The zone map places us on the fringe of z.6, and the definition always says, "average annual minimum temperature." Our average winter low is probably around -8F to -10F, but sometimes lower or higher. In the winter of 1993-1994, it was about -20, and for 1995-1996 it was -2F, 1996-1997 was 0F, 1997-1998 was +10F, and 1998-1999 was about -10F. Just for my rhododendrons' sake, I'd like to see the +10F of 1997-1998 every winter, so that I could grow all those nice West Coast varieties. So I guess that the average is -10F or so, and that puts us right on the edge.
I saw a book on bonsai, which had a USDA zone map along side of a "lowest temperatures ever observed, 1899-1939" map, and if you look carefully, the extreme lows are usually about 10F cooler than the average lows, in any given zone in the eastern U.S. The 0F isoline separates the coastal plain of the southeast from the rest of the eastern U.S., and just about defines the limit of the natural range of baldcypress and some bromeliads (Tillandsia), and it even cuts into Florida. Also, if you look at the lowest temperature ever observed in Florida, it's -2F, and it was recorded on February 13, 1899, in Tallahassee, which is well within z8. So I don't like to use zone ratings, because if a plant survived to -10F in z7 in Tennessee, then someone might rate it z6 and it could end up on the market in Michigan in the fringe of z6. That's why I think that a plant should be able to survive the severest winters in the warmer part of z5 to be rated z6. You really have to look at a "lowest temperature ever observed" map (and they're not very common), because otherwise if a plant is rated liberally to your zone, you could have a 50% chance of it surviving 1 year, 25% for 2 years, etc. I think that the zone map system should have been set up for extreme lows, not just average lows. If you look at the book Success with Rhododendrons and Azaleas (by H. Edward Reiley, 1992) you will see a list of different localities, their zone ratings, and their extreme lows. It reveals a lot. I really hate to see a plant called "hardy to -10F to 0F, zone 6." I'll try to grow something like that when I move to Tallahassee, FL, and then hope that we don't get -2F again! What if its hardiness is closer to 0F than to -10F? I would like to grow Yucca (Hesperoyucca) whipplei, and I have seen it rated to z9, z6, and z5. Needless to say, none of the plants ever appeared to survive into January. I'll keep trying, and if you know who rated that plant to z5, then let me know! Maybe he or she can help me. Well, enough of my soapbox.
New! Read Alicia Ellis' soapbox on my soapbox. Alicia Ellis is a Master Gardener here in Michigan. New!
