Hi Bob,
Let me see if I can make this more understandable (or more confusing)
. As you have figured out, the 'zones' are for the lowest temps not the high temps. So a plant with a zone range of 4-9 would be happy with minimum temps in those ranges. Usually that means that the plant needs a certain number of 'chill' hours (hours under 45 degrees). The zones were originally invented for farmers by U.S. Department of Agriculture. Fruit trees need chill hours (ie: peaches) or need temps above a low temp (ie: oranges).
Someone made the translation to everyday gardeners: think peonies. Peonies will be so happy in zone 3 to 7 (zone 8 is iffy). They need the cold to be happy. Or tulips... Tulips grow great in Reno but when I lived in California (zone 8), they lasted maybe 2 years at the most (and then they just don't come back).
But there is a high temp a plant will be happy with (remember that farmers are only interested in low temps). That's where the American Horticultural Society comes in. They have come up with a heat zone map that will be helpful on the other end:
http://www.ahs.org/gardening-r...
But not all zones are created equally. I live in Zone 6 or 7 but, our winters are dry and cold. The humidity often gets to zero with no snow. My friend, also zone 6 or 7 but in Minnesota, has 10 feet of snow every winter. Snow insulates. Without snow, our ground freezes to 24". In Minnesota, the freeze depth is 6".
Daisy
