The degree of frost hardiness achievable varies greatly with species and seed provenance. Such hardiness levels are fairly well understood for the commoner species. However, unseasonal sudden frosts in autumn or spring can cause considerable damage to plants that are not fully hardened, and may kill young plants. Eucalypts growing in locations that are normally very mild in winter are also at risk of damage by sudden frosts.
There is one well-documented instance in Scotland of deeply-frozen soil killing young eucalypts. The symptoms suggested that the effect was essentially due to severe drought stress, because the roots were unable to take up water. Damage to some young eucalyptus plantings made since 2005 in the UK and Ireland, particularly on lighter soils and in conditions in which the soil is unusually cold but not frozen, appears to have had a similar cause. The symptoms are often manifested as sudden dessication of shoots several weeks after the coldest weather.
The exceptionally long and cold winter of 2010-2011 caused much damage to eucalypts normally regarded as hardy, notably in the English midlands, parts of Yorkshire, southern and central Scotland and some normally mild parts of Ireland. There were also many examples of survival, including stands and many specimens of some of the less hardy species. In addition, trees of many species whose tops were killed or apparently killed have subsequently coppiced vigorously. Overall, the effects of that winter are a reminder of the greater risks of planting eucalypts in colder, inland areas, and that the more vigorous species tend to be less tolerant of severe cold. Whatever one’s attitude to risk, achieving a balance of benefits and risk with SRF using eucalypts is not straightforward. This situation is similar to that in other countries. A notable example is in south-west France, where severe winter weather in January 1985 nearly stopped what has since become a successful eucalyptus SRF programme.
At least two of the evergreen southern beeches with potential for SRF, Nothofagus dombeyi and N. betuloides, appear to have very good cold tolerance. Young trees in a trial south-east Ireland were largely defoliated by the extended severe winter of 2010-2011 (the local minimum temperature was -16°C), but the stems were largely undamaged and produced new foliage the following spring. Essentially, the response of the trees was to become deciduous in the exceptional cold conditions. In the same trial, one of the hardier eucalypts, E. glaucescens, suffered total shoot die-back some weeks after the cold period, but subsequently coppiced.