The same species behaves differently by cultivar group, so the planner resolves size, timing and support against the group rather than the species.
American Grape — hardiness zone 4–8
European Grape — soil ph 6–7, hardiness zone 6–10
Which of these numbers are soft
Every number above carries a confidence tier. These are the ones the corpus does not stand behind — estimated from general knowledge, or contested between sources. They are shown because hiding them would be the dishonest half of showing the rest.
Hardiness zone is the correct variable for perennials, and only for perennials.
suboptimal if ignored → advise
Mechanism: 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.
THE CEILING IS A SECOND AND DIFFERENT MECHANISM, and until 2026-07-26 this rule enforced it without naming it. A zone too WARM does not kill a perennial; it fails 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. Until ISSUES #13 was decided the band was enforced at both ends; now (option E) only the cold end gates — it alone follows from the definition of the zone system — and the warm end is carried as an ADVISORY, not a refusal.
AND THE WARM END IS A CULTIVAR'S ANSWER, NOT A SPECIES', which is the honest limit of what this rule can do. Chill requirement is published per variety and varies enormously within a species, so a species-level ceiling says "the modelled range does not reach your zone", never "no plant of this kind will crop for you". So the engine offers the plant above its band and advises choosing a cultivar rated for the zone rather than refusing it — the same move invariant 3 makes for soil, applied to chill.
Evidence: confirmed against 3 sources in the research-based literature.
the 3 sources
THE FLOOR IS TRUE BY DEFINITION and needs no citation: the USDA system defines a zone AS a band of average annual minimum winter temperature, so 'a plant colder-hardy than your zone survives your winter' is the definition restated. This rule was `verified_trivially` on that basis alone until 2026-07-26. THE CEILING WAS NEVER COVERED BY IT - a minimum-temperature definition says nothing about a plant failing for want of chill - and that gap is why ISSUES #13 exists. The pointer below closes it.
OSU Extension EC 1303, 'Growing blackberries in your home garden' (Bernadine Strik), catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/ec1303.pdf (read 2026-07-26). THE CEILING'S MECHANISM, STATED: '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.' A plant failing in a WARMER zone for want of chill - which is what this rule's upper bound enforces and what no source said until now. THE SAME PAGE MAKES IT A CULTIVAR PROPERTY: 'Even cultivars within the same type (trailing, erect or semierect) differ in fruit quality and flavor, appearance, tolerance to pests, cold hardiness, and plant longevity', and the remedy it gives is the one this rule's rejection text now repeats: 'Minimize risk by choosing types and cultivars that are adapted to your hardiness zone.' The page then publishes a table of sixteen cultivars EACH WITH ITS OWN ZONE RANGE, which is the strongest available evidence that a species-level band is an approximation. [read]
WHAT THIS DOES NOT SETTLE. The mechanism is now named, but the corpus still carries ONE band per species where the source carries one per cultivar, and 12 of 25 perennial zones have no citation at all. So the ceiling is explained without being precise. ISSUES #13 stays open on that, and the rejection text is deliberately phrased as 'the modelled range does not reach your zone' rather than as a prediction that nothing will crop.
Grapes fruit on new shoots that grow from last season's canes, and only a few well-placed canes are kept. Prune hard every dormant season - late winter, after the coldest weather but before the buds swell: removing about 80 to 90 percent of the previous year's growth is normal and necessary. Under-pruning is the common mistake and leaves a tangled vine with fewer flower buds and small, poor-quality fruit.
suboptimal if ignored → advise
Mechanism: Fruit forms only on shoots arising from last season's canes, so a vine left unpruned piles up old wood that no longer fruits while the crop crowds onto too many buds. Cutting back 80 to 90 percent of last year's growth concentrates the vine into a small number of well-placed, productive canes; prune too little and too many buds remain, so the vine over-produces small, poor fruit and becomes a tangled mass.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
Univ. of Illinois Extension, 'Pruning and Training of Grape Vines', extension.illinois.edu/small-fruits/pruning-and-training-grape-vines (read 2026-08-22): 'Grapes flower and produce fruit only on one-year-old canes'; 'Proper pruning often results in the removal of 80 to 90% of the wood'; 'Prune after the coldest part of the winter is past, but before the buds begin to swell. February and early March are usually the best times in Illinois'; 'Most gardeners do not prune severely enough'; 'most home arbors soon become a tangled mass of vines because of failure to prune heavily enough.'
Univ. of Maryland Extension, 'Training and Pruning Grapes', extension.umd.edu/resource/training-and-pruning-grapes (read 2026-08-22): 'Typically, 90 percent of the previous year's growth is removed during dormant pruning'; 'Failure to prune hard leads to fewer flower buds and small, poor-quality fruit. Insufficient pruning can also trigger a biennial bearing pattern, where plants skip a year between harvests'; 'Prune in March so you can determine the amount of winter damage'; 'Fruits form on second-year wood ...'; 'Grapes produce fruit clusters on canes that are two years old.'
remedy
Prune while the vine is fully dormant - after the coldest part of winter, before the buds begin to swell, around February to March. Remove roughly 80 to 90 percent of the previous year's wood, keeping a few strong, well-placed canes (and renewal spurs) according to your training system. Do not under-prune.
Another 114 rules in the corpus apply by situation rather than by species — rotation intervals, frost windows, sun, spacing and soil. Read the full evidence page.
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.
