Zones are based on average low in any given location. I really wish it was more intensive then that but there is so much that involves hardiness.... Generally speaking for plants it means the death of the plant but for some plants it isnt that easy (bamboo or plant that can survive under the snow or underground and resprout)

I believe the temprature causes the plant cells or water in the cells to explode.... This could only be in Broad leaved evergreens as well..

The graft may take a bit of hardiness due to the zone 5 rootstock but the scion will not effect the hardiness of the root stock. The root stock should survive in the situation you are talking about.

In regards to the other zone 5 graft it would need to be connected to the roots and if it is the zone 7 portion seperating the 2, the one not on contact with the roots wont survive.

As canadian said, the whole cold hardiness thing is very complex. Also a high altitude zone 5 in Arizona can be very different from a zone 5 in New York for example.

If anything, most hardiness ratings for plants are a bit conservative, but there can also be a fine line between something thriving and something dying outright. In some cases things like the length of the growing season, chill hours, or the number of growing degree days may be of more critical importance to fruiting plants than simple cold hardiness.

I think that finding out what is most suitable for your climate is probably one of the more intriguing parts of fruit growing.

The most important part is experimenting. You can do all the research you want to make a good choice but nothing beats just trying something.

The hardiness question comes up in the bamboo world quite often. The plant can loose its leaves and not have damage to the culms or buds, the plant can have damage to the culms (percentage of top kill) or die back to the ground and then regrow like a die back perennial. How do you classify bamboos hardiness when in theory it has 3 "levels" of hardiness? Even then how do you consider the insulating effect of snow or mulch?

Some plants can do the same thing. Paulownia, some eucalyptus and edible figs (F caricia) can all die back to the snow line in climate that would kill the above ground portion yet still regrow the following season (to an extent of course). I beleive in some cases here longevity issues are a problem, but some do in fact survive for a decent time. At least in the case of paulownia you end up with gigantic tropical looking leaves that are an excellent mulch!

I've posted links to several zone maps at AHS Plant Heat Zone Maps by State. - Vegetable Gardening Forum - GardenWeb, not just heat zone maps as the subject line says. I believe there are links to Canadian and Australian maps there too.

Ironically, I've no good links to Sunset Zones which are the only ones I know of that take into account, as best they can summer and winter temperatures as well as rainfall etc. The Subset Western book is good. I have never seen the Sunset Eastern book.

I've added a link to the above lists for Sunset Climate Zones. "[Sunset ] zones are a function of six geographic and climactic factors: latitude, elevation, ocean influence, continental air influence, mountains and hills, and local terrain."

This post was edited by albert_135 on Sat, Sep 14, 13 at 15:51