temperatures and plant cold hardiness/heat hardiness

i don't know if this is the correct forum for this question. and i am not sure how to verbalize it. but lots of times these days nurseries tell us the cold hardiness of plants by listing the degree it is hardy to. does this mean the actual temperature or does it mean wind chill temperature which is usually lower? and does anyone know where to go for information that can explain heat hardiness or sunset zones or whatever the opposite of cold hardiness is?

I heard somewhere the Agriculture Dept. or someone like that is developing a heat zone map. I'd love to see it if someone knows where to find it. It sure would help down here in Lower Hades.

Mama, hardiness zone gives the range of the average lowest ACTUAL temperature in a region. For me in zone 6, that would be between -10 and 0 degrees. It does not include wind chill.

Brigid, there is a Heat Zone map. It was designed by the American Horticultural Society (AHS), not the USDA, who did the hardiness zone map. It describes areas based on the number of days on average a region is above 86 degrees. This temp was chosen because that's where certain plants start undergoing stress and other breakdown. Here's a link:

http://www.monrovia.com/monroviaweb.nsf/8c104835579b67e18825685f006acdf8/f99d2ba9eb2a9ab58825692000737444!OpenDocument

Thanks, victor!

Edited to say I studied it a bit. I'm in 9, which has 120 - 150 days with temps above 86. They don't say how much above, and I assume that's without the heat index, which is a HUGE factor down here. Ah, well -- can't have everything, and that does give good info.

This message was edited Mar 7, 2007 3:55 PM

You're welcome. Yes - it's temperature alone - not heat index.

Nothern climes have one hidden advantage in summer for growing crops in that the daylight hours are longer.

Yes, I always thought that was Mother Nature's attempt at compensation - if you can't have longer seasons, at least you get more daylight hours!

And cool evenings so the plants can rest and look beautiful the next day.

True.

what's the heat index? is that the opposite of wind chill? meaning actual temp may be 100 but feels like 120?

i am zone 9 in the heat index too. althought when i type in my zip code it says AHS 8. we are a small town so i typed in the bigger town near me and it came back 8/9.
thanks victorgardener for the information.

Yes, heat index is sort of the opposite of wind chill. It combines temperature and humidity to give a sense of how uncomfortable 'it feels'. Remember, though, that the temperatures we are given are always in the shade so both temp and heat index are much higher in the sun!

thank you.

You're welcome.