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.
bloom window — estimated
ph range — estimated
hardiness zone — estimated
mature height — estimated
mature spread — estimated
spacing in row — estimated
frost tolerance — estimated
feeder class — estimated
pollinator value — estimated
Rules that apply to Blueberry
Well established
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.
Blueberries need strongly acidic soil, about pH 4.0 to 5.5. Above about 5.5 the soil is not acidic enough and the plant cannot take up iron, so the new leaves yellow between the veins (iron chlorosis). Test the soil before planting; if it is too high, lower it with elemental sulfur applied several months ahead, then re-test. A foliar iron spray only greens the leaves for a while and does not fix the soil.
suboptimal if ignored → advise
Mechanism: Blueberries are acid-loving plants adapted to low-pH soils. At soil pH above about 5.5, iron is not available to the roots even when it is present in the soil, and the plant develops interveinal chlorosis - yellowing between the veins - on new growth. Lowering the pH with elemental sulfur makes iron available again; spraying the leaves with iron greens them only temporarily and leaves the soil, and so the plant's long-term health, uncorrected.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
Univ. of Minnesota Extension, 'Growing blueberries in the home garden', extension.umn.edu/fruit/growing-blueberries-home-garden (read 2026-08-22): 'Blueberries require a pH between 4.0-5.5'; 'If the pH of the soil is over 5.5, then the soil is not acidic enough for blueberries'; 'Blueberry plants in soil with a pH above 5.5 will struggle to absorb the nutrients they need from the soil'; 'Chlorosis, or yellow discoloring of the leaves, is usually the first sign of a soil pH problem'; figure caption 'iron chlorosis caused by high soil pH'; 'Spraying plants with a foliar chelated iron fertilizer ... will temporarily green up the leaves, but it will not improve plant health in the long term'; 'It is best to amend the pH with sulfur the fall before planting, because it takes several months for sulfur to change the soil pH.'
Clemson HGIC, 'Blueberry', hgic.clemson.edu/factsheet/blueberry (read 2026-08-22): 'If the pH is above 6.0, select another planting site. If the soil pH is below 6.0 but above the 5.5 limit, apply wettable sulfur (90 percent sulfur)'; '... apply iron sulfate if needed to correct the yellowing of foliage'; 'Have a soil test taken in the fall before planting ... Any sulfur applications should be made at least three months prior to planting because it takes several months for sulfur to reduce the pH.'
remedy
Test the soil pH before planting. If it is above about 5.5, lower it with elemental sulfur applied months - ideally the season or the year - before planting, and re-test before you plant. If the soil is near neutral (above about 6.0) and hard to acidify, choose a different site. Do not rely on foliar iron sprays as a cure; they mask the yellowing without correcting the soil.
Another 111 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.
