Climate Zones.

Lee Poulsen
Thu, 27 Oct 2005 17:23:34 PDT
I like the Climate Zone concept. I think there's nothing wrong with the 
concept, but instead there are problems with how it is implemented and 
how it is used. There are of course a number of climatic parameters 
that all go into determining whether something will grow easily or not 
so easily or not at all in a specific location. And there are now 
starting to be maps for those parameters as well (such as heat index 
map). But I have to hand it to the USDA for thinking of the single best 
parameter to use in creating the first Zone map and then making the map 
and seeing what the result was. They could have picked average annual 
rainfall or annual average peak summer highest low temperature or 
something even more refined than that. But they decided that a 
particularly good value to look at as a determining factor in whether 
something could or would grow in a particular location or not was the 
average of the coldest temperature of each year averaged over 20 or 30 
consecutive years (or more). (Personally, I think the median coldest 
annual temperature would have been a better measure than the average 
since you would then know that half the years the likelihood would be 
that it could get colder than that temperature at least once during the 
year and half the years it would not get that cold throughout the year. 
But average is easier to calculate, especially back when they made the 
first Zone map.) What this kind of map doesn't show is the variance of 
that value, i.e., how much different than that value an annual coldest 
temperature got over the 30 year period of the average.

Let me give a practical example that I encountered when I first started 
getting into this kind of thing. One of my first plant loves was fruit 
trees, and when I was a teenager, I pulled out all the trees out of my 
parents' yard that the developer had planted when they built it. I 
replanted the entire lot with fruit or nut bearing trees (the "edible 
landscaping" concept). I wanted to plant at least one of every kind of 
fruit tree that would grow in that area (Austin, Texas, USA). So I got 
a list of all the recommended cultivars of each kind of fruit tree from 
the county agricultural extension office. This, among other 
responsibilities, is something that each county agricultural extension 
office in the USA was tasked to do quite a number of decades ago when 
the extension offices were first being set up nationwide. (Some do this 
better than others.) After I had done this, there was till room in the 
yard, and I began to wonder why some of the fruits in the supermarket 
weren't listed at all in the literature I had gotten. In particular, I 
wondered why orange trees weren't listed anywhere. I have numerous 
relatives in southern California (Riverside area), including aunts, 
uncles, and one set of grandparents, which we visited quite often while 
growing up. So I knew that temperature-wise, the weather seemed fairly 
similar all year round. And yet in the Riverside, California area, 
which back then was the major citrus growing area in California, and is 
still the location of the USDA's citrus research and citrus variety 
geneplasm growing fields, oranges grew everywhere. In Austin, Texas 
there were zero orange trees that I could see.

I learned that Austin was USDA Zone 8b (average minimum coldest night 
temp. between 15°F and 20°F) and Riverside was on the line between 
Zones 9a and 9b (so about 25°F). And that orange trees begin dying once 
the temperature goes into the low 20s °F beyond a certain length of 
time. So for 364 days a year an orange would grow just fine in Austin, 
Texas. But in more than half the winters, there would almost certainly 
come a night or two when the temperature would drop too low and the 
orange tree would be killed. As it turns out, some people have grown 
orange trees outside in Austin for a few years, especially when there 
have been a run of several years in a row with warmer than normal 
temperatures. But then along comes an average or colder than average 
winter and the tree is killed.

So it turns out that the coldest night temperature of the year is one 
of the major single parameters that determine whether an orange tree 
will survive in an otherwise hospitable climate. And this is true in 
Florida, Texas, and California.

Of course other factors do come into play. First there is the variance 
of the coldest temperature average. Just outside of Tucson, Arizona 
there are location that are also Zone 8b. However, they grow things 
there permanently that I never see in Austin, Texas. That's because in 
the Western USA the full range of the 30 years worth of coldest annual 
temperatures doesn't vary much from the average. Whereas in the central 
and eastern USA it can vary much more greatly. For example, there have 
been winters in Austin, Texas where it only got down to 30 or 31°F on 
the coldest night of the year. However, there have been other years 
where it has gotten down to 11 or 12°F, and in the 1980s it got down 
into the single digits (°F) on two occasions. In Tucson, Arizona, it 
always gets down into the 20s °F. But it rarely goes into the low or 
mid teens °F. It has also never only remained at or near freezing as 
the coldest temperature of the year. Here in Pasadena, California, the 
range of variability is even tighter. It seems that we always have one 
night where it hits 32°F about every other year. But it seems that even 
the warmest coldest-night of the year I've experienced here it still 
got down to 34°F. Only once did I see it hit 29°F in a 15-year period 
of time.

Another factor that seems almost as determinant about what will or 
won't grow is a harder-to-measure parameter that was the point of the 
"heat index" maps. Some catalogs try to indicate this by putting an 'S' 
or a 'W' after the the USDA Zone number (as in Zone 10aW or 8bS) 
meaning that something will survive the summers in Zone 10a areas in 
the western USA, but only Zone 8b areas in the southern USA. I found 
this to be true for example with fuchsia and sweet peas. They easily 
grow and flower in most areas of California, but to grow sweet peas in 
Austin, you had to time the planting very carefully if you wanted to 
see any flowers bloom before the plants died, and I was never able to 
get a fuchsia to even survive through a summer, let alone thrive. Even 
tomatoes stop producing new fruits during the peak hottest summer 
months in Austin--because the nights never cool down enough to let them 
form. (They've bred a few special heat tolerant varieties that can do 
it, and cherry tomatoes seem to already have that ability.) In 
California, even in areas where the days are hotter in summer than in 
Austin, the nights are still cooler and tomatoes fruit all summer long.

So while I know there are problems with Climate Zones, they also serve 
a very useful purpose in giving you likely boundaries of what to even 
try growing. The implementation problem I referred to at the beginning 
is that the maps that are made only have a certain resolution, so you 
don't see a lot of the detail that exists due to rapidly changing 
topography that occurs all over the western USA. But if you could 
accurately measure the temperatures in enough locations and had a fine 
enough resolution map to draw the contours on, you could actually make 
a more pertinent map of the western USA as well. I once played with 
some contouring software and got some actual values of "average annual 
extreme minimum temperature" for a part of a state (south Texas) and 
made a map that had USDA Zones in 1-degree steps. It was quite 
interesting to look at.

Then of course there is moisture, humidity, rainfall, wind, etc. But 
these aren't as determinant of a plant's survival as the above two are, 
and can be compensated for outdoors more easily than the above two. 
(Except for outdoor humidity, which seems to come into play for some 
bulbs--it's easier to provide more humidity than to make it less humid 

John, with the latest software and computing ability that exists these 
days, a virtual map could be made down to your street or house level, 
not just by zip code. But the number of actual temperature measuring 
locations is much much more sparse than that. So your detailed map 
would be highly interpolated to give you that kind of resolution, and 
hence have a much more likely chance of being inaccurate the further 
you were from the actual measurement locations. In the West, this is an 
even greater problem since climate parameters change much more rapidly 
with distance than in the East.

Okay, I think I wrote too much.

--Lee Poulsen
Pasadena area, California, USDA Zone 10a

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