Tea Party? Tree Party!!

60It’s all too easy to forget about our trees, particularly the deciduous species, once fall has earned its name and begins to turn toward winter. However, now is an excellent time to take stock of their growing conditions, pruning needs, and other cultural issues, many of which often overlap with our own aesthetic desires and basic requirements for safety and peace-of-mind.

For instance, pruning (and other interventions such as cabling) performed to alleviate hazards to human activities or habitations will almost invariably improve the life of the tree. The one obvious exception to this is the instance in which the hazard level demands complete removal of the tree, but it may be said that such a tree is living on borrowed time anyway.

Structural Pruning

Corrective pruning was performed on this maple to reduce co-dominant leaders and to encourage development of the central leader. Stems appear truncated at branch junctions where leaders have been pruned out to promote apical dominance.

Beyond hazard mitigation, pruning is motivated by two main objectives: improving the health of the plant and increasing its aesthetic value. Again, the means of achieving one goal usually fosters the second to a great degree. Pruning to reduce crossing branches and promote apical dominance (i.e., development of a strong central leader) is conducive to improving not only the health of a tree, but its architectural structure and beauty as well, since the trimmed-down product is more open and uncomplicated in disposition, and thus attains enhanced aesthetic legibility.

A fourth motivation for pruning is optimizing productivity, and is most applicable to orchard management, although many of the same principles will result in increased flowering in ornamentals as well. This may well be the most intensive type of pruning, both in detail and in frequency, since most modern fruit trees require annual pruning to control growth and maintain health and yield.

Bringing the forest to the trees

71We have become so accustomed to experiencing trees in the built environment that we often forget that they are sylvan entities, and that they draw the greatest benefit when we provide settings that mimic forest conditions to as reasonable a degree as is possible. In the most general terms, this means appropriately siting the tree: for instance, not planting a baldcypress in dry soil or a boreal species such as black spruce in a steamy lowland floodplain. However, in the most specific and practical terms, we ought to focus on the soil, the understory, and natural nutrient cycles in order to best provide for these proverbial fishes-out-of-water.

Much like an iceberg, a tree is as present within the soil as it is above, and in its pure fractal genius, nature has endowed trees with a dendritic architecture that mimics the form that reaches for the sky, as though the soil line were but a mirror dividing ego and alter-ego. (It bears noting that this mirror image is somewhat of a funhouse-reflection, both shallower in depth and broader in spread than its aerial counterpart.) A tree (making an attempt at) living in a hyper-urban environment, surrounded by impermeable hardscape, is about as divorced from its evolutionary origins as one can be, and understandably has more issues than National Geographic.

Conversely, though, understanding the plight of trees living in suburban settings seems to require a greater stretch of the imagination. I use the term suburban here in a very specific application referring to the growing conditions of the tree, without regard to the larger mise-en-scene. A native balsam fir growing deep in the backcountry can still be subject to typical suburban conditions and stresses if it is underplanted and surrounded by turfgrass. The human residents may be off the grid, but the tree might as well be living in Massapequa.

By far, the single greatest conflict we encounter vis-à-vis trees in the cultivated landscape is that they are more often than not situated in the midst of that perfect green blanket of American hagiography. One can be forgiven for not immediately grasping the inherent conflict, but it is really quite obvious if one considers where and how trees exist in the wild. There certainly is not a lawn anywhere in sight.

Typical damage to a trunk flare incurred by a lawn mower.
There are many reasons why turfgrass lawns are detrimental to trees. The most apparent is mechanical damage from mowers and string-trimmers, which can strip bark, cause irreparable damage to underlying tissues, and provide convenient entry to infection by any number of pathogens. Somewhat less obvious is stress incurred by both trees and turfgrass competing for water and nutrients in essentially the same stratum of soil.

