As I actually grow relatively few species, my experience is limited for the Cologne area. We are typically a Zone 8, but the last two winters have been severe with temps dipping down to -15°C for a week at a time with little snow cover. Enough to test the hardiness of some plants.
I found that Louisiana species cannot take this in the ground, but have no problem in water. I. fulva dies at -8°C in the ground, as do most hybrids. The soil is sandy clay and stays quite wet during the freeze-thaw cycles.
I. confusa and I. japonica do not survive any of my winters. The wet freeze-thaw does them in, I think.
Pacific Coast species, such as I. douglasiana and I. tenax have not trouble. I have a few more seedlings currently frozen in ice that look fine during the brief thaws.
I have yet to loose any aril species, despite the wet winters, but I only have I. sari, I. elegantissima, I. acutiloba, I. stolonifera, I. hoogiana, I. korolkowii, I. lineata and I. meda at the moment. These plants are in pots on a protected, south facing balcony.
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Jamie V.
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Köln (Cologne)
Germany
Zone 8
