How well do your plants tolerate cold temperatures?

The following is an encore edition of “In the Garden.”

IDAHO FALLS – Gardeners often talk about the hardiness of plants. 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 place to start is understanding USDA hardiness zones. For several decades, the U.S. Department of Agriculture has tracked weather and climate information across the country.

While previous 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 past 30 years. The last update to the map was in 2012. What started with ten hardiness zones has become 13 zones, each with levels “a” and “b”, making a total of 26 hardiness zones on the most recent map, with 5⁰ F being the average coldest temperature range for each zone (see map).

This information is most relevant to growing perennial, woody and herbaceous plants (which die back to the ground each winter). Zones in Idaho range from 3b to 7b. Zones in eastern Idaho range from 3b to 6a (perhaps 6b along the rivers).

Local application

Much of eastern Idaho is designated as Zone 5 (a or b). Please note that this information is based on the coldest average temperatures. Large portions of eastern Idaho will be significantly colder than the average low temperature shown on the map. (The year I got married, the temperature in Rexburg dropped to at least -35°F.)

Trees, shrubs and perennials tested under Zone 5 often fail in years when temperatures get very cold. This is one reason many peaches and sweet cherries, despite being rated for Zone 5, fail in areas north of Pocatello. This is also why many perennials are grown as annuals in eastern Idaho.

Microclimate

Urban heat island effects are common in urban areas. Courtyards and gardens in the city can be half a zone higher than the surrounding district gardens. Protection from wind can also improve a border plant’s ability to survive when it would otherwise be too cold. A south-facing fence or wall can absorb and reflect heat to help less hardy plants thrive.

My recommendation is that unless you have a sheltered, warm microclimate or are prepared to be disappointed, do not plant anything that is not approved for at least 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 in gardening. Other factors that can determine how well or whether a plant can thrive in your area include growing season temperatures, season length, precipitation, wind and soil characteristics.