How are USDA zones defined anyway?

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insipidtoast Santa Barbara, CA(Zone 10a) Dec 04, 2010

I see they look at low temperatures. Are these the average of all the annual lowest temperatures recorded for every year? Are they the Average of all the low temperatures for the winter months?

E.g. Zone 11 is defined as 40F Ok? What does that mean? That the temperature in Zone 11 never drops below 40F?

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ViburnumValley Scott County, KY(Zone 5b) Dec 04, 2010

Here is the US National Arboretum's brief explanation of the hardiness zone map:

http://www.usna.usda.gov/Hardzone/hrdzon3.html

The designations are based on average annual minimum temperatures. The zone maps are updated occasionally. The one I have signed by the director of the USNA is from 1990. I'm pretty sure that there is a newer rendition.

Zone 11 represents any area where the average annual minimum temperature is above 40 F (4.4 C).

I'm sure one could burrow even further into the derivation of this data. Check out USDA's website, etc.

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insipidtoast Santa Barbara, CA(Zone 10a) Dec 04, 2010

"the average annual minimum temperature"

What does that mean?

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ViburnumValley Scott County, KY(Zone 5b) Dec 04, 2010

There are lots of weather stations across this nation. They measure temperature throughout the year. Somewhere in all those measurements, there's going to be one that is the lowest (read: minimum) for that year.

Take that minimum temperature for each year for a span of time (let's stipulate 10 years), add them all up and divide by the number of years (10, in this case). That should give you "average annual minimum temperature."

On these zone maps, that average annual minimum temperature is often expressed as a range (say, -10F to 0F).

The zones are created by coalescing all the weather stations' measurements of average annual minimum temperature that are relatively similar or equal.

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carrielamont Milton, MA(Zone 6a) Dec 04, 2010

http://davesgarden.com/guides/articles/view/578/

This article might help.

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