Plant cold hardiness - It's the extremes, people!

When it comes to plant distributions based on temperature, it's the extremes that determine the limits.  Unfortunately, there also is a lot of misinformation, some might call it fraud, in the nursery business.  Now understand, there's a big difference between a plant that is hardy to zone 6 versus one that is hardy in zone 5, about 10 degrees (F) of cold tolerance.  And a lot more territory in which to sell a plant labeled zone 5.  TPP remarked in a workshop presentation that cedars (Cedrus) are not reliably hardy in zone 5.  A woman replied that she bought a Cedrus libani (cedar of Lebanon) that was labeled zone 5, oh yes, and it had died this year. According to a very authoritative source, one variety (var. stenocoma) is supposedly hardy "in the warmer parts of zone 5", which ain't here!  There is a lot of north-south territory in this part of zone 5 in the great Midwest.  The tree in question had been planted 4 years ago, and for the past three winters our minimum temperature has been about 0 F (32 degrees F below freezing for the rest of the world), but this year we've had temperatures below zero weekly for two months reaching a minimum of -17 F, nearly two cold hardiness zones, near the minimum expected temperature for zone 5 (-20 F), and certainly not in keeping with being the warmer part of zone 5, which is probably about 100 miles south of here. Even then how confident can you be that the tree in question was the "hardy variety"?  To label such a tree as hardy in zone 5 is a fraud.  This species of cedar prefers a zone 7 climate, but can handle zone 6, and with little exception, is not hardy in zone 5.  Now TPP regularly plants things that are zone 6 plants, and just as regularly some of them die, but not all.  Unfortunately because cedars are wonderful trees, cedars don't appear to be one of the tough ones. This winter will sort out the real cold hardiness.  One of TPP's range extension plants is Helianthemum (Cistaceae), a good Mediterranean shrub. Hopefully the snow cover has protected this low-growing plant, and the bunnies have ignored it. Some plants will die back and re-sprout from the base (e.g., Vitex). A sweet bay magnolia was struggling, and was replaced by a hardier variety, and hopefully it will survive.  Generally, it's best to select plants grown north of your area.  Plants grown further south are a poor bet; northwestern grown plants are nearly always losers.  It's like Californians that move to Chi-town. It is to laugh. Always ask about the nursery source. So while really cold winters are becoming less frequent, as you would expect with global warming, the extremes have not shifted northward as yet, so you pays you money and you takes you chances.