Plant Hardiness Zones

By Jan Cashman • Posted on February, 18th 2014

Plant Hardiness Zones—What do they mean?  Are they important?  By Jan Cashman 12/3/13

This winter, when you’re studying gardening magazines and seed catalogs to decide what to plant next spring, do you ever wonder about the “Zone” numbers next to plant names?   These numbers are supposed to tell us whether a plant will grow in an area.  The US Department of Agriculture has based these zones on average annual minimum temperatures during a period of years and put them on a map so anyone can easily tell what zone they live and garden in.

The USDA published their first Plant Hardiness Zone map in 1960 making 10 hardiness zones in the United States based upon 10 degree Fahrenheit gradients.  Then, in 1990 a major overhaul of the map was completed using temperature data from 1974 to 1986.  One new zone was added and the 10 degree gradients were broken down into 5 degree “A” and “B” zones, an improvement for us gardeners.  

In 2012 the USDA released a new map adding two new climate zones, 11 and 12. This map is available as an interactive GIS-based (Geographic Information System) for which a broadband Internet connection is recommended.  Users can type in their zip code when on the web site (planthardiness.ars.usda.gov) and find the hardiness zone for their area.  

Before the USDA did their 1990 overhaul, Bozeman was listed between hardiness Zones 3 and 4.  Because of warmer weather in the years 1974 to 1986, the revised 1990 map listed Bozeman as Zone 4B and we are still listed as Zone 4B on the 2012 version.

Bozeman and surrounding area gardeners and need to realize that these hardiness zones are only guidelines.  Temperature extremes, elevation, rainfall, humidity, length of growing season, and soil type  are not taken into account when determining these zones but are important when determining a plant’s ability to thrive in a certain area.

TEMPERATURE VARIATIONS:  Inland mountain climates have extreme temperature variations.  For instance, the Gallatin Valley might be 30 degrees below zero on a winter day and the next day, a Chinook wind will warm the air to 50 degrees.  These extremes can damage the tender buds of plants that have not fully reached dormancy.  On October 12, 2009, after a mild start to fall, the temperature dipped to a record low of 9 degrees.  The next spring many green ash, flowering crabs, and quaking aspen trees never leafed out.

RAINFALL AND HUMIDITY:  Bozeman’s average annual precipitation is 19.3 inches and Belgrade’s is 14.8;  humidity is low both summer and winter.  Evergreens such as white pine and balsam fir might be listed as Zone 3 but will not thrive here.  They grow better in locations where there is more humidity and winter cloud cover to protect their needles from winterburn.  

LENGTH OF GROWING SEASON:  In high elevations the growing season is short, fewer than 90 frost-free days in some places.   Some late-blooming perennial flowers listed as hardy in Zone 3 might grow OK in higher elevations but never bloom because the season is so short.  When planted at a high elevation, late season apples like Honeycrisp won’t have time to ripen.

SOIL TYPE AND pH:  The USDA hardiness zones do not take into account soil variations.  Many plants do not grow well in the heavy, poorly drained soils which are common to our area.  Plants such as blueberries, rhododendrons, hydrangeas, and even some maples, need acid soils to thrive, but our soils in the Valley tend to be alkaline, ranging in pH from 7.0 to 7.4, even higher in areas around Manhattan and Three Forks.

This winter, when you are planning your spring plantings, use USDA hardiness zones as a guide.  But also remember your soil type and our high and dry climate.  Make adjustments accordingly and your gardening will be more successful.