Plant Hardiness and USDA Zones

Americans have a Plant Hardiness Map created by the USDA. It is based on the average minimum temperature - that is, "the lowest average temperatures that can be expected each year in the United States, Canada, and Mexico."

Canada also has a Plant Hardiness Index and, like so many things which are used in both Canada and the US, have almost identical names and serve the same purpose ... they're different.

There's no way to paraphrase this, so I'll quote them. The formula to come up with the Plant Hardiness Index is:

Y = -67.62 + 1.734X₁ + 0.1868X₂ + 69.77X₃ + 1.256X₄ + 0.006119X₅ + 22.37X₆ - 0.01832X₇

where:
Y = estimated index of suitability
X₁ = monthly mean of the daily minimum temperatures (°C) of the coldest month
X₂ = mean frost free period above 0°C in days
X₃ = amount of rainfall (R) from June to November, inclusive, in terms of R/(R+a) where a=25.4 if R is in millimeters and a=1 if R is in inches
X₄ = monthly mean of the daily maximum temperatures (°C) of the warmest month
X₅ = winter factor expressed in terms of (0°C - X₁)R_jan where R_jan represents the rainfall in January expressed in mm
X₆ = mean maximum snow depth in terms of S/(S+a) where a=25.4 if S is in millimeters and a=1 if S is in inches
X₇ = maximum wind gust in (km/hr) in 30 years

I do love this country of mine. LOL If you need to figure out what your Canadian Plant Hardiness Index is, look up your city at The Weather Network. It's easier than trying to decipher those colourful charts. The harshest is 0 and the mildest (over on Vancouver Island) is 8. Halifax, Nova Scotia is in Zone 6 and has almost two more warm months than Zone 5 Truro, Nova Scotia, but two less warm months than Zone 6 Vancouver, British Columbia!  I'm realizing I have to stop saying "zone" because it can't be used by itself.

According to Environment Canada, frost-free days are the days before and after which there is a 50% chance of a light freeze (0 to -2C). This is the temperature at which tender plants are killed.

City Frost-free period Dates (I've added some Hardiness Zones to show)

Athabasca, AB - 88 days - June 1 to August 29

Calgary, AB - 114 days - May 23 to September 15 (Hardiness Zone 3a)

Edmonton, AB - 138 days -  May 7 to September 23 (Hardiness Zone 3a)

Grande Prairie AB - 117 days - May 18 to Sept. 13

Lethbridge AB - 123 days - May 17 to Sept. 18 (Hardiness Zone 3b)

Medicine Hat AB - 128 days - May 16 Sept. 22

Peace River AB - 99 days - May 26 Sept. 3

Red Deer AB - 106 days - May 25 Sept. 9

Abbotsford BC - 177 days - Apr. 24 Oct. 18

Chilliwack BC - 216 days - Apr. 6 Nov. 9

Dawson Creek BC - 84 days - Jun. 5 Aug. 29

Kamloops BC - 156 days - May 1 Oct. 5

Kelowna BC - 156 days - May 4 Oct. 8

Nanaimo BC - 171 Apr. 28 Oct. 17

Nelson BC - 159 May 4 Oct. 13

Port Alberni BC - 159 days - May 8 Oct. 15

Prince George BC - 91 days - Jun. 4 Sept. 3

Prince Rupert BC - 156 days - May 9 Oct. 13

Vancouver BC - 221 days - Mar. 28 Nov. 5 (Hardiness Zone 6b)

Victoria BC - 200 days - Apr. 19 Nov. 5

Brandon MB - 105 days - May 27 Sept. 10

Lynn Lake MB - 89 days - Jun. 8 Sept. 6

The Pas MB - 112 days - May 27 Sept. 17

Thompson MB - 61 days - Jun. 15 Aug. 16

Winnipeg MB - 119 days - May 25 Sept. 22 (Hardiness Zone 2b)

Bathurst NB - 129 days - May 19 Sept. 26

Edmundston NB - 112 days - May 28 Sept. 18 (Hardiness Zone 3a)

Fredericton NB - 124 days - May 20 Sept. 22(Hardiness Zone 4b)

