Minus seventeen degrees C corresponds approximately to 0 degrees F; that would put you in USDA zone 7, generally cited as minimum temperature 0 degrees F to +10 degrees F. If your winter temperatures regularly drop to minus seventeen degrees, it makes better sense to consider your site a zone 6 site. Jim McKenney Montgomery County, Maryland, USA, USDA zone 7, where temperatures as low as 0 F or minus seventeen C are fortunately experienced only once in many years.