Research and accepted practices

We’ve all heard the expression “experience is the best teacher,” and that is certainly true in arboriculture. Front line experience, coupled with scientific research, affects the accepted methods of care and, ultimately, tree health.

Pruning techniques are a specific area within arboriculture that is monitored and adapted as knowledge advances. What effect do recent findings have on accepted practices?

Educating consumers

Jim Meyers, a certified arborist, owns and operates Hedgehog Tree Care in Portland, Ore. He says that homeowners’ misconceptions about pruning present a challenge. Not being privy to current guidelines and research findings, many tend to rely upon outdated methods or hearsay. Customers may believe, for instance, that heavy pruning or wound dressing is essential. Meyers, along with other tree care professionals, must gently educate clients to promote both proper tree maintenance and good customer relations.

One way Meyers does that is to feature pointers on his company’s Web site, Other industry organizations provide educational leaflets for the public. The International Society of Arboriculture (ISA), for example, sells bulk copies of consumer educational brochures via its Web site ( to tree care firms. Its Web site also offers no-cost educational content for adults and children.

Photo by Steve Ausmus, courtesy of ARS.
Helping trees and power lines coexist is a common tree service task.

How are pruning practices changing?

Meyers said that standards have changed to accommodate the needs of individual species, rather than applying a one-size-fits-all philosophy. This allows arborists to make decisions about crown reduction, for example, based upon a particular species, rather than sticking with a prescribed percentage.

“Certain species will have a harder time dealing with a large reduction cut compared to others,” he added. “I think the industry is starting to see that less pruning is better for the health of the tree.”

Meyers hopes that the trend towards species-specific guidelines will continue and generate individual principles for hardwoods, conifers and fruit trees. He attributes many advances to Dr. Ed Gilman at the University of Florida, pointing out Gilman’s work on tree mechanics and structure.

Meyers said that Florida research convinced him that working with young trees to develop a central leader, with subordinate laterals, is important. He added that Gilman’s wind tests show that balanced foliage on the leader leads to greater stability and tolerance for wind; over-lifted trees did not demonstrate those qualities.

“I think, rather than thinning an entire tree to meet the standard one-quarter foliage, why not target a few heavy limbs with potential to compete with the dominant leader?” Meyers said. “This may do more for the tree in the long run without shocking it or forcing shoot growth.”

Talking with Dr. Ed Gilman

Gilman said that research projects aim to confirm best management practices, and today’s evaluation tools facilitate that. He said the case of the wind studies is an example. It originally seemed right to discourage over-lifting in care guidelines, but the research generated scientific evidence supporting that recommendation.

“Now we have structural data that says don’t do it; it promotes breakage in wind and possibly in ice and snow,” Gilman added.

The size of the cuts used in thinning was also evaluated in relation to stability during windstorms. Researchers learned that the size does matter; thinning by making 1.5 to 2-inch cuts helps reduce tree motion during high winds, leading to less damage and breakage.

Gilman said a surprising finding of the wind studies is that pruning on one part of a tree doesn’t affect the other portions of the tree. While the pruned area moves less, the movement in the unpruned region doesn’t change at all. Read more online at

Photo courtesy of Downey Tree.
Phillip Kelly takes a break after performing selective pruning on a red maple ( Acer rubrum).

Other University of Florida findings

Gilman shared another unexpected finding: the deeper a tree is planted, the shallower the roots develop and the opposite is also true.

“That was surprising,” Gilman added. “We planted some 12 inches deep and the roots came to the surface right away, but shallow planting produced deeper roots.”

He suggested that some practices might be modified based upon research. One study indicates that more flexibility may be appropriate in pruning guidelines. Varying reductions were made to evaluate growth, testing the guidelines that no more than 25 percent of any branch should be removed. Different limbs were pruned on one side as much as 80 percent, while the other side remained untouched. Results indicated that a more generalized guideline might be appropriate to accommodate other factors. For example, guidelines indicate that less foliage should be taken from mature trees, but in cases of limb defects, substantial reduction would lighten the load, reducing potential hazards.

Research in 2007 confirms current guidelines with regard to the angle of reduction cuts. Gilman said the angle doesn’t seem to be particularly important as related to decay behind the cut.

“The current standard is a good compromise because some species will die back near the angle,” Gilman said. “Err on the side that makes the cut smaller [exposing a smaller amount of wood]. Arborists should observe how particular species react to angles and adapt [accordingly].”

That brings us back to the importance of field experience, something Gilman values as crucial in tree management. Best practices are important, but Gilman emphasizes the need for flexible decision-making by arborists based on their knowledge and the details of the specific tree.

The author is a freelance writer based in Greensboro, N.C.

Additional University of Florida Research

Pruning lower branches of live oak (Quercus virginiana Mill.) cultivars and seedlings during nursery production: Balancing growth and efficiency: Gilman, P.J. Anderson and C. Harchick published findings in 2006 in the Journal of Environmental Horticulture [24: 201-206].

Production requires time for pruning to meet consumer demands, but pruning can slow growth, increasing time and expense of production. The scientists found no difference in caliper growth between trees with only the largest one or two low branches removed at each pruning, compared with trees having all lower branches shortened. Removing the largest one or two low branches at each pruning was the most efficient pruning method tested. As there was no difference in time required for pruning, the research team suggests removing branches early in the growing season to allow greater wound closure and a visually attractive product.

Branch union morphology affects decay following pruning: Gilman and J.C. Grabosky published findings in 2006 in Arboriculture & Urban Forestry [32(2): 74-79]. The following abstract was published online at

“Branch diameter relative to the trunk diameter (aspect ratio) affected the extent of discolored and decayed wood in the trunk of seedling-propagated red maple (Acer rubrum L.) after branch removal. More discoloration resulted from removing codominant stems than removing branches that were small compared to the trunk. Removing limbs that originated from lateral buds resulted in the same amount of discoloration and decay as removing suppressed limbs that were once the leader. This result provides indirect evidence that a small codominant stem suppressed by pruning techniques designed to slow its growth rate can result in a branch protection zone at the union. There was no relationship between presence of a bark inclusion and decay four years after making pruning cuts.”

Gilman and G. W. Knox addressed “crape murder” and published findings in the Journal of Arboriculture [30: 48-53] in 2004. Their four-year study indicates that allowing one sprout to grow larger in diameter than all others could improve structure on topped trees. Read more online at