Moncton NB - 125 days - May 24 Sept. 27 (Hardiness Zone 4b)

Saint John NB -  139 days - May 18 Oct. 4 (Hardiness Zone 5a)

Corner Brook NF - 142 days - May 22 Oct. 12 (Hardiness Zone 4b)

Gander NF - 123 days - Jun. 3 Oct. 5 (Hardiness Zone 4b)

Grand Falls NF - 115 days - Jun. 2 Sept. 26 (Hardiness Zone 4b)

St. John's NF - 131 days - Jun. 2 Oct. 12 (Hardiness Zone 5a)

Halifax NS - 166 days - May 6 Oct. 20 (Hardiness Zone 6)

Kentville NS - 141 days - May 16 Oct. 5 (Hardiness Zone 5a)

Sydney NS - 141 days - May 24 Oct. 13 (Hardiness Zone 5b)

Truro NS - 113 days - May 30 Sept. 21 (Hardiness Zone 5a)

Yarmouth NS - 169 days - May 1 Oct. 18 (Hardiness Zone 6)

Aklavik NT - 76 days - Jun. 13 Aug. 31

Fort Simpson NT - 81 days - Jun. 3 Aug. 24

Yellowknife NT - 110 days - May 27 Sept. 15 (Hardiness Zone 0)

Barrie ON - 112 days - May 26 Sept. 16

Hamilton ON - 168 days - Apr. 29 Oct. 15

Kapuskasing ON - 87 days - Jun. 12 Sept. 8 (Hardiness Index 2a)

Kingston ON - 160 days - May 2 Oct. 10 (Hardiness Index 5a)

Kitchener ON - 139 days - May 11 Sept. 29 (Hardiness Index 5a)

London ON - 151 days - May 9 Oct. 8 (Hardiness Index 6b)

Ottawa ON - 151 days - May 6 Oct. 5 (Hardiness Index 4b)

Owen Sound ON - 155 days - May 12 Oct. 15

Peterborough ON - 124 days - May 18 Sept. 20

Sudbury ON - 130 days - May 17 Sept. 25 (Hardiness Index 4a)

Thunder Bay ON - 105 days - Jun. 1 Sept. 15 (Hardiness Index 2b or 3a, hard to tell)

Timmins ON - 89 days - Jun. 8 Sept. 6 (Hardiness Index 2a)

Toronto ON - 149 days - May 9 Oct. 6 (Hardiness Index 6b)

Windsor ON - 179 days - Apr. 25 Oct. 22(Hardiness Index 7a)

Charlottetown PE - 150 days - May 17 Oct. 14 (Hardiness Index 5b)

Summerside PE - 162 days - May 9 Oct. 19 (Hardiness Index 5b)

Tignish PE - 138 days - May 23 Oct. 9 (Hardiness Index 5b)

Baie Comeau QC - 109 days - May 28 Sept. 15

Chicoutimi QC - 133 days - May 17 Sept. 30

Montreal QC - 156 days - May 3 Oct. 7

Quebec QC - 139 days - May 13 Sept. 29

Rimouski QC - 139 days - May 13 Sept. 30

Sherbrooke QC - 100 days - Jun. 1 Sept. 10

Tadoussac QC - 141 days - May 13 Oct. 2

Thetford Mines QC - 106 days - May 28 Sept. 14

Trois-Rivieres QC - 124 days - May 19 Sept. 23

Moose Jaw SK - 120 days - May 20 Sept. 18 (Zone 3a)

North Battleford SK - 120 days - May 19 Sept. 17

Prince Albert SK - 93 days - Jun. 2 Sept. 4 (Hardiness Zone 2a)

Regina SK - 111 days - May 21 Sept. 10

Saskatoon SK - 116 days - May 21 Sept. 15

Weyburn SK - 112 days - May 22 Sept. 12

Yorkton SK - 110 days - May 23 Sept. 11

Dawson YT - 62 days - Jun. 13 Aug. 17 (Zone 0a)

Watson Lake YT - 91 days - Jun. 2 Sept. 4 (Zone 0a)

The area where we're looking at land, by the way, is in Zone 5a with a frost-free period of 120-130 days.