Arboriculture & Urban Forestry 43(6): November 2017 midal, or columnar growth habits. Arborvitae study trees consisted of a species and cultivars (15) of American arborvitae and species and one cultivar of western arborvitae or western red cedar. Depending on availability, three to four single-tree replicates per species/cultivar were evaluated, with the exception of ‘Hetz Midget’, where five trees were used due to their small size (<1 m in height). A total of 67 arborvitae trees were included in the study. Table 1 contains a complete list of species/cultivars evaluated. Larval feeding damage was evaluated by spe- cies/cultivar, location in the canopy zone (lower, middle, upper), tree height (short or tall), tree growth habit (globose, columnar, pyramidal), foliage density (sparse to very dense), and cardi- nal direction (N, S, E, W). One branch sample, 20 cm long, was randomly selected from each of the four cardinal directions of each study tree at 1 m, 4 m, and >4 m above ground level. For trees shorter than 4 m, samples were taken at 30 cm, 1 m, and approximately 3 m. A total of 645 branch samples were taken from all study trees in the collection from 07 August 2000 through 06 February 2001. An additional 719 samples were taken in sum- mer of 2001. For trees above 4 m, a bucket truck was used to collect samples from portions of the tree canopy too high to reach with pole pruners. Branch samples collected in the field were brought back to the lab for inspection, where Tree S T. occidentalis ‘Hetz Midget’ T. occidentalis ‘Hetz Wintergreen’ T. occidentalis ‘Holmstrup’ T. occidentalis ‘Hoopesii’ T. occidentalis ‘Smaragd’ T. occidentalis ‘Spiralis’ T. occidentalis ‘Techny’ T. occidentalis ‘Umbraculifera’ T. occidentalis ‘Wagneri’ T. occidentalis ‘Wareana’ T. occidentalis ‘Waxen’ T. plicata T. plicata ‘Fastigiata’ z Tree height: S = short; T = tall. y Canopy density: SP = sparse canopy; MED = medium canopy; DEN = dense canopy; VDEN = very dense and compact canopy. x Growth habit: GLOB = globose; PYR = pyramidal; COL = columnar. T S S S T S S T T S S S S T T T 207 the total number of infested and non-infested branch tips was counted and recorded. The percent infested branch tips (IBTs) was cal- culated by dividing the number of infested tips by the total number of tips per branch. Percent IBTs was used as a proxy for compar- ing relative susceptibility among cultivars. Concurrent with branch sampling, each study tree was visually categorized for shape (globose, columnar, or pyramidal), height (short ≤4 m, tall ≥4 m), and canopy density (very dense, dense, medium, or sparse). Canopy density was deter- mined visually by observing the amount of light passing through the sprays and by measuring the depth of branching from the perimeter of the canopy (tip of sprays) into the interior of the plant. Differences in canopy density were very obvious to the naked eye, particularly for the very dense cultivars. In an attempt to estimate canopy density for both very dense and sparse cultivars, a few informal light penetration measurements were taken using a portable light meter (Extech® wide-range light meter-EA30, Extech Instru- ments, Waltham, Massachusetts, U.S.). The light meter was handheld inside the plant interior and amount of light penetrating the tree can- opy was measured (data not shown). A standard ruler was used to measure (nearest cm) the lin- ear distance from the spray tip to where foliage ended within the tree interior (data not shown). Table 1. Arborvitae (Thuja spp.) species/cultivars evaluated by tree height, canopy density, and growth habit. Species/Cultivar T. occidentalis T. occidentalis ‘Aurea’ T. occidentalis ‘Douglasii Aurea’ T. occidentalis ‘Globosa’ T. occidentalis ‘Gracilis’ heightz T Canopy densityy SP SP SP DEN DEN VDEN MED VDEN DEN DEN DEN DEN DEN DEN DEN DEN DEN DEN Growth habitx PYR PYR PYR GLOB GLOB GLOB PYR COL GLOB COL PYR PYR GLOB GLOB PYR PYR PYR PYR ©2017 International Society of Arboriculture
November 2017
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