Are there plans to update the current USDA plant hardiness zone map?


The new “normal” 1990-2020 temperatures were recently released. Do you have any idea if plans are being made to update the current plant hardiness zone?

I live in Portland near the Fore River Estuary. It is definitely a warmer “microclime” even though I can see the runway of the Jetport (5b) from the house. The National Arbor Society has Portland at zone 5-6. As you know, zone 6 in Maine is currently midcoast, south York county and a narrow strip in Saco, OOB, and Scarborough (seacoast/marsh area). I did a quick research of lows in Portland from 1990 on up (ignoring the few +1/+2 degree days (coldest was -17). The Portland average low was -8 which would put us in the Zone 6a spot. I realize it is all relative and not etched in stone but was wondering if anyone else has “played: with the numbers. I’m under no illusion my southern magnolia (which lives inside Nov-April) will be planted outside but I do enjoy “pushing the envelope” with experiments.

Pics are the southern magnolia about to bloom and the frost proof gardenia which is heavenly right now. I’ve kept it in the garage (about 40/5c) and also the past few years in a cool room. Worth the work!


Liz Stanley, Horticulture Community Education Assistant

Thank you for your question about the USDA Plant Hardiness Zone Map.

The interactive map was last published in 2012. We hope to inquire about updates so that home gardeners and commercial growers will have access to more accurate information.

In the meantime, overwintering your beautiful southern magnolia in the garage is a good idea!