I have copied the weather data from http://www.wrh.noaa.gov/boi/climo/days, for your viewing pleasure. NOAA is short for National Oceanic and Atmospheric Administration. In particular, this chart indicates the number of days the temperature was at zero degrees or below since 1990. And, while it doesn’t happen very often, you should be aware that it can, and probably will, drop below zero again. I offer this information because in the last 30 days I have heard we are in two different zones. We are either a zone 3A or we are zone 7. More than a little confusing to the new gardener, let alone this old gardener. Let’s discuss.
The Idaho Statesman published a special section, “Discover Treasure Valley” on October 30th. On page 60, it stated we are a zone 3a. Taking a guess, I believe that information probably came from the Sunset Western Garden Book. This is important: Sunset has their own ideas about zones- not part of the USDA program.
Prior to that, I heard a presentation where a gentleman proclaimed we have been upgraded to USDA Zone 7, according to the United States Department of Agriculture. The USDA designations are used by nurseries and landscape companies.
Here is the chart for days below zero:
1990-91 0 w- 0 8(-25) 6(-12) 14(-25)
1991-92 0 w+ 0 w+ 0 w+ 0 w+ 0
1992-93 0 w 0 w+ 1(0) w+ 0 w+ 1(0)
1993-94 0 w 1(0) 0 0 1(0)
1994-95 0 w+ 0 w 0 w 0 w 0
1995-96 0 0 c- 0 c- 0 c- 0
1996-97 0 0 0 0 0
1997-98 0 w+ 0 w+ 0 w+ 0 w+ 0
1998-99 0 2 c+ 0 c+ 0 c+ 2(-2)
1999-00 0 c 0 c+ 0 c+ 0 c+ 0
2000-01 0 0 0 0 0
2001-02 0 0 0 0 0
2003-04 0 w 0 w 0 w 0 w 0
2004-05 0 w- 0 w- 0 w- 0 0
totals 3 88 132 64 288
averages .02 0.7 1.1 0.5 2.3
The chart indicates in 1990 we had 14 days below zero and the coldest day recorded was minus 25. I remember it and it was ugly. In 2004, we had zero days at or below zero.
Then, taking all these minimums into account, you should go take a look at the National Arboretum website, for the USDA Hardiness Zones and Average Annual Minimum Temperature Range. Go to http://www.usna.usda.gov/Hardzone for more information.
When you add and subtract all this, you will see that as late as 1990 we had a winter classified as a Zone 4b. During the winter of 2004-2005, we would probably have been a Zone 8. The coldest days were January 5-6 and a measley 16 degrees. And let me muddy this up a bit more – toss in some microclimates – those hot spots on the south side up against the foundation or the deadly cold pockets on the north side where the sun don’t shine from November until March. And did I mention the kiss of death? Bitter cold and drying winter winds.
Let me be the first to tell you I shake in my boots when I think about global warming. However, I think something else is going on and it would best be classified as foolhardy (not to be confused with cold hardy) and it is called “Zone Denial.”Not the same as “Zone Envy” which is what I have upon visiting any area that doesn’t get below freezing.
Why do you care? Well, maybe YOU don’t. But if you are buying plant material for your garden/yard/landscape, it helps to know what zone you are in. For those of you who are adventurous and have plenty of cash for plant acquisitions, you will want to try some camellias and black mondo grass. I, personally, stick with the idea that it hurts to lose a tree that cost $200. Not to mention the glaring hole it makes in the landscape when you have to yank it out. Yes, I have a deodora cedar that doesn’t really belong here. But before I installed it, I knew of 2 deodoras on State Street which had survived the winter of 1990. And yes, I have an agave on the back patio, under cover, that I can wrap in polar fleece should it get really nippy.
So, consider yourself warned.