Ottawa's last and first frost dates

According to various sources:

Last frost:

May 1st to 10th (http://www.plantmaps.com/interactive-ontario-last-frost-date-map.php)
May 6th (http://www.veseys.com/ca/en/learn/reference/frost/canada)
May 7th (http://www.almanac.com/content/frost-chart-canada)
May 11th (http://www.omafra.gov.on.ca/english/crops/facts/climzoneveg.htm)


First frost:

October 1st to 10th (http://www.plantmaps.com/interactive-ontario-first-frost-date-map.php)
October 1st (http://www.omafra.gov.on.ca/english/crops/facts/climzoneveg.htm)
October 2nd (http://www.almanac.com/content/frost-chart-canada)
October 5th (http://www.veseys.com/ca/en/learn/reference/frost/canada)


Also came across these good articles about frost dates:  http://veggiepatchreimagined.blogspot.ca/2012/05/last-frost-date-watch-and-planting-out.html
http://ottawahortiphilia.blogspot.ca/2007/04/my-date-with-last-frost.html
https://ai731.wordpress.com/2012/03/20/spring-planting-dates-last-frost-and-fox-terriers/