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I live in Florida, but I have a friend who lives in Evensville, TN. She would like to know if she can leave canna, Elephant Ear, and ginger rhizomes in the ground over the winter, or should she dig them up and store them.
I have 'Bengal Tiger' cannas that stay in the ground year-round.
When we moved here, there were elephant ear (just the plain-jane green "taro" kind) left in the ground. I don't leave my purple-stem or fancy varieties in the ground - they're in pots, and they go in my GH over the winter, along with a pot of the canna just in case we get a really nasty winter (heavy rains followed by several days of freezing temps could do them in from winter rot more than cold temps.)
I don't think gingers can be left in situ either, especially if Evensville is veering anywhere near the Cumberland Plateau area, which is colder than we are. (We're due west of the Plateau, and northwest of Chattanooga.)
I am in zone 6b on the Cumberland Plateau. An hour and half drive from Chattanooga. My friend leaves her EE in the ground and has for the past 10 years. She has hers up against her house. i was surprised when I first saw it. I did not think they would survive some of our winters. The house may have created a micro-climate.
I have kept my cannas outside for the past 2 years with no problems.
Again, we have had 2 mild winters. Occasionally we have had winters where we should have been rated in zone 5.
I'm 7b and the cannas are almost invasive. So thick that they can't be dug and given away quick enough. I've been told NOT to even try to leave the ginger in the ground, that I have to dig it. I have one variety of elephant ears that had about naturalized on the side of the house. Found them when we cleaned out the overgrowth of wisteria and roses. They'd been there for at least 2 winters without our knowledge. I have other varieties that I've started, but will be bringing those inside. I'd rather get enough to seperate them before I try them in the ground. I'd rather have the backup overwintered in the garage, just in case.
Several of my neighbors have the plain EE's that stay in the ground year-round. I think the plain species is pretty hardy.
I don't take chances with my others, since they were a little harder to come by, but two big pots of them survived getting both dry and cold in my GH last winter (I lost a bunch of plants when we tripped a breaker during a particularly cold spell and knocked out my heaters.) I had all but given up hope on the pots of EEs - they looked dead as doornails this spring. But I tossed 'em in my bog filter and they started sending up new shoots within a few days.
I am in z7 about 50 miles east of Memphis. My hedychiums, cannas, and curcuma are planted in the ground and come back every year. The curcuma are in a bed by the house. The others are spread throughout the garden but are still somewhat sheltered by trees and a fence. In my experience rotting from excessive moisture when the plant is dormant can be a bigger problem than low temps, especially EE's. The Colocasia EE's like Illustris, Chicago Harliquen and Black Magic overwinter fine in the ground but I dig the Alocasia, like Hilo Beauty and Frydek varieties and the Xanthosoma varieties, because they are not hardy in this zone. This link can help with ID's on your EE's as to their type. http://www.agristarts.com/colocasia_main.htm
There is good information on the hardiness of marginal plants at the following site. I have never bought from tis nursery so cannot recommend their product one way or the other. But the owner has a special interest in the hardiness of marginal plants and has some good antedotal information about varieties that will overwinter outside under certian conditions.
The older I get the less tropical plants that need protection in the winter appeal to me. So anything that can be left outside for the winter is great. As others have pointed out your friend probably has different issues to deal with, like elevation, even if the zones are the same. Hope this information helps.