3 ms·

I'm really curious about the taxonomy of these zones. Why are there 13 zones with an 'a' and 'b' zone for each number? Why not just 26 zones?

bombcar 2y ago
The Plant Hardiness Zone Map (PHZM) is based on the average annual extreme minimum winter temperature, displayed as 10-degree F zones ranging from zone 1 (coldest) to zone 13 (warmest). Each zone is divided into half zones designated as ‘a’ and ‘b’. For example, 7a and 7b are 5-degree F increments representing the colder and warmer halves of zone 7, respectively. https://en.wikipedia.org/wiki/Hardiness_zone https://en.wikipedia.org/wiki/Hardiness_zone (It was originally the 13 bands, but as things got more detailed they realized splitting them made a difference, I suspect.)
hosh 2y ago
The “a” and “b” zones subdivides it. Zone 10 is tropical, and never gets frost. Each “a” and “b” represents a 5 degree range. You can also create microclimates, local areas where the zone effectively shifts up or down about 5 degrees. As an aside, there is a heat hardiness zone that rarely gets talked about.
- madcaptenor 2y ago
  You can find that if you scroll down the article far enough, or at https://ahsgardening.org/about-us/news-press/cool_timeline/heat-zone-map-developed/ https://ahsgardening.org/about-us/news-press/cool_timeline/h... - it's based on the number of days above 86 degrees F (30 degrees C). I would like to see these two maps overlaid on each other somehow.
VyseofArcadia 2y ago
Because it was initially divided into 13 zones. In 1990 they subdivided each zone into a and b zones. I'm not finding any rationale on why subdividing instead of renumbering, but it probably alleviated confusion at the time. If you're in zone 6b, but you're used to thinking of yourself as zone 6, you're still mostly right even if you forget whether you're in a or b. Easier to remember than suddenly you're in zone 13.
- jellicle 2y ago
  "This plant is zone 6" "Old numbering or new numbering?" "I don't know, just says zone 6" If you reuse your numbering system, you have to specify which version you're using. Using an all-new numbering scheme eliminates that.
  - justsid 2y ago
    Isn’t it the exact opposite? If it just says zone 6, it’s the old system because no a or b suffix. If the number space got doubled zone 6 would still exist but mean something completely different now. You can’t tell if it’s the old or new system at all because the first numbers all got re-used but with different meaning.

bombcar 2y ago

The Plant Hardiness Zone Map (PHZM) is based on the average annual extreme minimum winter temperature, displayed as 10-degree F zones ranging from zone 1 (coldest) to zone 13 (warmest). Each zone is divided into half zones designated as ‘a’ and ‘b’. For example, 7a and 7b are 5-degree F increments representing the colder and warmer halves of zone 7, respectively. https://en.wikipedia.org/wiki/Hardiness_zone https://en.wikipedia.org/wiki/Hardiness_zone (It was originally the 13 bands, but as things got more detailed they realized splitting them made a difference, I suspect.)

hosh 2y ago

The “a” and “b” zones subdivides it. Zone 10 is tropical, and never gets frost. Each “a” and “b” represents a 5 degree range. You can also create microclimates, local areas where the zone effectively shifts up or down about 5 degrees. As an aside, there is a heat hardiness zone that rarely gets talked about.

madcaptenor 2y ago
You can find that if you scroll down the article far enough, or at https://ahsgardening.org/about-us/news-press/cool_timeline/heat-zone-map-developed/ https://ahsgardening.org/about-us/news-press/cool_timeline/h... - it's based on the number of days above 86 degrees F (30 degrees C). I would like to see these two maps overlaid on each other somehow.

VyseofArcadia 2y ago

Because it was initially divided into 13 zones. In 1990 they subdivided each zone into a and b zones. I'm not finding any rationale on why subdividing instead of renumbering, but it probably alleviated confusion at the time. If you're in zone 6b, but you're used to thinking of yourself as zone 6, you're still mostly right even if you forget whether you're in a or b. Easier to remember than suddenly you're in zone 13.

jellicle 2y ago
"This plant is zone 6" "Old numbering or new numbering?" "I don't know, just says zone 6" If you reuse your numbering system, you have to specify which version you're using. Using an all-new numbering scheme eliminates that.
- justsid 2y ago
  Isn’t it the exact opposite? If it just says zone 6, it’s the old system because no a or b suffix. If the number space got doubled zone 6 would still exist but mean something completely different now. You can’t tell if it’s the old or new system at all because the first numbers all got re-used but with different meaning.