I bought a fan of Undefinable. Here is what the seller advised me to do with the plant,

"Let me share with you ... UNDEFINABLE will need and should be protected during the winter. Many up North have loss her due to the temps. Myself, I would plant UNDEFINABLE in a 10 gal squat pot and bring her into protection when freezing temps will occur. Or obtain a grow light and keep her in a protected area and use a grow light during the winter season. I would Grow UNDEFINABLE for a year or two (pretty fast grower), then divide her and test a fan (planted in Spring) growing in your winter conditions. I live in the South and I have to cover and protect all my UNDEFINABLE plants during our winter freezes or else I would loose them all. Hope this helps!"

Of course, I will follow her advice which I find it very helpful and sensible. Though I am just curious if there is anyone out there succeed in growing Undefinable outside in zone 6 or higher with just winter protection like heavy mulch? I am also trying to find out what temps may cause the plant's demise or weaken it that it struggles to grow the following year.

I have not bought as of yet. It is on my wish list. I spoke with the Hybridizer and asked that question since it would have been a hefty purchase for me. She said that it has been grown successfully up in the north (I am also in 6B) if fairs the best when your area has a good snow cover. If it is in an area that is very cold but no snow to insulate it and protect it from winds it doesn't do very well.
Hope this helps let me know how it does and what you decide to do. I think I am getting this coming year possibly.

Thanks Marcia! THe information is very helpful. I also posted on Facebook and got similar responses. Basically, it really needs a lot of protection to insure its survival through the winter. Some are able to grow it in the ground with heavy mulching in zone 5. As mine is a single fan, I am potting it up and growing it inside until spring. I got mine from a southern grower. There was rust showing on the foliage when I received it. In order to prevent its spread, I had to cut the foliage down to about 2 inches and then sprayed the whole plant with Daconil. I am planning to spray it every 3 days for the next 2 weeks and then once a week after that. It maybe overkill but better be careful than not. As it is a small plant, it does not require that much spray. Given what others have shared about growing it, I need to find the hottest and sunniest location in my yard for it. This is one daylily that likes plenty of sunshine and heat. THen in the winter, I will put heavy coverage of mulch around it.

Thanks for posting this info Karen. It's important for Northern growers to know which evergreens are hardy and which ones need special attention to survive. I wish you success keeping your Undefinable investment!