Understanding Heat Zone Maps
And How To Use Them

The heat zone map tracks the number of days in a year that the temperature rises above 86F (30C). They are not used as much as hardiness zones, but they are important in certain circumstances.

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A huge problem with plant hardiness zone maps (which only address cold) are that heat can be just as big a stress on a plant as cold, especially in the higher numbered hardiness zones.

Another issue is that some plants (citrus is a good example) require a certain number of heat days to bear fruit or become strong enough to survive the winter.

In areas such as the British Isles which don't get many heat days, those plants will not do well, even though they might seem to be growing in their proper hardiness zone.

On the other hand, in areas with too many heat days (and not enough cold days -- measured in chill hours), other fruits won't bear well (the raspberry is a good example)

Heat maps have been developed to address these problems.

The American Horticultural Society has a heat map of the lower 48 US states developed in the 1990's that you can download. However, with climate change, I wonder about the accuracy of each zone.

Here's a heat map for Australia (pdf file). Again, this was made in the 1990's.

I stumbled across some interactive heat maps for the US by state done by plantmaps.com -- the one I linked to is for Oklahoma.

The maps do not say when the data was collected, and I don't see any way to find a national map on this site, but you should be able to find your state's map by clicking the links to your right.

Until our global climate stabilizes, heat maps should be used as clues to how a plant might do in your area rather than relying on them. For more recent and detailed information, contact your local county extension office or nearest agricultural college.

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Protecting plants from frost - Outsmarting the planting zone map

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