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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!

The botanical images on this site were produced by The Photon Hunt.

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Copyright at Common Law by Manuel Flores