They are talking about a short term minimum, a one time short term event. But these aren't hard and fast numbers. It all depends on many factors such as stage of dormancy and prior temperatures.

Wind chill has no effect on trees or tree buds. People sometimes talk about cold winds drying out and damaging their trees. I'm not sure their's anything to that unless the soil is very dry. I'd say it's more likely just the cold. The wind blew like crazy in Amarillo. Never saw any damage to limbs or buds in 30 yrs growing fruit there.

The Fruitnut

I don't think there is any standard on how long or how many times in a given winter the low occurs. So some say plum tree may have been listed as not hardy in zone 5 because there was a winter where it was right by -20F for a long period, and that led to the declaration of that plum as not hardy. On the other hand if there was only a short period or two it may have been able to do fine. The lows tend to occur for very short periods if it is sunny the next day -- the low occurs about 7AM just as the sun is rising and at that point the temp pops up 10 degrees pretty fast. If it clouded up over the night on the other hand you can get a low that lasts for many hours. Sudden drops in temperature are also worse than slow drops. Maybe the problem with wind is not the drying out but that large quick temperature drops usually come with a strong north wind (they sure do around here). None of the hardiness ratings take these dimensions into account. So, they are only a rough guideline.

Scott

fruitnut, at sustained cold temperature the wind might not make much difference, but for a short period the wind may bring the tree down closer to the ambient temperature and much more quickly via forced convection.

Putting aside the effects of evaporation, its like cooking in an oven but the opposite. If you put something in with the oven temp at 350 it might take an hour for the internal food temperature to reach 180. Theoretically if you left it in long enough the middle of the food would also reach 350 but it would take several times as long.

If you put that food in a convection oven, it will heat much faster and cooking times are decreased. It increases the rate of heat transfer.

>When a tree or bush is rated at -10, does that meanYou need to find out how the source for that rating defines it themselves, what their rating is for exactly.

As always, if and when USDA Hardiness Zones come into it is important to realize and remember that with the USDA system -10 to 0, 0 to 10 etc. are average minimums expected in each Zone and not the coldest it gets there. So, if a plant is "Hardy to Zone 7" it better be hardy to well below 0 to 10 degrees F.