Abstract
Nine Callery pear (Pyrus calleryana) cultivars were planted under utility wires in 11 communities for evaluation as street trees. In most communities, 2 cultivars were alternated within each of several plots. Cooperators in the Municipal Tree Restoration Program used standardized methods to measure them annually at the end of the growing period for 3 years, and periodically afterwards until the ninth year in some cases. Significant differences were found in growth rate, height, crown width, and trunk diameter, but growth patterns and dimensions of most cultivars that were evaluated for 9 years were similar, with some notable exceptions. The height growth of ‘Autumn Blaze’ seems to have stalled at 5.6 m (18.5 ft) 9 years after transplanting, whereas most other cultivars exceeded 7.2 m (23.7 ft) and were still growing about 0.4 m (1.3 ft) per year. ‘Cleveland Select’ and ‘Whitehouse’ had narrower crowns and smaller trunks than others. All cultivars were in very good health except ‘Whitehouse’, whose foliage and twigs were injured somewhat in most years by unidentified insects and disease, possibly anthracnose or fireblight.
- Callery pear
- Pyrus calleryana
- Aristocrat-crat™
- ‘Autumn Blaze’
- ‘Bradford’
- ‘Capital’
- Cleveland Pride®
- ‘Cleveland Select’
- ‘Redspire’
- ‘Whitehouse’
- Valiant®
- street trees
- performance testing
Callery pear (Pyrus calleryana) cultivars are being evaluated as part of the Municipal Tree Restoration Program. MTRP encourages municipalities to improve their tree programs and provides information to help decision makers select appropriate cultivars for planting under utility wires. Free trees purchased with utility funds serve as an incentive for communities to participate. Initial results of Callery pear performance tests were reported previously (Gerhold and McElroy 1994).
Earlier research comparing landscape trees (Reisch et al. 1971; Ticknor 1971; Mower 1973; and Kozel 1974) led to the proposal of a cooperative performance testing system for street tree cultivars (Gerhold and Bartoe 1976; Gerhold 1985). The statistical design was based on measurements of 23 cultivars supplied by municipal arborists in Iowa, Michigan, Ohio, Pennsylvania, Vermont, and Washington (Bartoe 1977).
Methods
Eleven communities in Pennsylvania planted the trees represented in this report. Community representatives chose the planting sites with assistance by utility foresters, service foresters, and Extension urban foresters; 2 of these usually assisted each community
Each test planting consisted of 2 cultivars, except in Tioga where there were 3. A typical test consisted of 2 cultivars planted alternately within 4 to 10 plots that could contain 4 to 16 trees each—a total of 50 trees. All test trees were planted along streets and under electric conductors; the utility company arranged for removal of large trees that interfered with utility lines. Both cultivars for a community were ordered B&B from the same nursery, with a caliper of 4.4 or 5.1 cm (1.75 or 2 in.). Initial heights ranged from 2.8 to 5 m (9.3 to 16.5 ft); this wide range could be explained only partly by cultivar differences, so presumably growing conditions at the nurseries had a strong influence. ‘Redspire’ typically was 0.2 to 0.6 m (0.6 to 2 ft) taller than its companion cultivar; ‘Cleveland Select’ tended to be shorter, but not in all cases.
The cultivar tests were planted between 1988 and 1993. A trained cooperator inspected and measured the trees annually during the first 3 years, and then at 3-year intervals. During September or October, a service forester or Extension urban forester used standardized methods to measure tree height, trunk diameter, and crown width, and to classify foliage health, branch health, trunk health, maintenance needs, and an overall quality rating (Table 1*). Causes of damage such as disease, insects, drought, and mechanical injuries also were recorded.
Size, health, and overall ratings of Callery pear cultivars, derived from data collected until the sixth year to the ninth year after planting. Average trunk diameter, tree height, and crown width are in the most advanced year; foliage health, branch health, and overall ratings are averaged over all years.
