Let me start with the fact that I don't have a copy of 'Karl' yet. That said, you'll find differences in quoted heights and hardiness zones for virtually every plant you research, grass or otherwise.

First, zones. You can determine your zone yourself, and it's better if you do it, because you can amend it as your climate changes. Hardiness of a plant is usually measured by it's lowest tolerable temperature. You know, better than HCG or Bluestem, or the USDA, what your lowest temperature is. If it's hit (not averaged) -20 to -25°F during the past 3 years, you're in zone 4B. While you may have micro-climates in your garden that don't reach that low, you'd have to identify them (next to the house, etc..) before you could feel safe stretching the zone for a plant.

Both HCG and Bluestem have excellent reputations, but since Bluestem is really a specialist in grasses for cold climates, I'd believe their hardiness zone claims on grasses. My impression is they have reports from customers who are in the zones they claim, and are successfully growing the grasses.

Heights are another issue altogether. One of the reasons various resources vary in their estimates is some of them quote to the foliage and some to the plumes. Bluestem tells you both. That's another thing I like about them. Keep in mind, however, that grasses respond to their immediate conditions, just like any other plant. If they're happy, they maximize and even exceed their quoted dimensions. If they aren't, they don't. Shorter seasons, less sunlight, more or less water and nutrients will affect the way they grow. My advice would be if you have a very inflexible vacancy in the garden, don't fill it with a grass that is quoted right up to the dimensions of that slot. Pick a smaller one.

I haven't looked at either web site but one of the things you have to look for between canadian and US nurseries is what zone designations they use.
USDA and Canadian Zone maps differ by one zone designation. ie, a zone 5 in the US is a zone 6 in canada.
That said, many canadian nurseries use the USDA zones. Usually they will say which they use .,....

As for height ~ Donn is right on with what he says. I looked at your photos and I don't imagine 12 inches will matter one way or the other. Most of Karl Foesters height is in the bloom and that is not overpowering.
A.

I'm in zone 2B, and several clumps of Karl have proven hardy for a number of years in my yard in unprotected locations. So has Overdam and my newest Calamagrostis, Eldorado.

Karl with blooms approaches about 5.5' in height, as does Eldorado. Overdam is shorter, around 4'.

Good one Pudge!
I forgot to mention that ... Ornamental grasses are still relatively new and in most cases nurseries use zone ratings that are available. They don't talk to people like Pudge in the colder zones. We have such a valuable resource here on the OG forum.

So .. the moral of the story is; when in doubt try pushing at least one zone. AND ... ask someone on the OG Forum :o)

nb, zones were originally created for woody plants and have been extended to include herbacious plants whose zones can be stretched by choosing the right location or by mulching.

A.

As far as figuring out my zone based on my own experience, that may take a while. We just moved here at the end of March, so I haven't lived here through a winter yet. Though the guy at the tree farm I went to did say he has seen it get down to -32.

I am hesitant to "push one zone" as I do not want to plant things now, then have to replace them in the spring. Landscaping our entire yard has been quite expensive, so I want to be as cost efficient as possible. When I went shopping for perennials on the budget me and my husband agreed on, his comment upon seeing my purchases was, "That's all you got?!" I tried to console myself by telling myself that it will look much better once everything is blooming next year :)

I know that I can save money if I start everything from seed, but I am soooo tired of looking at a dirt yard that I need to see something growing!

One good thing about buying and planting in the fall is, you can get some great deals....most want to get rid of their stock before the real cold hits and for a reasonable price you can secure some stellar plants.

Blackie