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.
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 programs 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 communities. All test trees were planted along streets and under electric conductors. The utility company arranged for the removal 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 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, ‘Summer 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 standardized 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 calculated means and determined statistical differences between the cultivars at a particular location at the 5% level of probability. Each test location in every year constituted a separate experiment with plots providing replication within a community. These results, along with comments provided by cooperators, 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, sixth, or ninth year. ‘Ivory Silk’ is present in all 13 tests, ‘Regent’ is in six, and ‘Summer Snow’ in seven (Table 1). Survival ranged from 76% to 100% and averaged 90% or better in nine of the tests. In the 12th year, there were just a few significant differences in trunk diameter, height, and crown width and even fewer over the years in foliage health or branch health. Several community residents commented favorably about the trees, particularly about their creamy white flowers.
Size and health ratings of tree lilac cultivars, planted as street trees, two cultivars in each communityz.
The rate of growth in height was similar for all three cultivars, except that ‘Summer Snow’ grew more slowly until about the sixth year and then grew at about the same rate as the other cultivars (Figure 1). Average heights of the cultivars in the 12th year ranged from 4.8 to 5.1 m (15.8 to 16.8 ft). ‘Ivory Silk’ attained an average height in one community of 6.4 m (21.1 ft) in the 12th year. If the growth rate from year 9 to 12 continues, by the 18th year or later, the trees would attain a height of ≈7.6 m (25.1 ft) and could reach utility wires.
Average heights (m) of cultivars 3 to 12 years after planting.
The crowns of ‘Summer Snow’ on average were slightly wider than those of ‘Ivory Silk’ and ‘Regent’ was narrower (Figure 2) by ≈0.4 m (1.3 ft). ‘Regent’s divergence from the other two in crown width has been increasing since the sixth year.
Average widths (m) of cultivars 3 to 12 years after planting.
No serious disease or insect problems were reported. Dieback of branch tips was observed in several localities, and several observers attributed this to drought and other adverse site conditions. Trees growing in more spacious sites with better soils and soil moisture exhibited much better growth and healthier foliage. Leaves that were normal in size turned from dark or medium green to bright yellow earlier in the autumn than most other species, and this may have been a partial cause of lower foliage ratings.
CONCLUSIONS
‘Ivory Silk’, ‘Regent’, and ‘Summer Snow’ performed well as street trees, especially where site conditions were less stressful. Their moderate growth rate commends them where space is limited, and they are compatible with utility wires for 18 years or more. Minor health problems included dieback of branch tips and yellowing of foliage, especially after droughts.
Acknowledgments.
Financial support for the Municipal Tree Restoration Program was provided by Allegheny Energy, Asplundh Tree Expert Co., Baltimore Gas & Electric, Bartlett Tree Expert Co., Duquesne Light, Hazlett Tree Service, PECO Energy, FirstEnergy Corp., PPL Electric Utilities, and UGI Utilities. Service foresters of the Pennsylvania Bureau of Forestry and extension urban foresters of Penn State University assisted with community liaison and tree measurements.
- © 2007, International Society of Arboriculture. All rights reserved.
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