Tree Lilac Cultivars Tested as Street Trees: Initial Results

Methods

Ten communities in Pennsylvania and 2 in New York planted the tree lilacs represented in this report. Community representatives chose the planting sites with assistance from utility foresters, service foresters, and Extension urban foresters; 2 of these usually assisted each community. A typical cultivar performance 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 calipers of 4.4 to 5.1 cm (1.75 to 2 in.); heights ranged from 2.4 to 3.7 m (8 to 12 ft).

The cultivar tests were planted between 1987 and 1996. A trained cooperator inspected and measured the trees annually during the first 3 years, and then at 3-year intervals in most cases. 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 overall quality. 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 cultivars in a community 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

At most locations, the tree lilacs grew slowly for the first 3 or more years, and then the rate of height and diameter growth increased. The pattern of growth at Corry was typical of most locations (Figure 1); only at Lebanon did more rapid growth start in the second year after planting. Survival rates 6 or 9 years after planting were 88% to 100% at all locations, except one where trees were vandalized.

Figure 1.

Height growth of ‘Ivory Silk’ (IS) and ‘Summer Snow’ (SS) tree lilacs at Lebanon (Test 18) and Corry (Test 20).

Significant differences between cultivars in trunk diameter, tree height, and crown width were found at several locations (Table 1). In some cases (Corry and Camp Hill), these differences already were present at the time of planting, indicating they were not matched in size when they left the nursery. In the remaining comparisons the size differences were modest, except at Westfield where ‘Ivory Silk’ was 1.5 m (5 ft) taller than ‘Summer Snow’ 9 years after planting.

The foliage of all 3 cultivars tended to turn yellow in late summer before other species showed fall colors. The yellowing occurred especially on more stressful sites, whereas individual trees that had been well watered or growing in spacious areas remained green longer. Some cooperators regarded yellow leaves to be a sign of poor health. For this reason, foliage health ratings below 4.0 (more than 20% of foliage affected) were common, but there were few significant differences between cultivars and no severe disease or insect injuries.

Differences among cultivars in branch health, trunk health, and maintenance needs (not shown) were uncommon and transient. They were caused mainly by mechanical damage. Dieback of terminal branches commonly occurred during the first few years as trees were becoming established.

Overall ratings also were variable, inconsistent, and generally differed by less than 1.0. However at Athens, ‘Summer Snow’ was consistently rated above ‘Ivory Silk’. The service forester who measured these trees reported that he liked the yellow foliage of ‘Summer Snow’, which turned color earlier, and its somewhat larger, more robust crowns.

Conclusions

All 3 cultivars performed well, including ‘Summer Snow’, which is reputed to be slower growing but was not at all locations. The initial slow growth and even some dieback of these cultivars was the only real disadvantage, compared to other species used as street trees. The tendency of foliage to turn yellow before the onset of fall coloration by other species is perceived as a sickly appearance by some, but alternatively may be regarded as an attraction in the landscape.