Arboriculture & Urban Forestry 34(1): January 2008 trunk development (in particular, reaction wood formation) of several species. Harris (1984) reported that trunk staking is often unnecessary and can be detrimental to tree growth. The most recently published, peer-reviewed research compared the effect of three stabilization systems on the growth of Pyrus calleryana (Svihra et al. 1999). The investigators observed differences in tree height growth among the systems during the first year after planting; however, growth was similar in the second year, after removal of the systems, with differences in trunk taper persisting into the third year. No research literature was found comparing tree stabilization through rootball anchoring with trunk staking (one article from the United Kingdom mentioned using rootball anchoring but gave no posttransplant results). Of six arboricultural reference books consulted, all concur that stabilization should only be undertaken when necessary and not as a default practice (Gilman 1997; Watson and Himelick 1997; Lilly 2001; Harris et al. 2004; Watson and Himelick 2005; Whit- comb 2006). They further agree that in most cases, staking/ guying should be attached as low on the trunk as possible and left in place no more than 1 year. All references discuss the potential for the common form of guying—hose-covered wire— to cause trunk-girdling injury. They further suggest that guying materials be wide, smooth, flexible, and nonabrasive and, to help with problems of nonremoval, even photodegradable. Harris et al. (2004) categorize three types of tree staking: protective staking to keep away equipment, vehicles, and van- dals; anchor staking to stabilize roots or rootballs until new roots grow into the surrounding soil; and support staking for trees with trunks too weak to stand upright alone. Gilman (1997) describes both anchor and support staking and provides numerous ex- amples of products and systems that can be used. Watson and Himelick (1997, 2005) also discuss the alternative of stabilizing trees through products and systems that anchor rootballs. Whit- comb (2006) urges consideration of an eye screw attachment or guying system (in place of ties) not presented by other refer- ences. Whitcomb (2006) emphasizes that part of the problem, espe- cially for trees grown in containers, is excessive nitrogen appli- cation and close spacing of plants during production. These prac- tices encourage dense canopies and poorly tapered trunks. Such trees often will not stand upright when transplanted to the land- scape and thus require support staking. Whitcomb argues that trees produced in this manner are not acceptable nursery stock and should be rejected. In a search of the Internet, extension publications posted by universities contained the most up-to-date, research-based infor- mation, although several still listed hose-covered wire as the primary attachment or guying material. Articles posted on com- mercial sites and written by garden writers and other laypersons were less up-to-date and less accurate; however, an improvement in the quality of information was noted between the initial search conducted in 2002 and a follow-up search conducted in 2006. STABILIZATION MANUFACTURER SURVEY To gain perspective on tree stabilization in the United States, surveys were sent to 12 manufacturers of tree stabilization prod- ucts (100% reply response). Manufacturers (Table 1) were asked what products they made, how the products worked, whether they could be used for both field-grown (balled and burlapped) and container-grown trees, how rigidly the products held the tree, how long they recommended products be used in the field, 55 and what their long- and short-term goals were in developing their product relative to field performance. The following is a summary of manufacturer survey re- sponses: • 83% of the products are for aboveground use (“tree stak- ing”) and 17% are for belowground use (“rootball anchor- ing”). Within these product lines, 32% use wooden stakes, 32% use a rope or cable anchoring system, and the other 36% use a variety of other materials (metal or fiberglass poles, plastic guying, and so on); • 87% said their product could be used on both field and container rootballs, and 13% said only on field rootballs; and • 67% said their product was designed to allow some trunk movement, 22% said their product was designed to allow significant trunk movement, and 11% said their product was designed to hold the trunk rigidly in place. STABILIZATION PRACTITIONER SURVEY To gain perspective on how tree stabilization is implemented at planting, a practitioner survey was developed and distributed at several field days and conferences in the mid-Atlantic region and through regional and national trade publications (e.g., Tree Care Industry, Groundworks, Newsletter of the Virginia Nursery and Landscape Association, South Carolina Today, The Log). The following is a summary of over 300 responses received primarily from landscape contractors and arborists: • 81% of practitioners use an aboveground stabilization sys- tem with the most common being wooden stakes with hose- covered wire followed by wooden stakes with nylon strap guying. Of those who use belowground systems, the most commonly used system is a wooden frame with anchoring wires or cables followed by metal plates or spikes; • Practitioners were slightly more aware than manufacturers of the recommendation that trees not be held rigidly in place: 72% said they select products or systems that allow some trunk movement, 21% said they use products or sys- tems that allow significant trunk movement, and only 7% said they use products or systems that hold trunks rigidly in place; • Manufacturers and practitioners varied in their responses regarding short- and long-term criteria for product develop- ment and product/system selection, respectively (Table 2). Practitioners are most concerned with product impacts on tree growth and development rather than product mechan- ics; and • 71% of respondents said that they had observed damage (generally girdling) from stabilization systems being left in place too long. Other observations of damage or problems included poor root development, bark injury, trunk break- age, reduced trunk caliper, stem swelling above guying, suckering below guying, and foliage discoloration. RESEARCH TO TEST PRODUCTS AND SYSTEMS As a result of the increased number of tree stabilization products and systems in use by the green industry and the lack of pub- lished research evaluating the efficacy of the newer rootball anchoring products, two comparative research projects were ini- tiated in 2003. One was conducted at Virginia Tech’s Hampton ©2008 International Society of Arboriculture
January 2008
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