Growing Zones

[agongos]

Are growing zones based on actual temperatures, or on wind-chill temperatures?

We had -35* windchills yesterday, didn't spend a lot of time outdoors!

[applestar]

USDA Zones are based on minimum temperature likely to be experienced during the winter: http://www.usna.usda.gov/Hardzone/hzm-ne1.html

According to the above map and table, with the temperature at 24ºF inside a double floating cover and a 4mil plastic covered area in my garage in which I have a fluorescent light on 24/7 and am trying to overwinter a couple of containers of Pineapple Sage and a Heliotrope, as well as a tiny Japanese Maple seedling, and a few other things, it is USDA Zone 9a -- same as Houston, Texas and St. Augustine, Florida. That should mean next winter, I should be able to overwinter a dormant fig tree there.....

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OK, I just re-read your post and your question is about wind-chill temperature. Sometimes, my outdoor thermometer registers the wind-chill temperature that is being forecast/reported so....

In simple terms of considering whether a plant is hardy in my area or not, I always go with hardiness one zone colder than mine unless it's something I really really want and am willing to go extra length to protect it. Also, there are always micro-climate around your property, against the south side of the house wall, if you have masonry wall as opposed to aluminum or vinyl siding will make a difference too.

In addition, I try to purchase from nurseries that are same or one or two northern zones when cold hardiness is in question, and purchase from nurseries that are same or one or two southern zones when heat tolerance is in question.

-35ºF huh? BRRR!!!!

[wordwiz]

The USDA changed our zone, raising it from 5 to 6 this year. We came within five degrees of it being too cold for Zone 6 plants!

This is inference on my part, as I have not read anything definitive. Lots of growers, especially bananas and strawberries, talk about using mulch to protect the stems and upper most roots in real cold weather. That may suggest wind chill is considered but if it stays extremely cold for days, that coldness will penetrate the mulch. Back in '75-76, we had a week of frigid (for Cincy) temps including two nights where the official low was minus 25 but out in the country (we we lived) a few farmers reported minus 33 - without a wind chill. One day, the warmest it got was ten below. It was the first time since the OH River froze in most people's lifetime. (An exception was back in the early 20th Century when the river was not much more than a large creek.)

No one spoke about perennials dying that year, though. Of course, that may have been due to a very heavy snow blanket.

Mike

[agongos]

I guess some of you don't really understand what windchill is? Windchill is the combination of the actual temperature and the wind speed. Windchill can't register on a thermometer. Windchill is what it feels like outdoors. Example: Right now my thermometer reads -15 below zero. I went on line to a weather sight and it also said it was -15 below zero. It also said because of the 6 mph wind it feels like 25 below zero, which would be the windchill temperature, -25.
My original question was, are growing zones based on actual temperatures or on recorded windchill temps?

[applestar]

Thanks for clarifying I understand all that. I was also going to say that some weather sites lists windchill temp as "feels like" temp.

Please convince my thermometer that it can't register windchill.
All *I*m saying is that the actual temp has been, on occasion, as cold as the temps calculating in the windchill factor reported on weather sites.

...and as wordwiz said, local temperatures can be variable due to microclimates.

As far as I know, the zones are based on officially recorded air temps and not calculated windchill factors.

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P.S. Some "official" temps are recorded at weather stations that have nothing to do with where you live. Around here, one source of "official" temps are the ones measured at the Philadelphia Airport, which is, in addition to being an AIRPORT, located in a heavily industrial area off the Delaware River. I prefer take my cues from a weather station locatied near a county airstrip surrounded by farms. I also like Weatherbug and Wunder (weather underground) Weather for surrounding area temperature readings.