How cold-hardy are macadamia trees?

Whether or not you grow a macadamia tree in cooler climates comes down to frost limitation. Macadamia trees can survive short-term temperatures of 25°F (-4°C), but flowers and nuts will die if temperatures drop to 28°F (-2°C). If a macadamia tree freezes for several days, the wood can die back to the ground level resulting in multiple years without nuts. Site selection in USDA zones 9-11 is critical if you want to survive.

Flowers abort at 32°F (0°C), making spring frosts disastrous. One Central California grower lost 80% of the blooms during a recent May cold snap and now uses smudge pots on critical nights that raise the temperature to 5°F. In combination with wind machines, she can raise the temperature to 8°F.

Rootstocks can greatly affect cold hardiness in plant species. From trials at the University of Hawaii, Beaumont, when grafted onto roots of M. tetraphylla, survived down to a temperature of 23°F. In comparison, a pure M. integrifolia died at 27°F. Based on this, it is best to ask or verify the rootstock lineage if the nursery trees are to be planted in areas of marginal temperatures.

Microclimates protect orchards. Plant 15 feet from south-facing walls that absorb and radiate daytime heat. A Texas farm grows macadamia nuts in zone 8b with the help of stone walls and the use of reflective ground covers. The temp at night stays 6°F warmer than an adjacent open field.

Maintaining post-frost care minimizes disease in your trees. You should wait until new growth begins to prune off the dead wood. You can spray copper fungicide on cuts to help prevent cankers. A farmer from Florida saved 30 trees in this manner, even though 50% of all branches were black following a historic freeze of 24°F.

Read the full article: How to Grow Macadamia Trees: A Complete Care Guide