Callery Pear Cultivars Tested as Street Trees: Second Report

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.

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).

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Figure 1.

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.