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.
nitrogen fixed per season — estimated
nitrogen reaching neighbours the same season — contested
Rules that apply to White Clover
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.
Do not plant legumes in ground that held legumes within 3 years; the root rots build where they repeat, and the worst of them outlast any garden rotation.
costly if ignored → warn
Mechanism: Bean and pea root rots (Fusarium solani, Rhizoctonia, Pythium, Aphanomyces) and white mold (Sclerotinia sclerotiorum) survive in soil as spores and sclerotia and build where legumes follow legumes; a three-year break holds the common ones down. Fusarium needs longer, and Aphanomyces oospores and white mold's sclerotia persist for many years, so rotation reduces but cannot clear them.
Evidence: confirmed against 4 sources in the research-based literature.
UMN Extension, Fruit and Vegetable News, 'Crop planning deep dive: Mind your Peas and Beans' (2021), blog-fruit-vegetable-ipm.extension.umn.edu/2021/04/2021-consideration-minding-your-peas.html (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Rotate away from legumes for more than 3 years'; 'More than 6 years is needed for some diseases (Fusarium, some damping off pathogens).'
UConn IPM, 'Molds and Mildews of Beans in Connecticut', ipm.cahnr.uconn.edu/molds-and-mildews-of-beans-in-connecticut (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): white mold - 'Rotate away from susceptible crops for 7 years', it affects beans and 'practically all vegetable crops'; downy mildew - 'Rotate with other crops for 2 years.'
NDSU Agriculture, 'Field Pea and Lentil Root Rot Risk, Diagnosis and Resources', ndsu.edu/agriculture/ag-hub/field-pea-and-lentil-root-rot-risk-diagnosis-and-resources (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Fields with a long history of pulses in close rotations (every 2-3 years) are at greatest risk'; 'Chickpea, soybean, and faba bean are not susceptible to Aphanomyces euteiches and thus can be included in a rotation with pea and lentil to break up the disease cycle.'
remedy
Plant legumes in different ground, or wait until 3 years have passed since this ground last held them. Where root rot has shown, make it longer, and keep the ground draining freely: every one of these rots is worse in wet soil.
A legume fixes atmospheric nitrogen only in symbiosis with rhizobia specific to its cross-inoculation group; ground with no recent history of that group may carry too few of them to nodulate well.
suboptimal if ignored → advise
Mechanism: Nitrogen fixation happens in root nodules formed by Rhizobium or Bradyrhizobium bacteria specific to each legume group. On ground that has not grown that legume group before - a new bed, a soil-less mix, a long gap in the rotation, a fumigated or strongly acid soil - the matching strain can be absent or sparse, so nodulation and fixation are reduced until a population builds.
Evidence: confirmed against 1 source in the research-based literature.
the source
Penn State Extension, 'Inoculation of Forage and Grain Legumes', extension.psu.edu/inoculation-of-forage-and-grain-legumes (read 2026-08-14): 'Rhizobia bacteria are fairly specific as to which legumes they will infect, form nodules on the roots of, and for which they will fix nitrogen'; 'Legumes effectively nodulated by the same Rhizobia species are termed cross-inoculation groups'; 'Inoculation is recommended when the legume being planted has not been grown in that field in the past three years or with every planting of a high-value crop'; 'Inside the nodule, an enzyme called nitrogenase drives the N fixation reaction.'
remedy
On first-time legume ground, coat the seed at sowing with the inoculant matched to the crop's group (garden peas and beans, soybean, and clover each take a different one). Ground that has grown that legume recently usually needs none. It is a cheap one-time step - advised where the history is absent, never required.
Another 110 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.
