Each plant’s "Zone fit" shows two readings: your zone today, and your zone around 2050. The first is where your winters sit now. The second takes that same starting point and adds only the warming the climate is projected to bring to your area by mid-century.

Why add only the shift, instead of showing a climate model’s raw future number? The models read the typical winter month, not the single coldest night, so their absolute zone runs about 2–3 zones warmer than a true winter-hardiness zone. That offset is consistent over time — so the change a model projects between today and 2050 is reliable even though its absolute value is not. We keep that trustworthy shift and anchor it to your real present-day zone, rather than trusting the model’s biased baseline.

A hardiness zone is set by the coldest night you can typically expect each year — the temperature a plant has to survive. In the US and Canada we use the official published maps (USDA and NRCan) for "today".

Everywhere else there is no single official map, so we compute the true coldest night directly from daily climate data (NASA NEX-GDDP-CMIP6) rather than a seasonal average — which is the more accurate measure of what a plant must endure. For the 2050 trajectory we layer the projected warming on top, the same way as the US/Canada path.

Heat has two sides, and they answer different questions. Intensity is how hot the hottest part of summer gets. Frequency is how many days a year cross into real heat — the standard here is the number of days above 86 °F (30 °C), which is what the American Horticultural Society Plant Heat-Zone map measures.

A place can have a high peak but few hot days, or a milder peak with a long hot season — so we show both where we can, rather than collapsing them into one number. Current AHS heat-zone day counts are Plotwright-computed from CHELSA-W5E5 daily maximum-temperature data for 1981-2010. They use the public AHS day-count definition; they are not copied from the official AHS map.

Present-day and trajectory climate comes from CHELSA v2.1 (a high-resolution global climate dataset from the Swiss Federal Institute WSL). Current AHS heat-days come from CHELSA-W5E5 daily tasmax, also CC0. Daily coldest-night projections and the future AHS heat-day import path use NASA NEX-GDDP-CMIP6 — daily, downscaled projections averaged across five climate models.

Future numbers use the SSP3-7.0 scenario — a middle-to-high path that assumes slow, fragmented climate action. It is a deliberately sober default, not a worst case. All values are 20–30-year averages, so a single hot or cold year never moves them.

Published-authority "today" winter zones come from the USDA Plant Hardiness Zone Map (US) and Natural Resources Canada (Canada). Heat-zone numbers use the AHS Plant Heat-Zone framework, but Plotwright computes the location values from open climate data.

When two numbers look like they disagree, it is usually because they answer different questions — not because one is wrong. "Coldest night" vs "typical winter", or "how hot" vs "how often". Rather than hide that behind a single averaged figure, we show both and say plainly what each one means. The honest comparison is more useful than false precision.

CHELSA v2.1 — Karger et al., Swiss Federal Institute WSL (CC0 1.0).

CHELSA-W5E5 daily tasmax — ISIMIP / CHELSA-W5E5 (CC0 1.0).

NASA NEX-GDDP-CMIP6 — Thrasher et al., NASA Earth Exchange (CC-BY-SA 4.0).

USDA Plant Hardiness Zone Map (2023); Natural Resources Canada Plant Hardiness Zones.

AHS Plant Heat-Zone framework — American Horticultural Society day-count definition.