182 Gerhold: Tree Lilac Cultivars As Street Trees Arboriculture & Urban Forestry 2007. 33(3):182–184. Tree Lilac (Syringa reticulata) Cultivars Tested As Street Trees: Second Report Henry D. Gerhold Abstract. Through the Municipal Tree Restoration Program, cooperators planted three tree lilac (Syringa reticulata) cultivars in 13 Pennsylvania and New York, U.S. communities for evaluation as street trees, comparing two in each community. Standardized measurements in years 1, 2, 3, 6, 9, and 12 revealed that ‘Ivory Silk’, ‘Regent’, and ‘Summer Snow’ grew well and remained healthy at all locations. Average heights of the cultivars in the 12th year ranged from 4.8 to 5.1 m (15.8 to 16.8 ft). Differences in average height and crown width were small. In one community, the tallest cultivar, ‘Ivory Silk’, attained an average height of 6.4 m (21.1 ft) in the 12th year. ‘Summer Snow’ grew more slowly in height initially, and ‘Regent’ had narrower crowns. Foliage health ratings were somewhat lower than those of other species in some years, probably reflecting earlier yellowing of leaves in the autumn. If the tree lilac cultivars continue to grow at the same rate, they would not grow into overhead utility wires until the 18th year or later. Key Words. ‘Ivory Silk’; performance testing; ‘Regent’; street trees; ‘Summer Snow’; Syringa reticulate; tree lilac. In 1987, the Municipal Tree Restoration Program (MTRP) began evaluating tree lilac (Syringa reticulata) cultivars and other species as street trees, mainly in Pennsylvania, U.S. MTRP encourages municipalities to improve their tree pro- grams by demonstration plantings of free trees, and through research provides helpful information for selecting species and cultivars that are compatible with overhead utility wires. A previous report (Gerhold 1999) of initial results from the tree lilac tests confirmed that they grew slowly during the first 3 years and then somewhat faster but still at a moderate rate. MATERIALS AND METHODS Plants and Experimental Design Cooperators planted the three tree lilac cultivars between 1987 and 2000 in 12 Pennsylvania communities and one in New York in United States Department of Agriculture plant hardiness zones 5 and 6. Representatives of the communities chose the planting sites with the advice of utility foresters, service foresters in the Bureau of Forestry, or Penn State extension urban foresters. Each test planting consisted of two cultivars typically planted alternately within four to 10 plots that contained 4 to 16 trees each, for a total of 50 trees in most of the commu- nities. All test trees were planted along streets and under electric conductors. The utility company arranged for the re- moval of large trees in the plots that interfered with utility lines, thus making space for planting in some of the plots. Plant Materials The test trees came from several nurseries as balled-and- burlapped stock, and the same nursery always supplied both ©2007 International Society of Arboriculture cultivars for a community. Initial trunk calipers at 15 cm (6 in) above grade ranged from 4.4 to 5.1 cm (1.76 to 2 in) and tree heights from 2.26 to 3.45 m (7.5 to 11.4 ft). The wide range in heights presumably could be explained by genetic differences among cultivars and diverse cultural differences among nurseries that supplied the trees. In most cases, ‘Sum- mer Snow’ originally was shorter than its companion cultivar. Measurements and Analysis A trained cooperator inspected and measured the trees soon after planting, then annually during the first 3 years, and at 3-year intervals afterward. During September or October, a service forester or extension urban forester, using standard- ized procedures, measured tree height, trunk diameter, and crown width; classified the health of foliage, branches, and trunk separately; and rated maintenance needs and overall tree quality. Causes of injuries such as diseases, insects, drought, or mechanical damage were identified and recorded when possible. An analysis of variance (using MINITAB’s General Linear Model) of quantitative data collected in a given year calcu- lated means and determined statistical differences between the cultivars at a particular location at the 5% level of prob- ability. Each test location in every year constituted a separate experiment with plots providing replication within a commu- nity. These results, along with comments provided by coop- erators, were used to characterize performance of the cultivars. RESULTS AND DISCUSSION Eight of the tree lilac performance tests have been measured in the 12th or 13th year, and the others have attained the third,
May 2007
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