The same species behaves differently by cultivar group, so the planner resolves size, timing and support against the group rather than the species.
Hardneck Garlic — bulb, spacing in row 15 cm6 in, mature height 60–75 cm24–30 in, mature spread 15–20 cm6–8 in
Softneck Garlic — bulb, spacing in row 13 cm5 in, mature height 50–60 cm20–24 in, mature spread 15–20 cm6–8 in
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 — estimated
hardneck: mature height — estimated
hardneck: mature spread — estimated
softneck: mature height — estimated
softneck: mature spread — estimated
Rules that apply to Garlic
A good hunch
Alliums and carrot family are unrelated to nightshades/cucurbits/brassicas and can break those families' rotation cycles — but they are not "low-pressure" and need their own rotation.
suboptimal if ignored → advise
Mechanism: They host none of the big-three families' soilborne pathogens, so following those crops with an allium or apiaceous crop breaks the host cycle. But they are not inherently low-pressure: allium white rot (Sclerotium cepivorum) is allium-specific, its sclerotia persist 15+ years, and it cannot be eradicated once established; apiaceous crops carry their own soilborne diseases.
Evidence: confirmed against 1 source in the research-based literature.
the source
UMass Extension, 'Alliums, White Rot', ag.umass.edu/vegetable/fact-sheets/alliums-white-rot (read 2026-07-11): 'Sclerotium cepivorum is specific to allium species'; sclerotia 'may remain viable in soil for 15 years or more without allium hosts'; 'Currently, there is no way to eradicate white rot from a field where it has become 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 alliums in ground that held alliums within 4 years, and know that the worst allium disease outlasts any rotation.
costly if ignored → warn
Mechanism: Fusarium basal rot (Fusarium oxysporum f. sp. cepae) and pink root survive in soil as spores and on other plants' roots, and their inoculum builds where alliums repeat; a four-year break starves them. White rot (Sclerotium cepivorum) is allium-specific and its sclerotia stay viable 15 to 40 years, so rotation reduces but cannot clear it.
Evidence: confirmed against 4 sources in the research-based literature.
the 4 sources
UMass Extension, 'Alliums, Fusarium Basal Rot', umass.edu/agriculture-food-environment/vegetable/fact-sheets/alliums-fusarium-basal-rot (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Rotate away from alliums for at least 4 years'; 'Fusarium oxysporum f. sp. cepae is a soil-borne pathogen that overwinters as chlamydospores (thick-walled survival spores) buried in the soil'; affects garlic, leeks and onion.
Colorado State University Extension, 'Soil-Borne Diseases of Onion', extension.colostate.edu/resource/soil-borne-diseases-of-onion (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Do not plant onions more than once every five years in a field'; its Table 1 lists a rotation of less than 4 years as favouring damping-off, smut, pink root, Fusarium basal rot and stem and bulb nematode; 'Soil-borne fungal pathogens and nematodes can persist for many years in previously infected onions.'
Univ. of Maryland Extension, 'White Rot of Onion and Garlic', extension.umd.edu/resource/white-rot-onion-and-garlic (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Rotation alone will not control white rot because sclerotia can survive in the soil for 20-40 years'; 'white rot only infects Allium species.'
UMass Extension, 'Alliums, White Rot', umass.edu/agriculture-food-environment/vegetable/fact-sheets/alliums-white-rot (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'White rot survives in soil in the form of sclerotia, which are dense masses of fungal tissue. They may remain viable in soil for 15 years or more without allium hosts.'
remedy
Plant alliums in different ground, or wait until 4 years have passed since this ground last held them. If white rot has ever shown here, treat this ground as lost to alliums and grow them elsewhere.
Onions and garlic lifted for storage are cured first - dried down in a warm, airy, shaded place for about two to four weeks until the neck is tight and dry and the outer scales are papery. Bulbs stored uncured, or poorly cured, rot in storage instead of keeping.
suboptimal if ignored → advise
Mechanism: At harvest the bulb's neck and outer scales are still moist living tissue and the neck is an open channel into the bulb, so curing in warm moving air dries the neck shut and the outer scales to a papery protective skin, removing the wet tissue that storage rots colonise and sealing the neck they enter by; an uncured bulb goes into storage with a moist open neck and soft scales, and decays.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
UMN Extension, 'Growing onions in home gardens', extension.umn.edu/vegetables/growing-onions (read 2026-08-21): 'Curing is essential if you plan to store your onions'; 'Keep the onions in a warm (75F - 90F), well-ventilated area for two to four weeks, until outer bulb scales are dry and the neck is tight'; 'Poor curing will result in decay during storage.'