An analysis of variance (MINITAB General Linear Model) was conducted on each type of quantitative data from the 2 (or 3) cultivars in a test planting to calculate means and determine significance of differences. Each test location in every year was treated as a separate experiment with plots providing replication. These results, along with written comments of cooperators, were used to characterize performance of the cultivars.
Results
The main differences found among the cultivars 6 to 9 years after transplanting were in trunk diameters, heights, and width of crowns (Table 1). Aristocrat™ was largest in diameter at breast height (dbh), and ‘Cleveland Select’ and ‘Whitehouse’ were smallest but sturdy enough. Heights differed among locations, but growth rates were quite similar especially from years 3 to 9 (Figure 1). The most notable exception was ‘Autumn Blaze’, whose height was 5.5 m (18 ft) in the sixth year and 5.6 m (18.5 ft) in the ninth year at the only location where it was evaluated. In comparison, heights of most other cultivars varied from 7.2 m (23.7 ft) to 8.5 m (27.7 ft) in the ninth year, and their growth rates of about 0.4 m (1.3 ft ) per year showed no sign of slowing down. Most crown widths in the ninth year ranged from 4.9 m (16.2 ft) to 5.9 m (19.3 ft), except those of ‘Cleveland Select’ (3.8 to 4.6 m, 12.4 to 15.0 ft) and ‘Whitehouse’ (2.9 to 4.7 m, 9.6 ft to 15.5 ft).
Average height growth of Aristocrat™ (Ar), ‘Cleveland Select’ (CS), and ‘Redspire’ (Rd) Callery pears.
All of the cultivars except ‘Whitehouse’ were very healthy throughout the evaluations, as indicated by foliage and branch ratings above 4.0. Insects, diseases, and drought injured the leaves of ‘Whitehouse’ according to observations; anthracnose or fireblight were suspected as causes but not verified. These injuries explained the 2 lower overall ratings of ‘Whitehouse’ compared to ‘Bradford’ and ‘Cleveland Select’. ‘Cleveland Select’ was the only other cultivar whose overall ratings were more than 1.0 unit lower than comparison cultivars ‘Capital’ and Valiant® at 2 locations. The service forester who evaluated the trees at both of these locations regarded the branching habit and leaf color of ‘Cleveland Select’ to be somewhat inferior. At 5 other locations, ‘Cleveland Select’ was given high overall ratings similar to other cultivars.
Conclusions
Callery pear cultivars that grew well and remained healthy at several locations through the ninth year after transplanting included Aristocrat, ‘Cleveland Select’, and ‘Redspire’. Others that can be recommended based on less extensive evaluations are ‘Autumn Blaze’, ‘Capital’, Cleveland Pride®, and Valiant. ‘Bradford’ also performed well at 1 location, where it has not yet suffered limb breakage that has occurred frequently elsewhere.
Several cultivars have narrow crowns or lower mature heights that commend them for restricted spaces, for example under utility wires or near buildings. ‘Autumn Blaze’ apparently will not grow as tall as the others. ‘Cleveland Select’ and ‘Whitehouse’ have narrow crowns, but the latter has suffered foliage injuries and some twig dieback.
Acknowledgments.
Financial support for the Municipal Tree Restoration Program was provided by utility companies through the Pennsylvania Electric Energy Research Council and by donations of arboricultural firms: ACRT, Allegheny Power Systems, Asplundh Tree Expert Co., Baltimore Gas & Electric Co., Bartlett Tree Expert Co., Davey Tree Expert Co., Duquesne Light, Environmental Consultants Inc., GPU Energy, Hazlett Tree Service, Penn Power, Pennsylvania Power & Light, and UGI Corporation. Service Foresters of the Pennsylvania Bureau of Forestry and the Maryland Department of Natural Resources, and Extension Urban Foresters of Penn State University assisted with community liaison and tree measurements.
Footnotes
↵* Tables and figure for this article begin on page 58.
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