Just as the term “polar vortex” was beginning to trip off the tongue, Evanstonians learned another term from last week’s extreme weather: radial shakes. That is the name for the cracks that appeared in about 130 parkway sycamore trees and London plane trees, a close relative of the sycamore.
A resident in the 2200 block of Dodge Avenue who alerted the City to a large split in one of the parkway sycamore trees set off a City-wide inspection of the 303 sycamore trees and London plane trees – in the public areas of Evanston.
City Arborist Mark Younger found that several trees on that block and others in other parts of the City were split, said Paul D’Agostino, assistant director of Public Works, in a Jan. 10 memo to City Council. Some of the splits were shallow – affecting only about three inches of the trunk – but others were quite deep, extending all the way to the center of the tree. “Forestry staff discussed the problem, but no one had ever seen this type of extensive splitting before,” said Mr. D’Agostino.
Staff at Morton Arboretum, “suggested that the recent cold, combined with bright sunshine may be the culprit, leading to frost cracks, also known as radial shakes.” They referred Mr. D’Agostino to literature on the subject, including the book “Arboriculture” by Richard Harris.
The book said that both sycamores and London plane trees are “commonly affected by these cracks” and that they “usually occur in the bark and wood parallel to the grain and extend to the center of the trunk.”
Mr. D’Agostino said he is confident that the cracks are radial shakes, also called “frost cracks.”
While cold alone does not create a frost crack, it can aggravate a wound or “seam” formed by the rapid growth of London plane and sycamore trees. Typically these seams heal over each year’s growth, said Mr. D’Agostino, resulting in a ridge along the seam. The extreme cold that set in on Jan. 5 “caused areas within the trunks and some of the major limbs that already had an internal defect (in most cases, these growth seams) to split all the way out through the bark and appear as cracks,” he said.
Last week’s inspection turned up three categories of trees, said Mr. D’Agostino: those with full depth/multiple radial shakes (60 trees); those with minor/shallow radial shakes (76 trees); and those that have no visible radial shakes (167 trees). Nearly all of the major splits are exactly where the growth seams were already present, and in many cases they have extended far beyond the original seam defect.
Cracks in some of the trees have already resealed themselves, said Mr. D’Agostino. The 60 trees with “full depth multiple radial shakes cannot be saved, because of the structural damage and because the openings would continually expose the tree to decay “which cannot be stopped.”
City crews will continue to monitor and to periodically re-inspect the 76 trees with minor damage “especially if we get more sub-zero temperatures,” said Mr. D’Agostino. He also said that, although City crews did not have time to inspect all the parkway trees, which number in the thousands, crews had noticed some damage to some lindens and at least one oak tree. These will be monitored as well.