Hardiness Zone

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ramanmu Chennai, India Jun 01, 2010

Just tossed my hat into the gardening ring very very recently so, I wouldn't be surprised if what I'm about to ask makes you chuckle.

What is the meaning of the upper limit on a (US) hardiness zone for a plant? For instance, in this week's Dave's weekly garden newsletter, it says that Clematis "Blue Light" 's cold hardiness zone is:

Cold hardiness: Zone 4a to Zone 9b

So, why is Zone 9b specified? If this Clematis cultivar is cold hardy to Zone 4a, clearly (that is, as I 'understand' it) it is cold hardy in all zones (numerically and lexicographically) greater than Zone 4a since,

Zone Y > Zone X

implies

minimum temperature (Zone Y) > minimum temperature (Zone X).

So, does specifying the upper hardiness zone, Zone 9b in this instance, mean that the plant/cultivar will not survive in the higher zones (such as Zones, 10, 11, etc?)

I am, needless to say, confused.

Thanks in advance for all clarifications.

Post #7848399
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ecrane3 Dublin, CA(Zone 9a) Jun 01, 2010

Technically speaking, the hardiness zones only refer to winter cold, so anything that's hardy in zone 4 is not going to be killed by winter cold in any zone warmer than that. However, winter cold's not the only thing that can kill plants--some can't take a lot of summer heat either so sometimes people will use an upper zone limit that way to indicate a plant that can't handle hot summers. So if that information is correct, then it would imply that it would not like your hot summers and would be unlikely to do well. Do doublecheck the info though--Plant Files relies on user input, so there are sometimes errors. I don't know enough about Clematis to say whether they'd do well in your area or not.

Post #7848941
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Grantman31 West Palm Beach, FL Jun 21, 2010

In addition to what ecrane stated, the other thing you have to consider is dormancy. Some plants require dormancy - a state in which minimum cold temperatures are required. Thus, in hotter climates, plants won't experience such temperatures required to go dormant, thus killing the plant.

Post #7907821
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