Is Boise ID and the Treasure Valley truly a USDA Zone 7 Growing Area?

The  recent release of the revised USDA Zone Hardiness Map puts Boise in Zone 7A. I am going on record as ROFL (Rolling on the floor laughing). Anyone here experience the winter of 1990? Sure, it was 22 years ago. Minus 25 for days. Many days. OK, I understand that  is the exception to the rule, but let me tell you this: be prepared to lose most Z7 plants every few years. If you can afford that, by all means, go right ahead. If you don’t mind hauling plants in and out or covering them during a cold snap, by all means, have at it. I stand by my recommendation that expensive  specimen plants and trees should be able to go withstand a Z5 winter. Yes, I do. 
I wrote to Scott Dorval, one of the valley’s true meteorologists, to get his take on this. Here it is (reprinted w/his permission).
While some of the record lows in Boise are frigid Boise does not go below zero that often. Boise did go below zero in Dec 2009 but that was the first time since 1998. From 1973-1993 Boise went below zero 15 out of 20 of those years. In the last 20 years Boise went to zero or below 5 of 20 of those years.
You can get specific monthly and daily records at :  http://www.nws.noaa.gov/climate/index.php?wfo=boi
1)click  Preliminary Monthly Climate Data (CF6
2) click Boise or any location offered
3) click  Archived Data:
These will only go back 5 years or so.
You can also get lots of other Boise climatic data at:
Here is a the Boise “below zero” data that comes from the above link:
              Number of Days Minimum Temperature <= 0 F             

monthly/annual lows in parentheses
length of record in brackets

            November  December  January   February       Annual

 1879-80       0       1(-4)       0         0          1(-4) 

 1880-81       0         0         0         0            0 
 1881-82       0         0         0         0            0
 1882-83       0       1(-4)     7(-27)    7(-12)      15(-27)
 1883-84       0         0         0       4(-8)        4(-8)
 1884-85       0       1(-3)     2(-7)       0          3(-7)

 1885-86       0         0       1(-1)       0          1(-1)
 1886-87       0         0         0         0            0
 1887-88       0         0      14(-28)      0         14(-28)
 1888-89       0         0         0         0            0  
 1889-90       0         0       7(-12)      0          7(-12)

 1890-91       0         0         0         0            0   
 1891-92       0         0         0         0            0
 1892-93       0         0         0         0            0
 1893-94       0         0       2(-1)       0          2(-1)
 1894-95       0         0         0         0            0

 1895-96       0         0 w+      0 w+      0 w+         0
 1896-97       0 w+      0         0         0            0
 1897-98       0 c       0 c       0 c       0 c          0
 1898-99       0 c       0 c       0 c     4(-9) c      4(-9)
 1899-00       0 c-      0 w       0 w       0 w          0

 1900-01       0 w+      0         0         0            0
 1901-02       0 w+      0 c       0 c     4(-8) c      4(-8) 
 1902-03       0 w       0 w       0 w       0 w          0
 1903-04       0 c       0 c+      0 c+      0 c+         0
 1904-05       0 w-      0 w+      0 w+    1(-2) w+     1(-2) 

 1905-06       0 w+      0 w+      0 w+      0 w+         0
 1906-07       0 c+      0 c-      0 c-      0 c-         0
 1907-08       0         0         0         0            0
 1908-09       0 c+      0 w-      0 w-      0 w-         0 
 1909-10       0 c       0 c+    5(-8) c+    0 c+       5(-8)

 1910-11       0 c+      0 c       0 c       0 c          0
 1911-12       0 w+      0 w+    2(-5) w+    0 w+       2(-5)
 1912-13       0 w-      0 w       0 w       0 w          0
 1913-14       0 w+      0 w-      0 w-      0 w-         0
 1914-15       0 w+      0 w+      0 w+      0 w+         0

 1915-16       0 w     1(-2) c-  1(-2) c-    0 c-       2(-2) 
 1916-17       0 c+      0 c+      0 c+      0 c+         0
 1917-18       0 c+      0 c+      0 c+      0 c+         0
 1918-19       0 w-      0 w+      0 w+      0 w+         0
 1919-20       0 w     5(-7) w-    0 w-      0 w-       5(-7)

 1920-21       0         0 c+      0 c+      0 c+         0
 1921-22       0 c+    5(-10) c+   0 c+      0 c+       5(-10)
 1922-23       0 c     1(-2) c     0 c       0 c        1(-2)
 1923-24       0 w+      0 w-      0 w-      0 w-         0 
 1924-25       0 c+   11(-18) c+   0 c+      0 c+      11(-18)

 1925-26       0 w+      0 w+      0 w+      0 w+         0
 1926-27       0 c-    1(-3) c-  2(-2) c-    0 c-       3(-3)
 1927-28       0 w-      0         0         0            0  
 1928-29       0 c+      0 c+    1(-7) c+  2(-1) c+     3(-7)
 1929-30       0 c     6(-17) c+   0 c+      0  c+      6(-17)

 1930-31       0         0         0         0            0
 1931-32       0 c-      0 w-      0 w-      0 w-         0
 1932-33       0 w-     5(-7) w-   0 w-    4(-13) w-    9(-13)
 1933-34       0 c-      0 c       0 c       0 c          0     
 1934-35       0         0 c-    1(-6) c-    0 c-       1(-6)

