I would have agreed with you until last winter. We have been a Zone 5a for the past 5 years with less rain than normal. Last winter Zone 3a came back with a blast. This summer has been cooler and wetter than recent years.

I am waiting for next winter.

Allen

I don't know if it is global warming or not but I haven't had a zone 8a winter in a long time, (at least 8 years and counting).
Half my years are zone 8b and the other 9a. I believe we are having global warming when I finally see a zone 9b.
Now that would be nice.

Wes ... here (and you know where "here" is) we recorded a winter low of -3.7 celcius on January 22 - 08. That would qualify as a 9b winter certainly but I won't be planting any King Palms 😆

Cheers, Barrie.

The climate had better be changing! I've been planting my gardens based on a 3-5C increase in the average temperature! That will put me on the cusp of 6b/7a (I think, unless the extreme lows remain the same)...
Acubua, Trachycarpus, Sabal, Rhapidophyllum, Phyllostachys, are not really what you think of when you think of zone 4b/5a gardens...
An interesting thing to note is that when you look at most of the Ontario weather stations and map average and/or extreme minimum temperature, year to year, you can see a distinct warming trend... Not good for dealing with invasive species, but not bad for those of us who like to 'push' our zones!