The USDA Plant Hardiness Zones Have Changed – Here’s Why That Matters

Let’s take a little trip back in time…

When I was a kid, I celebrated Thanksgiving with unusual aplomb. Part of it was recognizing the few days off from school as a prelude to a longer winter break, but the memories I hold most dear involve bundling up and taking a post-dinner hike into the woods.

Did you know the USDA Hardiness Zone map went through some changes a few years ago? Find out how this might affect your garden. |

An increasingly rare snowy Thanksgiving day. Photo by Matt Suwak.

Here I would skirt the tree lines separating farmers’ fields and stomp my way through the snow.

Eventually I’d find myself standing square in the center of a barren field, surrounded by yellow and gray grasses and bordered by leafless trees. Soon I’d hear that goofy and somehow soothing call of Canadian geese flying south towards warmer climates.

It was a favorite tradition, but it’s a rare happening to see snow on Thanksgiving nowadays. As a gardener, these climatic changes are vital to understanding the evolving state of our yards and our gardens.

Changes in the Garden

Those folks who’ve been gardening since the 2000s and before have probably noticed changes in growing patterns between then and now.

A rose facing the chill of the first snow |

Annuals that ought to be toast by September are blooming through to November. Just a few weeks ago, I witnessed very confused apple trees and forsythia pushing out a late-autumn bloom.

Sometimes it’s a sight of joy and excitement, and other times it’s a disconcerting observation when you’re wondering why your peppers are pushing new flowers in the first week of November. If you’ve seen these changes firsthand, you’re not alone.

Tomatoes not yet ready to be picked |

Larger harvests are a nice byproduct of warmer temperatures.

A Look at the USDA Growing Zone Map

The United States Department of Agriculture has been a reliable authority for determining what plants can grow where in the United States. It relies on decades of records of weather patterns to determine the average highs and lows of any specific area.

The desert rose in bloom |

Plants like the desert rose (Adenium) will be more likely to bloom unassisted in certain areas.

The USDA Plant Hardiness Zone Map’s origins stretch back to the 1960s, when Henry Skinner of the United States National Arboretum first devised a working map of plant hardiness zones. His original vision has transmuted into the USDA zones we’re familiar with today.

Back in 1990, a growing database of climatic records prompted the first major change to this map, and in 2012 the map was changed again.

But How Does It Work?

The plant hardiness zone map is separated into regions by average low temperature. The regions range from 1 to 13, with each single number broken into an “a” or “b” category.

Each digit between 1 and 13 represents a 10-degree difference in average low temperature, while the “a” or “b” narrows this temperature gap down to 5-degree increments.

An unexpected snowfall on a garden |

Unexpected snowfalls go hand-in-hand with fluctuating USDA climate zones.

For example, Zone 7 has an average low temperature of 0 to 10 degrees fahrenheit; 7a has a low of 0 to 5 degrees, and 7b has a low of 5 to 10 degrees. Zone 6 has an average low between -10 and 0 degrees, and zone 8 has an average low between 10 and 20 degrees.

The Climes, They Are A-Changin’

Although our planet is definitely getting warmer, this change in the USDA plant hardiness measurements is officially attributed to more accurate temperature measurements, and a system that takes into account a location’s closeness to bodies of water, elevation, and other micro-climate effects.

The warmer weather is arriving |

It’s getting warm out there!

However, just because a 6b is now considered a 7a, certain plants still can’t grow in these warmer locations. If you started your garden before 2012, these subtle changes in climate could explain why harvest times and dates of first and last blooms have shifted.

Canna lilies basking in the warm spring sun |

Sensitive to the cold, canna lilies may respond well to warmer climates.

As always, the gardener must take into account the natural habitat of any plant they wish to grow (read more about when full sun doesn’t really mean full sun). Some plants thrive in hot weather, but also demand a humid and moist environment.

So, the next time you’re shopping for plants and reading the tags like a dutiful gardener, keep in mind that the climes, they are a-changin’.

Dry weather can make life hard for a plant |

Longer summers and warmer weather can exacerbate weather patterns on a large-scale basis, like the prolonged California drought in 2016.

For a great tool to see exactly what hardiness zone you live in, visit the USDA Plant Hardiness Zone Map website and pop in your zip code to find out. Keep these changes and this map in mind when planning and planting your first vegetable garden.

Watering the garden |

Hotter weather means extra watering.

What differences have you noted in your garden in the last few years with the consistently warmer weather? Share in the comments!

Snowy road photo by Matt Suwak, © Ask the Experts, LLC. ALL RIGHTS RESERVED. See our TOS for more details. Uncredited photos: Shutterstock.

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