 1935-36       0 c       0         0       2(-3)        2(-3)
 1936-37       0 w-      0 c-    5(-14) c-   0 c-       5(-14)
 1937-38       0         0 c       0 c       0 c          0
 1938-39       0 c+      0 c+      0 c+      0 c+         0
 1939-40       0 c       0 w       0 w       0 w          0 

 1940-41       0 w+      0 w+      0 w+      0 w+         0
 1941-42       0 w+     1(0) w+  6(-13) w+   0 w+       7(-13)
 1942-43       0 c       0 c+    2(-10) c+   0 c+       2(-10)
 1943-44       0 c       0 w-      0 w-      0 w-         0
 1944-45       0 c-      0 c-      0 c-      0 c-         0

 1945-46       0 c-      0         0         0            0
 1946-47       0 w       0 w     3(-10) w    0 w        3(-10)
 1947-48       0 c       0         0         0            0
 1948-49       0       1(0) w-   9(-7) w-    0 w-      10(-7)
 1949-50       0       1(-1) c+  2(-17) c+ 3(-10) c+    6(-17)

 1950-51       0 c+      0 c+      0 c+      0 c+         0
 1951-52       0 w+      0 c+    1(-2) c+    0 c+       1(-2)
 1952-53       0         0 w       0 w       0 w          0
 1953-54       0 w       0         0         0            0
 1954-55       0 c-      0 c       0 c       0 c          0

 1955-56     1(-3) c+    0 c+    2(-5) c+  2(-6) c+     5(-6)
 1956-57       0 c+      0 c-    5(-9) c-    0 c-       5(-9) 
 1957-58       0 w       0 w+      0 w+      0 w+         0
 1958-59       0         0 w+      0 w+      0 w+         0 
 1959-60       0 c       0 c-    2(-4) c-    0 c-       2(-4)

 1960-61       0 c-      0         0         0            0
 1961-62       0 c-      0 c+    5(-12) c+   0 c+       5(-12)
 1962-63       0 c+      0 c     3(-7) c     0 c        3(-7)
 1963-64       0 w+      0 w       0 w       0 w          0
 1964-65       0 c+      0         0         0            0

 1965-66       0 w+      0 w       0 w       0 w          0
 1966-67       0         0 c+      0 c+      0 c+         0 
 1967-68       0         0 c-      0 c-      0 c-         0
 1968-69       0         0 w+      0 w+      0 w+         0
 1969-70       0 w       0 w       0 w       0 w          0

 1970-71       0 c+      0 c+      0 c+      0 c+         0
 1971-72       0 c+      0 c-      0 c-      0 c-         0
 1972-73       0 w+   10(-23) w+   0 w+      0 w+      10(-23)
 1973-74       0 c+      0 c+    4(-5) c+    0 c+       4(-5)
 1974-75       0 c+      0       2(-5)       0          2(-5)

 1975-76       0 c+      0 c+      0 c+      0 c+         0
 1976-77       0         0 w-    5(-14) w-   0 w-       5(-14)
 1977-78       0 w+      0 w+      0 w+      0 w+        0
 1978-79       0       3(-9)     7(-12)    3(-12)      13(-12)
 1979-80       0         0         0       4(-7)        4(-7)

 1980-81       0         0         0         0            0
 1981-82       0         0 c     5(-10) c   2(0) c      7(-10)
 1982-83       0 w+    1(0) w+   2(-2) w+    0  w+      3(-2)
 1983-84       0 c-    6(-18)    5(-13)      0         11(-18)
 1984-85       0       4(-6)     2(-7)     4(-12)      10(-12)

 1985-86       3(-3)    20(-9)     0         0         23(-9)
 1986-87       0 w-      0 w+      0 w+    1(0) w+      1(0)
 1987-88       0 w       0 w-      0 w-    3(-2) w-     3(-2)
 1988-89       0 c+      0 c+    1(-4) c+  8(-15) c+    9(-15)
 1989-90       0 c-      0 w+      0 w+      0 w+         0

 1990-91       0 w-      8(-25) w+ 6(-12)    0         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 w           0
 2005-06       0         0         0         0             0

 2006-07       0 w+      0 w+      0 w       0             0
 2007-08       0 c+      0 c+      0 c+      0 c+          0 
 2008-09       0         0         0 c-      0 c           0
 2009-10       0         4 w       0 w+      0 w           4
 2010-11       0 c+      0 c+      0 c+      0 c+          0

totals         3[131]   96[131]  132[131]   64[131]      292[131]

averages       .02       0.7      1.0        0.5          2.2
This entry was posted in Featured, Journal entries. Bookmark the permalink.

One Response to Is Boise ID and the Treasure Valley truly a USDA Zone 7 Growing Area?

  1. Cold, but could be worse. I don’t know about “many days” at -25F:
    http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?id1022
    Left column, scroll to the bottom > Period of Record Data Tables > Daily Summary Stats (~55 KB). Look at Dec 1990, Jan 1991…I saw (4) <-20F lows, (3) -19 to -10F lows, and (4) -9 to 0F lows. And highs those days from -6 to +14F.

    But your dips are colder and longer than our dips in Z6b to Z8a. Sounds like their period of record is accurate in making you 7a, but the other factors are why I prefer Sunset zones…Sunset Z3a makes more sense for you (like Fanta Se, Reno, Salt Lake, Grand Jct). You can be conservative but still do better than Z5 (Denver); there is a huge difference in plants between a place that gets quick hits at -20 to -10F (you), vs those that get that most years (Denver, Omaha).

Comments are closed.