July 5, 2020

What’s Your Zone?

ME zone map from USDA• By Janine Pineo •

The USDA has issued the first update to the USDA Plant Hardiness Zone Map since 1990.

There are some significant changes, including portions of coastal Maine now considered Zone 6, which is comparable to living in New Jersey in the 1990 map. Comparable for temperatures, that is.

To the uninitiated, the zone map is a quick way to determine whether that posie you bought on impulse would survive a Maine winter. But not just any Maine winter. If you live in Aroostook County, that plant labeled Zone 5 likely will succumb to the cold, while if you live in, say, Bar Harbor, the plant will likely be OK since Bar Harbor is now considered Zone 6A.

As for Aroostook County, the shift has been warmer, too, with the 1990 Zone 3b designation now mostly changed to the 4a rating.

Meaning Maine is warmer than it used to be. And the state is not alone, for the USDA has added Zones 12 and 13 to the map. That means the lowest average minimum temperature in those areas was either 50-60 degrees (Zone 12) or 60-70 degrees (Zone 13).

Granted, those places aren’t in Maine, but it is a sign of a warmer world.

Here’s how the mapping works. Each zone is a 10-degree Fahrenheit range. And then each zone is further split into 5-degree bands A and B, with B being the warmer of the two.

According to the USDA announcement, “Plant hardiness zone designations represent the average annual extreme minimum temperatures at a given location during a particular time period. They do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time.”

The USDA explains the general trend in the temperature shift: “Compared to the 1990 version, zone boundaries in this edition of the map have shifted in many areas. The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States. This is mostly a result of using temperature data from a longer and more recent time period; the new map uses data measured at weather stations during the 30-year period 1976-2005. In contrast, the 1990 map was based on temperature data from only a 13-year period of 1974-1986.”

But there were also other influences in the changes. First, more “sophisticated methods” for calculating between established weather stations were employed. The USDA wrote that these “include algorithms that considered for the first time such factors as changes in elevation, nearness to large bodies of water, and position on the terrain, such as valley bottoms and ridge tops.”

Plus, there are more weather stations providing more temperature data.

With all that said, how do you use the zone map?

With a grain of salt.

Since these are averages based on data from weather stations that may not even be located in your vicinity and from formulas based on elevation and so forth, these zones are not 100 percent accurate. An average also means there was a higher number and a lower, and it’s the lower number you need to worry about.

Zone maps are helpful in getting you in the ballpark, but it is important to be aware of the microclimate in your area. Plants growing against the foundation of your house have a warmer microclimate than a spot on top of an open hilly area.

The USDA release does address this with a sage piece of advice: “If your hardiness zone has changed in this edition of the USDA Plant Hardiness Zone Map (PHZM), it does not mean you should start pulling plants out of your garden or change what you are growing. What is thriving in your yard will most likely continue to thrive.”

The agency redeems itself somewhat with this comment: “You also could have pockets within your garden that are warmer or cooler than the general zone for your area or for the rest of your yard, such as a sheltered area in front of a south-facing wall or a low spot where cold air pools first. No hardiness zone map can take the place of the detailed knowledge that gardeners pick up about their own gardens through hands-on experience.”

And that’s the point.

Who uses the zone maps?

From the USDA: While about 80 million American gardeners, as well as those who grow and breed plants, are the largest users of the USDA Plant Hardiness Zone Map, many others need this hardiness zone information. For example, the USDA Risk Management Agency uses the USDA plant hardiness zone designations to set some crop insurance standards. Scientists use the plant hardiness zones as a data layer in many research models such as modeling the spread of exotic weeds and insects.

US zone map for the lower 48 from the USDA