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Does warmer weather mean a new plant zone for you?

Posted by David Epstein  January 31, 2012 06:08 AM

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Last week the USDA put out a new plant zone map for the country. They divide the contiguous United States into 11 zones that help gardeners choose what to grow in their area. The map is now more precise and reflects microclimates, heat islands, prevailing wind, elevation and generally better data. According to the USDA the map should not be used to make conclusions about climate change. The new zones are based on temperatures for the period 1976-2005, updating the 1990 version of the map, which used temperatures from 1974-1986. So what does this mean to you? Can you plant orange and palm trees in your yard? The answer is of course no, but, based on the new map you can probably try some new varieties of plants that you might have been reluctant to try in the past.

National Zones.jpg

All plants thrive under certain ideal weather conditions and have ranges of temperature they tolerate. If a plant can grow in zones 8a to 6a and you plant it in zone 5b (too cold), over time it will likely die. Remember, one important aspect about the zone map is that it reflects averages. A zone 6b area can still experience zone 5b cold. That happened the winter of 2003/2004 and the frigid air killed some plants grown at the northern edge of their zone range. You might have lost a butterfly bush or Japanese holly that winter. Think about this winter's snowfall and cold. I wouldn't want to make any long term plans about golfing in Worcester in January just because its been a zone 7 winter this year. So, the question you might ask is how can you use the new zone map? The answer is it depends on how you garden and where you live. If you are into experimenting, then those of you who now fall in a warmer zone could try a new plant that you thought might not be hardy. Just remember, the plant could live for 5 years without any problem, but if we get one abnormally cold winter, you will often lose marginally hardy plants in a given zone.

One aspect of meteorology the new zone map reflects much better is the urban heat island affect. That is the concept that buildings, sidewalks, roads etc. all emit the heat they absorb during the day, back to the air at night. This process slows down the cooling of the cities and thus the coldest temperatures in Boston, New York and other urban areas are not longer as cold as they were 40 years ago. If you look at the map for Massachusetts you will notice quite nicely that zone 6b now runs west to about the route 128 belt. Untitled.jpg It's no coincidence, those areas inside the belt are the most populated and thus impacted the most by the heat island effect. For example, Winthrop, according to the new zone map, is now in zone 7a. That new zone designation is most likely the result of both the heat island and better mapping of the water around the peninsula.

Even inside the zone you are in, there are subzones because of microclimates. For example, I am on the edge of zone 6a and 6b. I have successfully grown 7b plants against the house in sheltered areas. The heat of the house acts to create a tiny microclimate that is great for trying marginally hardy plants. Conversely, you may live in an area that is typically colder than surrounding areas. Norwood would be a great example. The temperature at the Norwood airport consistently run 5-10 degrees colder than surrounding areas on clear calm mornings. Although Norwood, according the USDA, is in zone 6b, I would be careful about too many marginally hardy plants in my yard there.

The bottom line is that better data and better technology has enabled the USDA to create a more specific and user friendly plant zone map. If you are an avid gardener, chances are you already know what works in your own backyard. Stick with what has worked, but it might be fun to experiment too. Just keep in the back of your mind that the plant you try may die in a colder winter. If you are new to gardening this map will help you decide what plants are best suited for your area. Click here for an interactive look at the map.

Finally, I agree with the USDA that this map shouldn't be used as proof of climate change or its cause. The map is reflective of new data and algorithms (math), that create a more accurate map. A good scientist wouldn't make conclusions about climate change using the previous 1990 version of the plant zone map. That is because the 1999 map and the 2012 map were not created with the same variables. To me, it's like saying we have more hurricanes now than there was back before 1960. Before 1960 we had no satellites and some percentage of storms in the ocean went unnoticed or were not identified as hurricanes. So it stands to reason if we now count hurricanes that would have been missed before satellites, the yearly number will be on average higher.

Follow me on Twitter at @growingwisdom and check out my latest videos at GrowingWisdom.com

This blog is not written or edited by Boston.com or the Boston Globe.
The author is solely responsible for the content.

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About the author

David Epstein has been a professional meteorologist and horticulturalist for three decades. David spent 16 years at WCVB in Boston and currently freelances for WGME in Portland, ME. In 2006, More »
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