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Your USDA Zone and Why It’s Important

Posted on 11 October 2014   fruit trees

Your USDA Zone and Why It’s Important

hardiness zones

For every fruit tree and plant on our website, we list the USDA zones where the tree or bush grows best. So what on earth is a USDA zone anyway?

 

The USDA zones are the divisions of the country that the United States Department of Agriculture (USDA) has defined based on specific climatic conditions that impact plant growth. The main climatic factor evaluated by the USDA in developing the zones was the minimum temperature reached in that particular area. The USDA has each zone divided in two – with both an a and b sub-zone for each numbered zone. For example, we live in USDA zone 8. But we are considered to be on the southern half of zone 8, so we are more specifically in zone 8b.

 

Most nurseries list the best zones for growing the specific plants they sell. If a plant is listed as hardy to zone 5, that means it can tolerate a temperature down to -10º F. Other plants, including some that we sell, are not cold hardy, so you may see a recommendation to not grow a plant “above USDA zone 7.” Each of these recommendations is based on an average minimum temperature below which the plant will be harmed.

 

One problem with the USDA zones is that they don’t take into account summer heat, an issue that is especially important in the South. Many nurseries list fruit tree varieties that grow to USDA zone 8, which includes much of the South. The problem with these recommendations is that many of these trees come from Michigan or some other place that doesn’t have the high temperatures those of us in the South deal with in the summer. While these trees may be hardy here based on the average minimum temperature in winter, can they face days of 95º F heat with 90% humidity? Probably not!

 

The American Horticultural Society (AHS) recently released a heat zone map that is excellent and provides details of how hot it gets across the United States. In conjunction with the USDA hardiness zone map, most gardeners can make a smart choice about what type of fruit tree or other plant to grow in their area.

 HeatZoneMap1-548x344

Compare the USDA map with the AHS heat map (sorry the resolution isn’t better on that image – click the map for a better image). On the heat map, the colors are more or less consistent across the Southeast. This is a pretty accurate description of the climate in the area – and one the helps us know the temperatures that our plants need to be able to thrive in.

 

A garden book publisher called Sunset also has its own hardiness zone map, with over 20 distinct zones described and mapped. They publish books about what plants grow in each zone.

 

The USDA hardiness zones are helpful and are the most widely used tool to describe where a plant can grow, but they are not the only tool available.

 

Have questions about any of this? Email us, or let us know in the comments. Did we forget something useful in this blog post that you’d like to add? Leave a comment and let us know. Thanks for reading!

3 Comments

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