However, in terms of the overall micro-environment of the tree, we need to look underground for our answers, and preferably with a powerful compound microscope, because the most basic biological disparity between forest and grassland ecotypes is in the composition of the community of soil microbes associated with each environment. Those microbes are symbionts that have evolved with different types of plants to benefit from the conditions created by the particular plant, and in return, aid the plant in nutrient acquisition, pathogen exclusion, and other functions that foster overall vigor of the plant – which, of course, is then better able to provide optimal conditions for its microbial flora and fauna.

A very apt analogy can be drawn from the many services provided to us by our own microbial residents, whether their address is epidermal or alimentary in nature. Similarly, just as Lactobacillus acidophilus is not going to be of any help to a termite, the microbes associated with turfgrass do more harm than good to a tree.

62Components of a healthy forest floor include various kinds of leaf litter, decomposing wood, mosses and native woodland plants such as the glossy-leaved, evergreen perennial goldthread (Coptis trifolia), a native medicinal herb.

The answer is accessed by looking at and around wild trees. What we find is decomposing plant material, and plenty of it: a mélange of leaf litter, old twigs, bark, and wood, all contributing to the gradual creation of humus, the ambrosia of woodland plants great and small.

With this understanding, removing the turf ought to be a no-brainer, but the solution is incomplete without some replacement that proffers ameliorated biological function. Again, the answer is accessed by looking at and around wild trees. What we find is decomposing plant material, and plenty of it: a mélange of leaf litter, old twigs, bark, and wood, all contributing to the gradual creation of humus, the ambrosia of woodland plants great and small. This key cycle is what is most glaringly absent in the life of the suburbanized tree, but it can be most easily approximated by mulching within the drip-area of the tree. While the population of microbes may not be distributed in a way beneficial to the tree at first, it will shift and diversify over time so as to fall closer in line with what one would expect to find in nature. To dip into a postmodern cliché, it’s a bit of a Field of Dreams situation.

The resulting mulch bed – which ought to have a diameter about 10 times that of the trunk at breast height (commonly abbreviated as d.b.h.) – can be simply that, as is often appropriate with a small ornamental such as a crabapple, or it can be a garden in its own right. Most shade perennials (and certainly the woodlanders among them) coexist quite politely with tree roots, and from a designer’s perspective do not arouse the same sense of discontinuity that is present with an underplanting of lawn.

While such projects may seem daunting, they need not be so. In fact, one need not even remove the turf; it can simply be smothered into soil-building biomass by layering a biodegradable barrier such as newsprint or cardboard over it before mulching. One can still plant the area with groundcover perennials after the grass below has died and decomposed, a process which consumes only a few months of the growing season. Areas mulched in such a way in either the autumn or the spring are normally ready for planting by late in the subsequent summer.

While such projects may seem daunting, they need not be so. In fact, one need not even remove the turf; it can simply be smothered into soil-building biomass by layering a biodegradable barrier such as newsprint or cardboard over it before mulching. One can still plant the area with groundcover perennials after the grass below has died and decomposed, a process which consumes only a few months of the growing season. Areas mulched in such a way in either the autumn or the spring are normally ready for planting by late in the subsequent summer.

Treats for trees

We often ask, or rather demand, that trees grow for us in suboptimal conditions. It is not realistic to expect that every tree in the landscape will find its home in a mulch ring or garden bed. Similarly, part of the fun of horticulture is found in pushing the envelope in terms of what we can and should grow in our cultivated landscapes. In short, sometimes suboptimal siting is inevitable, and in such cases, trees need all the help they can get.

One way to provide this assistance is through so-called “deep-root” fertilization, by which one injects a targeted dose of nutrients directly into the zone populated by a tree’s feeder roots. This treatment gives the tree the raw materials it requires to develop in a healthy and resilient manner, better able to withstand the caprices of an often harsh environment. Since this passing season brought a great deal of stress in the forms of a persistent dry spell, late freezes, and disease issues, fertilization of important landscape trees is highly recommended to prevent those stresses from accumulating into a loss of vigor in the future.

By Terrence Trapp, Director of Horticulture

November 14th, 2011

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