegetable
gardens could not be more pleased. However, when dealing with our trees
(and shrubs) we need to know that any pruning encourages more growth. As
we approach the winter season, promoting growth - even inadvertently or
out of ignorance - is reckless. A hard freeze when a tree is not fully
dormant causes tremendous die back of smaller branches and shoots.
Within shoot tips, a natural
growth-retarding chemical creates the condition we call apical dominance.
The apical dominance factor keeps lower axillary buds from growing. Remove
upper shoot tips and lower buds will develop.
The scenario of daytime highs
in the mid-70s to low- to mid-80s coupled with adequate rain, even into
mid December, will keep many trees from entering dormancy. If such a warm
spell is followed by a hard freeze, serious damage will occur. Such a scenario
is not improbable. Our warming climate now demands we delay any fall pruning
until dormancy is initiated by a hard freeze. Those who wish to take chances
with their or their clients’ oaks should read, or re-read, ‘The Black Swan:
The Impact of the Highly Improbable' by Nassim Nicholas Taleb. A black
swan event of the meteorological kind, however unlikely, is still possible.
The problem of delayed dormancy
is particularly acute with certain forms of Crepe Myrtle and oaks with
persistent leaves. What are commonly called, Live Oaks, are not actually
evergreen. The leaves from one growing season persist until the following
spring. Most oaks with persistent leaves are grouped in the ‘White Oak’
subgenus. In the ‘Red Oak’ subgenus, trees that keep their leaves into
the winter season, usually with some fall color, are said to be tardily
deciduous.
I cultivate several of the
persistent-leafed oaks of the cloud forest and mild montane zones of Mexico.
They are native to elevations where spring-like temperatures prevail all
year and, south of the Tropic of Cancer, where seasonal day length fluctuations
are minimal, they grow in response to rainfall, practically year-round.
If thoroughly acclimated, these oaks are completely cold-hardy in San Antonio.
Like with the Crepe Myrtle cultivars derived from crosses with Lagerstroemia
fauriei and Lagerstroemia subcostata, I withhold all supplemental
irrigation and fertilization after Labor Day. We also do not do any pruning
after mid-August.
Quercus insignis is
my favorite of these. In its high elevation habitat in the state of Veracruz,
it can develop acorns as large as a tennis ball.
From close observation of
native and exotic oaks with persistent leaves, I have come to realize they
are completely dormant only after a hard freeze. In the absence of a hard
freeze, delaying pruning until January should be a safe strategy.
Giraffe Pruning
A tendency not to leave well
enough alone is laudable in the material world but problematic in the natural
world. Natural beauty that is lauded in song and verse is not good enough
for some. While there might not be anything more lovely than a tree, many
in our area feel that trees, especially Live Oaks, might have way too may
leaves and require their kindest of cuts.
The practitioners of “artistic”
pruning somehow believe trees grow supernumerary leaves on superfluous
branches. Actually, trees grow the precise number of leaves for their needs
– no more, no less. There is an exquisite balance between nutrient production
in leaves (by photosynthesis) and nutrient expenditures for growth and
absorption of water from the soil by the roots and the mycorrhizae that
forage on behalf of those roots
Primum non nocere,
(first, to not harm, in Latin), while not from the Hippocratic Oath as
commonly supposed, is actually from Hippocrates' Epidemics. This
ancient precept should be followed by tree surgeons as well as physicians.
Thinning the canopy of Live Oaks, especially old trees in their declining
years, stresses them severely. It is just as stressful as the herbivory
of giraffes on the acacias of the Serengeti.
Is this sculpting of trees,
to accentuate their main branches, really necessary? If this is to be called
art, it is an art that deprives trees of needed nutrients, stresses them
severely and, performed in the fall in our warming climate, encourages
growth when new growth should be discouraged.
Seneca’s Latin translation
of another phrase from Hippocrates is also appropriate here. Ars longa,
vita brevis (art is long, life is short) has nothing to do with the
visual or performing arts. In the original, Hippocrates was referring to
the healing arts. He was saying that the art of medicine required a lifetime
to master. From the perspective of trees, we could reverse it and state,
ars
brevis, vita longa.
May your trees live long,
prospering under your benign neglect!
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