Currant

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.

UMass Extension, 'Currants and Gooseberries' (New England Small Fruit Management Guide), umass.edu/agriculture-food-environment (read 2026-08-05): 'Some New England states regulate aspects of Ribes spp. production within their boundaries due to their role as alternate hosts of White Pine Blister Rust (Cronartium ribicola)'; 'This disease is the primary reason for limited commercial production of Ribes in North America'; 'Most black currants (Ribes nigrum) and white pines (Pinus strobus) are extremely susceptible'; resistant cultivars named include 'Consort', 'Crusader', 'Ben Sarek', and 'Titania'.

USU Extension, 'How to Grow Red Currants in Your Garden', extension.usu.edu/yardandgarden/research/red-currants-in-the-garden (read 2026-08-05): 'In the early 1900s, currants were banned in the northern United States to prevent the spread of white pine blister rust. However for most states, including Utah, the ban was lifted in 1966'; 'Most red currant varieties are actually immune to the disease, with black currant being much more susceptible.' USU gooseberry page: 'White pine blister rust requires both a Ribes species and white pine to complete its life cycle.'

Iowa State Univ. Extension, 'Growing Currants and Gooseberries in the Home Garden', yardandgarden.extension.iastate.edu/how-to/growing-currants-and-gooseberries-home-garden (read 2026-08-05): 'Several states, however, still ban the planting and cultivation of black currants'; 'Consort ... along with Crusader and Coronet, are resistant to white pine blister rust.'

UMN Extension, 'Growing currants and gooseberries in the home garden', extension.umn.edu/fruit/growing-currants-and-gooseberries-home-garden (read 2026-08-05): 'Currants, gooseberries and other plants in the Ribes genus play a part in white pine blister rust'; 'Blister rust is little more than a nuisance to the grower of currants, but can be devastating to white pine trees'; 'Planting remains restricted in some areas of the U.S.'; 'Crusader ... Immune to blister rust'; 'Titania ... immune to blister rust.'

Penn State Extension, 'Home Fruit Plantings: Gooseberry and Currant Variety Selection', extension.psu.edu/home-fruit-plantings-gooseberry-and-currant-variety-selection (read 2026-08-05): 'Black currant (Ribes nigrum) is the most susceptible, though resistant varieties are available'; 'Titania ... Has very high resistance to white pine blister rust.'

NOT PROMOTED TO VERIFIED (invariant 4): the quotes above were gathered and quoted, not yet confirmed against the live pages. Five extension sources agree on the host relationship, the state regulation, black currant's susceptibility, and the resistant-cultivar remedy; promotion is one source-read away, the other rules here path.