I would like to know when a certain area is classed a "Zone 6" for example. Does this include windchill factor...or is it just the base number? It could be -10 c and feel like -20 c with the windchill....does that need to be taken to acount for plant hardiness also?
Just the actual temperature. Wind chill doesn't count, as it only affects things which are warmer than the surrounding air (i.e., people). Plants are at the same temperature as the air, so they don't get additional cooling from the moving air compared to still air. Plants can get wind and frost burn due to desiccation by cold dry winds, but that's a separate issue to windchill.
It is sometimes more useful to state the particular plant you're interested in, and ask who is growing it and where, to get some idea of determine it's actual hardiness. Zone ratings are notoriously poorly-researched... at the colder end of the scale, I mean. Additionally, intolerance of wet conditions (i.e. "winter wet" or poor drainage) is often misconstrued as lack of hardiness.
Apart from the 1990 USDA Hardiness Zones map being inaccurately drawn in the mountainous west, where unlike in the eastern states winter climates do not occur in nice tidy waves with the level of cold evenly distributed from top to bottom and left to right, the common problem with using them is inaccurate interpretation and representation of the temperature ranges assigned to each. Often if a plant kills out between 0 and 10 degrees F. for instance it will then be said to be hardy to Zone 7. 0 to 10 degrees F. is not the range of record lows for Zone 7, it is the range of average annual minimum temperatures. Some years Zone 7 will get below 0 degrees F. During those years a plant hardy to between 0 and 10 degrees F. will be killed in Zone 7. And so on. Here in Zone 8 plants hardy to only 10 to 20 degrees F. are offered to us as suitable for our area by local outlets using information provided by wholesale suppliers making statements like "Hardy to USDA Zone 8, 10 to 20 degrees F." Canada has its own separate hardiness zoning system that is somewhat different from the USDA one, this could also be a source of confusion as the USDA hardiness zones are drawn over Canada as well.
