Tree Forms and Structure:
A wise man once said, “The danger in communication is the illusion that it occurred.” The statement reflects an all too common truth, that the words (terms) we use to communicate are NOT always mutually agreed upon by those we communicate with. This becomes particularly troublesome in technical discussions, like those between landscape architects or municipalities when specifying tree structures and forms to be planted by landscape contractors. Science uses Latin; an essentially dead language, when communicating the names of everything to the names of organism to biological structures. Landscape horticulture has no such luxury in our communications.
“Standard” and “Multiple Trunk” are terms regularly used to describe the trunk structure of trees used in landscape designs. While such terms are liberally used in the desert southwest within the design, construction and horticultural communities, there are no generally agreed upon definitions or descriptions that relates the terms to actual tree forms. Clear, precise terms are needed if growers, contractors and landscape architects are to successfully and effectively communicate their individual tree needs and comply with requirements stipulated by municipalities.
American Hort publishes, "American Standards for Nursery Stock" which attempts to set standards and define terms for nursery professionals nationwide. Their definitions are heavily weighted toward tree species, in commercial nursery production, that are commonly found growing wild in woodlands from the eastern to northwestern United States. Hortus defines standard as a woody plant cultured over a few growing seasons to replicate a small tree form. In many tree species, including desert species, we see gradations in trunk characteristics ranging from true single trunk trees (e.g. Acacia stenophylla) to clump form multiple trunk trees (e.g. Cascalote, Caesalpinia cacalaco, and to some extent Desert Willows Chilopsis linearis). Many terms have been used to describe the intermediate tree forms between these two extremes. Examples include low breaker, naturals, triples, clump, shrub form and others. The variety of terms and the absence of general definitions have contributed to confusion both within design studios and landscape construction sites. Florida's Nurserymen and Growers Association in 1955 implemented some of the original grades and standards adopted by nurserymen. Setting grades and standards for nursery stock was instrumental in better communicating the landscape designer's visions to the grower, buyer and seller.
To further complicate the discussion there are specialty nurseries that train trees into forms not typical of their normal growth habit. They include standard (e.g. rose trees, oleander trees), braided stems (e.g. Ficus benjamina), poodles (e.g. Privet), espalier (e.g. Pyracantha), topiary (e.g. Taxus), and bonsai (e.g. Juniperus, Pinus) to name a few.
All these efforts, while well intentioned, have met with limited success. Given the diversity of regional landscapes, the enormous number of plant species offered for commercial sale nationally, the range of differing sized shapes and containers offered for sale, developing widely acceptable and useful standards is a formidable task.
The simplest and most descriptive terms for the commonly planted desert trees in today's landscape palette are "single leader" and "multiple" trunk (or leader). It is clear that the word "standard" refers to a "single leader trunk" tree with the initial scaffold branches originating at three to six feet above the soil line. The distance to the first branch will vary depending upon the proportions (ratio of height to width) desired by the landscape architect, the particular landscape setting where the tree is to be installed, the natural growth habit of the tree species and the production methods commonly used by the grower (for example keeping temporary branches). It would be clearer if such trees were referred to as “single trunk” since this is what they are. The natural form of the vast majority of desert adapted trees is some variation on a multiple trunk; greater detail is needed in describing these forms. In some respects the word "multiple trunk" seems to over simplify the assortment of forms to be found within these species. These variations include clump multi, shrub multiple, low breaking multiple, and low breaking single.
The diagram, shown previously, of the most popular forms of deserts trees offered for sale, recommends term to use when specifying these forms on landscape plans or when ordering trees. While these terms are not universal, within the desert southwest landscape community, they are straight forward and clearly descriptive of the form(s) desired for installation.
Efforts to clarify tree specification terms will ultimately be of little long term value if pruning and maintenance practices, after installation, fail to appreciate the natural form of the tree. This “education” piece of the puzzle is possibly more critical, and more challenging to communicate, than the specification piece and far more difficult to control. Communities’ (states, cities, counties, HOAs etc) commitment to the development of vibrant and regionally appropriate urban forests depends on an informed focus on both specification and long term care of trees in the developed landscape. The absence of appropriate, long term maintenance programs will compromise the desired appearance and the longevity of the urban forest and negate the measures taken in tree selection, design and construction.