TDA 130 A Slippery Slope
Tree Damage Alert No 130
3rd Feb 2009
A Slippery Slope.
“Britain has suffered the coldest start to winter for more than three decades. And we are all set to shiver for some time yet.” “Research by the Met Office reveals the average temperature for the start of December has been 1.7ºC compared with 4.7ºC for the same time over the last 32 years.” Both of these quotations from the Daily Express for December 13th 2008 are supported, if unscientifically, by the last time we considered it appropriate to warn about a potential for winter damage being in 1998 (TDA 52).
That low temperatures have been locally significant for plants has been confirmed by several reports from the West Country that Acacias, probably Acacia dealbata, have been severely damaged this winter. We have not received reports of such casualties for many years.
Warnings about the winter being cold should not be dismissed too easily. Although most species will survive the temperatures to date, low temperatures signify possible icy ground conditions and a potential for ‘delayed’ damage which may not become evident until spring when trees leaf only partially, or may not leaf at all.
In the 1980s icy roads were treated with salt at what emerged as poorly, or even uncalibrated rates causing considerable damage to mature urban roadside trees. This, we understand, has been rectified with lorries gritting roads now reported to deliver appropriate doses of salt. So should there be concern about a risk of salt damage to town trees this year?
Road salting was found to be only one contributory factor. Spreading salt on footpaths and in pedestrian areas by the shovelful, particularly when this was associated with snow clearance was found to be damaging. The amounts of salt spread by uncalibrated ‘shovel-and-throw’, in some instances over the same area by more than one Department, supplemented by applications made by the occupants of adjacent properties can be many times greater than the recommended dose. When salt laden snow thaws and the salt laden water enters the soil there can be seriously damaging loads of salt deposited in the rooting zones of trees.
A single application of salt should pose negligible threat to trees. The problem arises when a cold spell is prolonged, or recurring, and multiple applications of salt are made. On some occasions applications are made in anticipation that temperatures may fall. Such ‘pre-emptive’ applications may add salt unnecessarily compounding the risk of damage. However, rainfall following a thaw may wash salt off road and pavement surfaces directly into drains rather than allowing slow infiltration into the ground, and so reduce the threat of toxic levels building up in the soil.
Fluctuating temperatures and generally low rainfall combined with the observation “Winter isn’t done with us for the time being” (TV Weatherman) suggests there is a high risk of damage to trees this winter from repeated salt applications and now snow clearance. If the risk of damage is to be reduced it is important that salt applications are calibrated and not duplicated unnecessarily. Also cleared snow should not be stacked around the bases of trees.
Derek Patch, and Ben Holding, AAIS, Farnham, Surrey
References
Tree Damage Alert 52 Will there be De-Icing Salt Damage this Winter?
Dobson, M.C., (1991) De-icing Salt Damage to Trees and Shrubs. Forestry Commission Bulletin 101. HMSO London
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