Hardiness zone is the correct variable for perennials
Hardiness zone is the correct variable for perennials, and only for perennials.
Well established A person has checked it against its sources. If you skip this, it still grows, just less well. The planner suggests it.
How it works
Zones encode average annual minimum winter temperature, which governs whether a woody perennial survives to spring. It is the wrong quantity for an annual, which never sees winter. A zone that is too warm is a second, different mechanism. Mild winters do not kill a perennial; they fail to chill it. Deciduous fruit crops need a winter dormancy period to break bud and set fruit, so in a zone milder than a plant is adapted to it may survive and never crop. Only the cold end is enforced, because only winter survival follows from what a zone measures; the warm end gets advice and is never refused. That warm limit is a property of the cultivar, and that limits what this rule can do. Chill requirement is published per variety and varies enormously within a species, so a species-level ceiling says "the modeled range does not reach your zone", never "no plant of this kind will crop for you". So above its range the planner still offers the plant, with advice to choose a cultivar rated for your zone.
The evidence
Evidence: checked against 1 source in the research-based literature.
The source
Winter cold-hardy cultivars adapted to the colder regions of Oregon (such as zones 5 to 7), may not grow normally in regions of the Willamette Valley (zone 8) where winters are warmer and plants receive less winter chill.
Plus 2 working notes on how the sources were read. Every note is in the data download.
Plants it applies to · 61
Fruit & berries · 33, Herbs · 13, Cover crops & soil builders · 5, Perennial vegetables · 4, Flowers · 4, Onions & garlic · 2
Fruit & berries · 33
Herbs · 13
Cover crops & soil builders · 5
Flowers · 4
Plant teams that rest on it
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How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- Wording made clearer
- Sources updated
- Wording made clearer
- Details updated
- Added to the rules
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Milpa Gardens. “Hardiness zone is the correct variable for perennials, and only for perennials.” Evidence grade: Well established. https://milpa.garden/rules/hardiness-zone-correct-variable-perennials-only-perennials/
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How we grade evidence
- Well established
- The rule names its mechanism and is settled, by sources a person has read and checked or by plain arithmetic and geometry.
- Promising
- The mechanism is named and the literature backs it. A few questions are still open.
- A good hunch
- Plausible by its mechanism, but field studies are thin, so treat it as a lean.
- Contested
- The evidence points both ways, or no study has tested it, so the planner does not enforce it.
- Refuted
- The claim did not survive its sources. It is kept so the folklore can be answered.