Hardiness Zone Misconceptions: Notation, Boundaries, and Cross-Border Comparison
The letters are not a parallel A/B series that crosses somewhere — the number is the unit and a/b are just its cold and warm halves. Shared boundary numbers are dividing lines, not overlap. A slash like '5b/6a' is informal shorthand. Most consequentially, Canadian zones are not USDA zones despite identical-looking labels, so cross-border comparisons and plant tags mislead. The number is an extreme, not an average, and not a survival guarantee.
Corrections for the hardiness zone material, so the reference chunks can stay declarative. ## Reading the label **The letters are not a parallel series.** A natural misreading of "6b" and "9a" is that there are two independent scales — an A-series (A1, A2, A3…) and a B-series (B1, B2, B3…) — that might cross somewhere, so that "9a" and "15b" would be separate things. There is no such structure. The **number is the unit** and the letter is only ever the cold half (*a*) or warm half (*b*) of that one number. The letter never runs on its own track, never appears without its number, and never takes a value other than a or b. There is no "a9" distinct from "9a", and nothing like "15b" exists because the numbers only run about 1 to 13. Read it as *[number][half]* and the whole system is one ordered line: …5a, 5b, 6a, 6b, 7a, 7b… **Shared boundary numbers are not overlap.** Zone 6a runs −23 to −21 °C and 6b runs −21 to −18 °C. The −21 is not in both zones — it is the single dividing line between them, the same way "0–10" and "10–20" age brackets share the number 10 without anyone being in both. The zones tile the scale edge to edge with no gaps and no overlaps, which is a strength: no temperature falls into no zone. **A slash like "5b/6a" is informal shorthand, not a system notation.** It means a location sits close to the boundary between two adjacent zones — near enough that different data years or different parts of a city land on either side. Any given spot has exactly one zone. ## Cross-border comparison **Canadian zones are not USDA zones.** This is the error most likely to cause real damage, and it routinely goes unnoticed because the labels look identical. Canada uses its own multivariate system running 0a to 9a; the USDA system is based on minimum temperature alone. The offset is roughly one zone, with Canadian numbers running warmer. See Canadian Plant Hardiness Zones Are Not USDA Zones. The practical consequences: a plant tag rated by one system misleads under the other, and casual comparisons between a Canadian and an American zone number are not comparing like with like — the real temperature gap may be considerably larger than the numbers imply. **The system is not global.** The UK's RHS ratings and the western US Sunset zones use different criteria again. A zone number without its system is ambiguous. ## What the number means **It is an extreme, not an average.** Zone 6b does not describe typical winter temperature; it describes the coldest night of a typical year. Mild-winter places can have cold zones because of occasional hard nights. **It is not a survival guarantee.** The figure is an average of annual extremes, so an unusual winter can go below it and kill a correctly-zoned plant. It is a first-pass filter. **A shared zone does not mean similar gardening.** Summer heat, rainfall, humidity, season length and snow cover are all absent from the number. See What Hardiness Zones Don't Tell You. **Zone maps change.** The USDA map was rebuilt in 2023 and Canada's in 2025, both shifting many locations warmer. Older books, plant tags and websites may state a different zone for the same place, and neither figure is wrong — they are different vintages.