Reference
Data & methods
The plant pages keep it plain: your zone now, and where it is headed. This page is for anyone who wants the logic behind the numbers.
The simple rule
Each plant page shows Now plus one selected future window. The default future is 2050, represented by the 2041-2070 climate window. The alternate late-century view is 2100, represented by 2071-2100. These are 30-year climate normals, not exact single-year forecasts.
The future number is always calculated the same way: today's best baseline plus the projected change. If Plotwright cannot make that comparison honestly, it does not show the future value yet.
Winter hardiness
A hardiness zone is set by the winter cold a plant has to survive. In the United States, Now uses the USDA Plant Hardiness Zone Map. In Canada, Now uses Natural Resources Canada plant hardiness zones. Outside those official map areas, Plotwright labels the value as derived instead of official.
For future hardiness, Plotwright keeps the Now baseline and adds the projected hardiness-zone shift for the selected future window. That keeps the present-day standard and the future estimate on the same footing.
Heat tolerance
Heat uses the AHS Plant Heat-Zone framework: average annual days above 86 F (30 C). That is a heat-frequency measure, different from winter hardiness and different from peak summer intensity.
Now AHS heat-zone values are Plotwright-computed from observed daily maximum-temperature data. They use the public AHS day-count definition; they are not copied from the official AHS map.
Future AHS heat-zone values follow the same rule as hardiness: observed Now plus the projected change in hot days. When that comparison cannot be made consistently, the plant page shows the current heat zone only.
Technical sources
Published Now values come from USDA and NRCan where those standards cover the location. Derived Now values come from open climate data. Future shifts come from climate-model ensembles, so one model's quirks do not dominate the result.
The technical source stack includes CHELSA, CHELSA-W5E5, and NASA NEX-GDDP-CMIP6. Future windows use the SSP3-7.0 scenario: a sober middle-to-high emissions pathway, not the worst case.
Why some values are marked derived
Not every country publishes the same kind of plant-zone map, and heat zones are not published globally as a current machine-readable baseline. When Plotwright computes a value itself, the card says so. When a future value would not be comparable to the current baseline, the card shows the current value only.
Glossary
Hardiness zone
A winter-survival zone based on the coldest temperatures a plant is expected to endure. Lower numbers are colder.
USDA Plant Hardiness Zone
The standard United States hardiness scale, based on average annual extreme minimum winter temperature.
NRCan Plant Hardiness Zone
Canada's plant hardiness system from Natural Resources Canada. It combines several climate variables and is the standard Canadian reference.
AHS Plant Heat-Zone
A 1-12 heat-frequency scale from the American Horticultural Society, based on average annual days above 86 F (30 C). Plotwright computes location values from open climate data.
Climate model
A computer simulation of the climate system. Plotwright uses model ensembles so one model's quirks do not dominate the result.
SSP3-7.0
A standard future-emissions pathway with slow, uneven climate action. Plotwright uses it as a sober default rather than a best or worst case.
Baseline vs projection
Baseline is the current climate reference value. Projection is the selected future climate window. When sources differ, Plotwright anchors to the baseline and adds the modelled change.
Sources
USDA Plant Hardiness Zone Map (2023).
Natural Resources Canada Plant Hardiness Zones.
AHS Plant Heat-Zone framework and day-count definition.
CHELSA v2.1 and CHELSA-W5E5 daily tasmax.
NASA NEX-GDDP-CMIP6 daily downscaled climate projections.
Plotwright climate baseline and projection contract.