[Toronto6A6B]

Here's the cold spells stats for Toronto I found in Environment Canada's website.

Toronto

2003
Jan 17/-16.8
Jan 22/-18.5
Jan 23/-17.4
Jan 24/-14.7
Jan 26/-15.9
Jan 27/-20
Feb 11/-19.1
Feb 13/-13.6
Feb 15/-15.4
Feb 16/-18.2
Feb 17/-13
Feb 24/-12.8
Feb 25/-15.7
Feb 26/-13.7
Mar 3/-23.3

2004
Jan 9/-22
Jan 10/-20.7
Jan 14/-21
Jan 15/-20.9
Jan 16/-21
Jan 17/-13
Jan 20/-13.5
Jan 21/-13.7
Jan 23/-13.8
Jan 24/-17.2
Jan 25/-17
Feb 15/-16.8

2005
Jan 18/-21.8
Jan 20/-17.4
Jan 21/-22.7
Jan 22/-22.9
Jan 23/-20.2
Jan 24/-15.2
Jan 27/-20.3
Jan 28/-15

2006
Jan/Feb all above -13C

2007
Jan 26/-15.7
Feb 6/-13
Feb 13/-16.5
Feb 14/-14.7
Feb 15/-18.4
Feb 19/-14
Mar 6/-20.7

2008
Jan 3/-15.1
Feb 11/-16.7
Feb 27/-14.4
Feb 28/-16.5

2009
Jan 14/-19.1
Jan 15/-16
Jan 16/-15.3
Jan 17/-17.4
Jan 21/-14.5
Jan 24/-17
Jan 26/-13.2
Jan 31/-13.3
Feb 4/-14.7
Feb 5/-17.3
Feb 28/-15.1
Mar 3/-14.8

2010
Jan 2/-16.3
Jan 3/-14.6
Jan 9/-13.9
Jan 29/-14.8
Jan 30/-17.9

2011
Jan 17/-14.1
Jan 23/-18.2
Jan 24/-18.2
Jan 31/-16.2
Feb 8/-14.1
Feb 11/-14

2012
Jan 3/-14.2
Jan 15/-14.4

With global warming in mind and with South facing backyard and some protection, I think Toronto can grow windmill palms.
Note: Young windmill palms is hardy to only -8C according to Wiki; Mature windmill palms can survive colder temperatures, please check Wiki for details.