[NAFEX] USDA Zones chill hours & heat etc

Martha Davis martha_davis at earthlink.net
Sun Dec 11 14:01:59 EST 2011

Hi all,

I hope I don't sound like I'm ranting too much.  Mostly I'm trying to
facilitate clear communication.

Minimum temperatures, which is the USDA zone basis, are different from
chill hours, and both are different than amount of summer heat (growing
degree days) which is different than summer maximums.  In some parts of the
country (I think low elevation midwest and east) they all move together,
but not in others, typically a lot of the west.  For some fruits it appears
that strong sun can substitute for summer heat, but probably not always. I
live where my average minimum temp is somewhere in the 0 to -10 range right
on the edge of zone 5/6, that really only says something about cold
hardiness, nothing about how much chill hours I want or get or how much
heat. I want trees with a high chill requirement (hours between 32-45 F)
ideally around a thousand because we get erratic winter warmups and it is
easy to satisfy low chill requirements with a week of weather in 35-70 F
temp range in February before it drops down to the teens & lower and then
bounces up enough to start bloom and down some more.  Even though I am at
the border of 5/6, because we cool into the 50ties on a lot of summer
nights I still don't get enough heating degree day units to grow most
tomatoes well, which has implications for many fruit crops. If you can
distinguish between all these things in your experience or your reference
it is helpful to be more specific. It may also help when you move to a new
climate area especially without experienced people around.  There are
reasonable chill hour maps on line, and some heat zone maps (based on
summer maxs) the degree day ones are harder to find because they are often
crop specific.  Just knowing whether it cools off much on summer nights is
useful.  Min max thermometers are always good, it can be very educational
to look at your minimums for summer nights depending on where you are.
 Just looking at USDA Zones and American Horticultural Society heat zones
Chicago looks a lot like high elevation NM. The winter mins and summer maxs
are fairly similar.  When you look at variation in winter temps, desirable
chill hours and growing degree days, or even just summer minimums they are
different worlds and the plants know it.  It has been a real education
moving here.  I look to Maine and western Oregon to get variety ideas
regarding tolerance for low growing degree days, IL for season length, and
winter minimums, and experimenting for a lot of other things.

Martha  Davis

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