How well do your plants tolerate cold temperatures?

Gardeners often talk about plant hardiness. This term refers to a plant’s ability to tolerate cold temperatures. How do you know which plants will survive in eastern Idaho?

A good starting point is to understand the USDA Hardiness Zones. For several decades the United States Department of Agriculture has been tracking weather and climate information across the country.

While earlier maps were created, the USDA hardiness zone map was first published in 1960. The hardiness zone is based on the average coldest temperature of a given region over the previous 30 years. The most recent update to the map was in 2012. What began as ten hardiness zones has become 13 zones, each with an ‘a’ & ‘b’ level, making a total of 26 hardiness zones on the most recent map with a 5⁰ F average coldest temperature range for each zone (see the map).

This information is most pertinent when growing perennials, woody and herbaceous (die down to the ground each winter). The zones in Idaho range from 3b to 7b. Eastern Idaho zones range from 3b to 6a (maybe 6b along the rivers).

Local application

Much of eastern Idaho is labeled as zone 5 (a or b). Keep in mind that these are based on the average coldest temperatures. Much of eastern Idaho will get much colder than the average lowest temperature indicated on the map. (The year I got married the temperature dropped to at least -35⁰ F in Rexburg.)

Trees, shrubs and perennials that are tested to zone 5 will often fail in those years when temperatures get very cold. This is one of the reasons that many peaches and sweet cherries, although rated for zone 5, will fail in areas north of Pocatello. This is also why many herbaceous perennials are grown as annuals in eastern Idaho.

Microclimates

Municipal areas will often experience urban heat island effects. Yards and gardens in the city may be a half zone higher than surrounding county gardens. Protection from wind may also improve a marginal plant’s ability to survive what would otherwise be too cold. A south-facing fence or wall may absorb and reflect heat to help less hardy plants succeed.

My recommendation is that unless you have a protected, warm microclimate, or you are prepared to be disappointed, do not plant anything that is not at least rated for zone 4 in the Upper Snake River Valley and zone 5 in Bannock and Power Counties.

Understanding your hardiness zone is an important first step to gardening. Other factors that may determine how well, or if, a plant may thrive in your area are growing season temperatures, season length, precipitation, wind and soil characteristics.

Horticulture Educator Ron Patterson can be reached at (208) 529-1390.