Clemson HGIC, 'Onion, Leek, Shallot, & Garlic', hgic.clemson.edu/factsheet/onion-leek-shallot-garlic (read 2026-08-21): onion - 'Thoroughly air-dry bulbs in a shaded area before storage', 'To avoid storage diseases, be sure onions are well-cured before storing'; garlic - 'Cure in a warm, shady place with good air movement', 'Allow bulbs to dry until the neck is dry and the outer skin is papery, approximately two to three weeks', 'Bulb rot can also occur in storage from bruising and insufficient drying.'
remedy
After lifting, spread the bulbs in a single layer or hang them in bundles in a warm, shaded, well-ventilated place out of direct sun and rain, and leave them until the neck is dry and tight and the outer skin is papery - roughly two to four weeks for onions (a warm 75-90 F speeds it), two to three weeks for garlic. Only then trim, and keep the cured bulbs in mesh bags or shallow boxes in a cool, dry, well-ventilated spot. Handle gently throughout, as bruising also seeds storage rot.
Garlic is planted in autumn, not spring - the cloves go in the ground several weeks before it freezes, root, and overwinter. The weeks of winter cold below about 40 F vernalize the clove, and that cold is what triggers it to form a bulb the following spring as the days lengthen. Planted in spring, garlic misses the cold and bulbs poorly.
suboptimal if ignored → advise
Mechanism: A garlic clove will not form a bulb until it has been vernalized - held for roughly six to eight weeks below about 40 F. The bulb itself does not develop during that cold; it is only induced by it, and forms in spring as days lengthen and temperatures rise. Fall planting delivers the cold naturally over winter with the clove already rooted, so growth resumes early and a full bulb sizes up by early summer. It is the same biennial vernalization that prematurely bolts a spring-sown cool-season crop, but with the sign reversed: there the accumulated cold buttons the plant into an early flower and ruins it; here, in a crop sown in autumn to be harvested as a bulb, that same cold is the intended trigger.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
UGA Extension, 'Garlic Production for the Gardener' (C854), fieldreport.caes.uga.edu/publications/C854 (read 2026-08-22): 'Late September through November is the time to plant garlic in Georgia'; 'Garlic cloves require a period of 6-8 weeks of cool weather after planting (below 40 F) to undergo vernalization ... by low winter temperatures'; 'Although vernalized, no inflorescence or lateral buds (that later form the bulb) are developed until early spring with the onset of lengthening days and suitable temperatures'; 'Hard-necked garlics may grow in Georgia but generally prefer the cold winters and long, cool springs of more northern climates. Soft-necked garlics are well-adapted to the more temperate climate of the South.'
Iowa State Univ. Extension, 'Growing Garlic in the Home Garden', yardandgarden.extension.iastate.edu/how-to/growing-garlic-home-garden (read 2026-08-22): 'Plant cloves ... in the fall (October to early November)'; 'The highest yields are obtained from the largest cloves planted in the fall'; 'Plant large cloves 1 to 1.5 inches deep with the pointed side up, 3 to 5 inches apart within rows spaced 18 to 24 inches apart'; 'Fall-planted garlic should be mulched in November with a 4- to 6-inch layer of weed-seed-free straw to help prevent winter injury.'
remedy
Plant cloves in the fall - roughly late September through November in most regions, several weeks before the ground freezes so they root before winter - pointed end up, about 1 to 3 inches deep and 4 to 6 inches apart, using the largest cloves for the largest bulbs. Once the ground has chilled, mulch with a few inches of weed-free straw to blunt winter heaving and injury. Match the type to the winter: hard-necked garlics prefer cold-winter regions, soft-necked are better adapted to mild-winter areas.
Another 107 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.
