Home > Great Big Greenhouse Blog > April 2016 > IN THE DIRT with DOUG - UNDERSTANDING THE PLANT HARDINESS ZONES

IN THE DIRT with DOUG - UNDERSTANDING THE PLANT HARDINESS ZONES

This blog is being prompted by the many questions that I get from customers who want to know if a particular plant is hardy for our area.  To understand the reason behind the hardiness questions, I think it is important to understand the history behind our two plant zone maps.
HISTORY:  The initial hardiness zone maps for the United States were developed during the 1920’s and 30’s.  The first USDA map was published in 1960.  In 1990 an overhaul of the map was completed.  The primary reason for this overhaul was because some plants were dying in what was first thought to be hardy plants for certain zones.  In 2002 the plant hardiness zone map was reviewed by the American Horticultural Society.  This project revealed that many of the hardiness zones had moved northward reflecting a general warming trend.  The USDA revised our current plant zone map in 2012, which showed again a general warming trend across the country.
                Here, in the Richmond area, we are considered Zone 7.  More specifically, we are considered Zone 7a since the AHS determined that the various zones needed to be subdivided to reflect a more accurate hardiness plant zone.  Zone 7a represents the area that plants can survive as cold as zero degrees F.  The state of Virginia is somewhat unique in that we have four different zones, ranging from 5a in the western mountains to 8a on the Eastern Shore.  Also, we have a small area near Petersburg that is considered warmer and a Zone 8a.  And, we have a small area in Goochland County that is considered colder and a Zone 6b.
                Most of this information is presented on the plant id label.
                Coldness is not the only factor determining whether our plants will survive and thrive in our region.
                How many of you are aware that we now have a heat hardiness zone map to determine if certain plants can tolerate our heat of the summer?  Richmond and surrounding area is in Heat Zone 6.  Between 1974 and 1995 the National Weather Service compiled statistics on daily high temperatures.  In Heat Zone 6 we average between 46 and 60 days of 86 degrees F and higher each year. 
                 We are all aware of the impact that heat has on our plants.  This information was used to analyze the affect that heat has to the performance of plants in regions throughout the United States.  Heat damage can first appear in many different parts of the plant.  Flower buds may wither, leaves may droop or become more attractive to insects, and roots may cease growing.  Plant death from heat is slow and lingering.
                So, when we think of some plants that live further north, such as the beautiful Copper Beech trees or the European Mountain Ash trees or the flowering Potentilla shrubs, we think that if they can live in the colder north that they surely can grow in the Richmond area.  But, no – these plants hate our summer heat and dryness.  In a nutshell, this is why understanding the heat tolerance of plants is just as important as understanding the cold hardiness.
 
Posted: 4/13/2016 by Doug Hensel | with 0 comment(s)
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