Ruibarbo

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 (amended 2026-09-24). The mechanism is named, but the corpus carries ONE band per species where the source carries one per cultivar, so the warm limit is explained without being precise. That is why, since 2026-07-26, the planner treats the warm end as advice rather than a refusal: above a species' band it still offers the plant and says to look for a cultivar rated for your zone, never that nothing will crop. Every species band now records what its sources say, though most are still marked estimated.

UW-Madison / Wisconsin Master Gardener (Susan Mahr), 'Rhubarb, Rheum rhabarbarum', mastergardener.extension.wisc.edu/files/2018/04/rhubarb.pdf (read 2026-08-05): 'The leaves, however, contain a toxic amount of oxalic acid and are poisonous. The ensuing cramps, nausea and sometimes death from ingestion suppressed interest in the plant for about two hundred years.'

NDSU Extension, 'Dakota Gardener: Is Frost-Damaged Rhubarb Toxic?', ag.ndsu.edu/news/columns/dakota-gardener/dakota-gardener-is-frost-damaged-rhubarb-toxic (read 2026-08-05), names BOTH compounds and the edible/toxic split outright: 'The leaves contain oxalic acid which can damage kidney function'; 'recent scientific studies show the presence of other compounds such as anthraquinone glycosides that may contribute to the toxicity of the leaves'; 'Rhubarb stems (petioles) which resemble celery stalks are the edible portion of the plant because they contain only trace amounts of toxic compounds.' The SAME page is why the frost->toxin belief is excluded: 'A hard freeze is widely believed to cause the movement of toxic compounds from the leaves into the stems' ... 'I could not find any scientific studies to confirm or disprove this belief.'

U. of Illinois Extension (Good Growing), 'To rhubarb or not to rhubarb?', extension.illinois.edu/blogs/good-growing/2023-06-08-rhubarb-or-not-rhubarb (read 2026-08-05): 'Do not consume leaf blades, they contain large amounts of oxalic acid which can limit the body's ability to take up calcium.'

UMN Extension, 'Growing rhubarb in home gardens', extension.umn.edu/vegetables/growing-rhubarb (read 2026-08-05): 'Rhubarb leaves are toxic. Do not eat them.'

NC State Extension Gardener Plant Toolbox, 'Rheum rhabarbarum', plants.ces.ncsu.edu/plants/rheum-rhabarbarum/ (read 2026-08-05): Poison Severity 'Medium'; Poison Parts 'Leaves'; Toxic Principle 'Anthraquinone glycosides and soluble oxalates'; Poison Symptoms 'TOXIC ONLY IF LARGE QUANTITIES EATEN. Abdominal cramps, burning of mouth and throat, headache, weakness, nausea and vomiting, coma; may be fatal'.