The Hardiness of Trees


There is an internationally recognised rating for tree hardiness based on the USDA (United States Department of Agriculture) Plant Hardiness Zone system. This rates trees survival as follows:

 

Zone 1

 Below -45 °C

Zone 2

 - 45 °C to - 40 °C

Zone 3

 - 40 °C to - 35 °C

Zone 4

 - 35 °C to - 29 °C

Zone 5

 - 29 °C to - 23 °C

Zone 6

 - 23 °C to - 17 °C

Zone 7

 - 17 °C to - 12 °C

Zone 8

 - 12 °C to - 6 °C

Zone 9

 - 6 °C to - 1 °C

Zone 10

 - 1 °C to 4 °C

Zone 11

 Above 4 °C

 

The UK climate is so dominated by the changeable mild Gulf Stream that winter injury is not a subject for continual debate as it would be on a continental land mass and the table above is rarely used as a guide to hardiness. Zones 7 - 11 are those that affect us in normal winters and possibly Zones 5 and 6 in extreme winters.

 

Wind (exposure): easterly (dry) and westerly (wet) are as much and possibly more important a consideration as low temperatures when deciding what will survive in the UK. We also do not have predictable 'hardening off' periods as we enter winter and damage to trees and other plant life can often occur on soft un-ripened wood that would survive quite happily if affected later in the winter.

 

Hardiness is only a concern for very few of the trees we grow; look out for the 'frost' symbol on the labels which means that a hard frost can sometimes cause dieback in shoots on some varieties. Most of these trees can be pruned back into healthy wood and grow away perfectly happily the next season.