Heat Damage
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Heat damage is less obvious than cold damage. It typically shows up as a plant that simply does not thrive, and instead slowly declines and dies over a period of years. In general, plants begin sustaining damage at 86 degrees F., with the number of days above this threshold having a cumulative effect. Extended periods of such high temperatures appear to be more harmful than the maximum day-time temperatures, as do high night-time temperatures.
The American Horticultural Society developed a heat zone map similar to the USDA cold hardiness map. It maps 12 heat zones to show the cumulative number of days above 86 degrees F. While heat hardiness using these heat zones is beginning to be published for specific plants, just as for cold hardiness zones, the heat zone information is not yet as readily available. Many deciduous hybrid azaleas appear to be more sensitive to high temperatures than evergreen hybrids. Hybridizers are working to improve the heat tolerance of deciduous hybrids by crossing them with the very heat-tolerant native R. austrinum. |