Hi! I am thinking about purchasing a couple of Myrtus communis. Temperature in winter here usually stays between -5C and +3C. But, sometimes fall to -10C and one year fell down to -20C for 2 days, but this was only one time in many years.

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Now, seems like nobody agrees with the temperature supported by the Myrtus communis! :-)

https://www.rhs.org.uk/plants/details?plantid=1305 says hardiness is -5C to -10C (H4).
http://fr.hortipedia.com/wiki/Myrtus_communis it says that it is USDA Zone 10.
http://www.gerbeaud.com/jardin/fiches/myrte.php states -10C

In comes the vendor and tell me the plant should stay inside the house at +7C.

Well, anybody has experience with wintering the Myrtus communis outside home and in below 0C temperatures? :-)