ONE OF THOSE WINTERS!
Having just discussed a weather map (zone what??) issue I ended up by commenting -- ALWAYS consider the plants originating habitat. My Zone 7 in the the Pacific Northwest is not the same as the Zone 7 of Oklahoma: neither in the length of the the cold or the humidity or lack thereof Add snowfall coverage and winter colds are mitigated. Then again what are your SUMMERS like?
Gunnera comes from the moist highland mountains of S. America and does well here in the Pacific Northwest. It probably will not do well in the Mid-West. On eBay someone asked me for a better climate zone map. I too would like that, but suspect that it simply doesn't exist. Zone maps are for regions and do not include altitude, length of winter temperatures, slope exposure, sun, wind etc. My Zone 7 plantings in Clallam Co. WA are mostly affected by altitude, secondarily because of tree shade. Our Zn. 7 has a winter beginning in Dec. and lasts until mid Feb. and usually relaxes after that. All told we have some 2.5 or three months of very cold weather.
The same Zone 7 in Oklahoma might have two extra months of Zone 7 weather. I suspect the best answer is to address the regional Farm bureau, your Ag extension office, some good nursery or even Wunderbrund that collects seasonal weather records for your zip code. Where I happen to live, there exist at least four totally different climate zones within just 15 miles.
Some variations are due to precipitation, altitude or proximity to the moderating effects of the ocean. No govt. map can provide such a detailed "google" footprint of climate zones. One customer of mine said it best. She was in her 70's and commented on my remark to different zones on our own properties. She said; "I just took all my clothes off and stood in different places in the garden." I loved it!
My own place has a five to ten degree difference in temperature depending on winter sun exposure and WIND. One other thing to mention when planting. Weather maps generally work on the lows. They neglect to factor in day heat degrees or even UV exposure. Denver could be 20 F. in the morning and rise to 75 F. in the afternoon. The back trunks of the trees will be frozen, while the south side warms up and the trunk splits between ... often causing the plant to die.
(C) Skyline Nursery.