Decoding Hardiness Zones & Frost-Free Dates

As a community garden staff member, I have the privilege of visiting all ten of Garden City Harvest’s community gardens on a weekly basis. It’s great witnessing the progress people have made in their plots, and how the gardens are growing as a whole. Some plots are flourishing with starts nearly six inches high, whereas others have yet to sow seeds, having only tackled the task of cleaning out last year’s straw and prepping soil.  At this point, either tempo is acceptable, it’s the contrast, the personalities of the plots that is interesting. The past few weeks have been pretty dreamy here in Missoula, drawing us outside and together as a community. The anticipation as thick as the BBQ smoke which wafts through our sleepy neighborhood streets.  I say this while, at 4:26 PM on Monday May 9th, it’s currently 46 degrees and raining. It’s downright chilly and I very much doubt that many community gardeners are schlepping through the mud at the moment. Which is also, completely acceptable. Spring is peculiar, my least favorite season for its lack of predictability. Every year I’m surprised yet again that one day it can be so nice and another so frigid! And the juxtaposition only intensifies it. Given the spring flux, it’s a challenge to know just when to plant in Missoula.
So here we are going through the motions of a Missoula spring. Let us visit the United States Department of Agriculture (USDA) Hardiness Zone Map.

2012 USDA hardiness zoning map, courtesy of the United States Department of Agriculture,

And more specifically, Montana. Courtesy of

For those of you unfamiliar, “the 2012 USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones.”[1]  It took the USDA twelve years to update plant hardiness zoning from the previous 1990 map, with much impetus from plant scientists across the country. This raises concern given the reality of climate change and is certainly something to consider. Garden blogger Elizabeth Licata, writes more about this on Garden Rant.

The National Arbor Day Foundation creates more updated hardiness zone maps. The most recent is 2015, and provides an accurate representation of zoning given the changing climate in North America.



The most recent 2015 Arbor Day Foundation Hardiness Zones Map and changes in climate and effects on hardiness zones. Images courtesy of

Missoula falls in zone 5, and can handle certain plants of zone 4 and 6. This is something to consider when planting, as well as first and last frost dates. Make sure to check your seed packets for which zones the plants are best suited for – - this applies to fruits, veggies, trees, bushes and flowers.  The MSU Extension Service offers an interactive garden calculator, as well as many resources on weather in Missoula County and planting.

As Genevieve mentioned, in the Real Dirt’s past post on first frost free dates and scheduling, entitled “Community Gardens: Average Frost Free Date May 19th,” the first frost-free date marks a safe point to plant warmer season crops such as “cukes, melons, summer squash, winter squash, tomatoes, eggplants, and peppers.” We’ve had an especially warm spring this year, and rumors have floated through the gardening world of an earlier frost-free date. The first frost-free date is calculated on an average, based upon thirty year averages taken from the Missoula International Airport, and compiled by the National Climatic Data Center. With that in mind, the May 19th date is not set in stone, and allows some wiggle room for seasonal temperature variation. Consider using cold-covers for a few weeks during this interim period of fluctuating temperatures. And remember, despite all, plants are more resilient than we think.

For more information, visit:

The United States Department of Agriculture 

The MSU Extension Service

Arbor Day Foundation


[1]USDA Plant Hardiness Zone Map,” USDA Agricultural Research Service, accessed May 11, 2016,