Handwarmers placed inside conventional insulating tree wraps increased trunk temperatures and improved tree survival under freeze conditions. Handwarmers generate heat by oxidation of Fe powder. In freeze-chamber tests with air temperature as low as –7.1C for 4 hours, wraps plus handwarmers kept trunk temperatures above freezing. Handwarmers increased minimum temperatures by 7C during a one-night freeze. Benefit of the handwarmer decreased the second night of a simulated two-night freeze but still increased minimum temperature by 1.3C. Tree survival was significantly improved by handwarmers in the freeze-chamber tests. In a field test during a mild freeze, handwarmers increased the minimum temperature by 3.5C the first night but provided no benefit the second night.
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