In November 2023, the United States Department of Agriculture released a revised, updated map of plant hardiness zones. The revised map may have implications for Butler County gardeners when purchasing this season’s plants and adding new cultivars to your landscape.

You may be familiar with plant hardiness zones from reading seed and plant catalogs and plant tags tucked in the pots of plants purchased at nurseries. Plant hardiness zones are specific to cold and can be used as a guideline for determining the tolerance of a perennial plant to cold weather and injury from the cold. Periods of cold weather during fall when plants are just going dormant, and spring when plants are just emerging from dormancy, can injure, weaken, or kill plants not able to withstand those changes. Prolonged, severe cold in winter also can damage twigs, roots, and buds.

Plant hardiness zones are a standardized risk management tool used by gardeners and nursery growers for deciding which plants will survive the winter in their geographic area. Plant hardiness zones are especially useful for growing horticultural varieties whose genetic lineage may not be adapted to the region in which they are grown. Conversely, if you grow native plants, referring to a plant hardiness zone map is not necessary because native plants are adapted to tolerate the regional growing conditions where it is native.

Plant hardiness zones are created using averages of the lowest annual winter temperatures recorded at specific locations every year over 30 years. The maps were first created in 1927 and over time, have evolved to include 13, sequentially numbered zones with each zone representing 10 degrees. Sub zones of 5 degrees were added to the maps in 1960 and designated with “a” or “b.” Map revisions in 2012 standardized map colors for zones and sub zones and used computer modeling.

Since 1927, the data collected have become more accurate with additional records from more weather stations, improved computer modeling and finer scale resolution. The data are technically reviewed by climatologists, meteorologists, and nursery owners among others. The new 2023 maps are now online and feature customizable layers allowing a user to search by zip code for their specific hardiness zone.

With the revision of the map in 2023, more than half of the United States moved into the next warmer half zone. For example, much of Zone 6a area shifted to Zone 6b. In Butler County, however; although the 30-year average lowest annual winter temperatures for our area increased +3 degrees, we did not shift sub zones and remain designated as Zone 6. The northern and central portions of the County are within sub zone 6a:  -10 degrees to -5 degrees and the southern portion of the County is sub zone 6b:  -5 degrees to 0 degrees.

What do these designations mean for Butler County growers? Since the zone/sub zone designations did not change, there should not be any adjustment for growing plants if the plant tag determines that the plant is hardy to Zone 6. Read plant tags, noting that even though the hardiness map zones revisions were made in November 2023, the plant tags for this growing season may not be updated. Also, be aware that if you find a new plant for 2024, the plant tags may not be as accurate as those for older, established plants that have a longer history of growing data behind them.

Given that the lowest annual winter temperature average is 3 degrees higher than it was at the previous map revision in 2012, if you are a gambling gardener, you could consider trying to grow a few plants that would be “marginal” in our region. If you find something that is growable at Zone 7a for example, give it a try!

Just don’t invest your whole garden or landscape in marginal plants. Also bear in mind that cold hardiness is not the only condition to use when considering growing an out of zone plant: wind exposure, snow cover, slope, soil drainage, sun exposure, and radiant heat, such as heat from your home for example, will affect your success. Like the tools in your garden shed, the USDA Plant Hardiness Zone Map is another tool to help you be a successful gardener.

For more information on the USDA Plant Hardiness Zone Map revisions or to download a copy of the new map, please visit planthardiness.ars.usda.gov.

If you have questions about hardiness zones and growing plants, call the Master Gardeners of Butler County Garden Hotline at 724-287 4761, ext. 7, or email the Master Gardeners at butlermg@psu.edu.

Mary Alice Koeneke is a Penn State Extension Butler County Master Gardener.

Mary Alice Koeneke