The zone are from the USDA and they are really used mostly for the average
last date of frost in a given area.

I'm afraid you don't have it quite right. The USDA climate zones are not
related to frost dates. These zones are only related to the average minimum
winter temperature. In zone 9 your average minimum winter temperature is
between 20 and 30 F.
I had a discussion with Diane Valevanis recently. I was over at Bill's for a
workshop on April 21 and I said, "Only two more weeks to frost-free date." She
disputed that, because their spring frost-free date isn't until the second week
in May, and they aren't in the clear until almost Memorial Day. I went home &
looked it up. I'm in USDA Zone 5, Sunset Zone 40, while Rochester is in USDA
Zone 6, Sunset Zone 39. Sure enough, their spring frost-free date is later than
ours, & so is their fall frost-free date. It is probably due to being closer to
Lake Ontario, so the air & water warm up slower in the spring & cool down
slower in the fall. Yet because the ground never freezes as cold in Zone 6 as
it does in Zone 5, spring blooming bulbs & trees are always earlier in Zone 6.
I tried to access the draft of the new USDA Zone map, which is a PDF file, but
my computer choked on it.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)