The same species behaves differently by cultivar group, so the planner resolves size, timing and support against the group rather than the species.
Onion — bulb, spacing in row 12 cm5 in, mature height 45 cm18 in, mature spread 15 cm6 in, days to maturity 90–120
Shallot — bulb, spacing in row 15 cm6 in, mature height 45 cm18 in, mature spread 15 cm6 in, days to maturity 90–120
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.
shallot: days to maturity — estimated
shallot: mature height — estimated
shallot: mature spread — estimated
ph range — estimated
sowing depth — estimated
Rules that apply to Onion
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.'
Mechanism: Bulbing fires when daylength crosses a cultivar's critical threshold (short ~10-12h, intermediate ~12-14h, long ~14-16h). A short-day cultivar grown too far north crosses its low threshold early and bulbs prematurely on small plants, so bulbs stay small. A long-day cultivar grown too far south never reaches its high threshold and fails to bulb. Changeover is around the 35th-38th parallel.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
NC State Extension, 'Bulb Onions', content.ces.ncsu.edu/bulb-onions (read 2026-07-11): 'onions will not form bulbs until daylength reaches the cultivar's critical threshold. Short-day onions bulb at approximately 10-12 hours of daylength ... Intermediate-day onions bulb at approximately 12-14 hours ... Long-day onions bulb at 14-16 hours of daylength.'
Univ. of Illinois Extension (The Humble Gardener), 'Growing Onions in Central Illinois', extension.illinois.edu/blogs/humble-gardener/2018-03-19-growing-onions-central-illinois (read 2026-07-11): 'Long day varieties are for northern growing areas (above the 35th latitude); short-day varieties are for southern. If short-day varieties are grown in our zone, they won't do well. They will not form bulbs of any decent size. And since the long day onions need more daylight, they will not do well' (in the south).
remedy
Choose long-day onions in the north and short-day in the south; the planner picks the group from your plot's latitude.
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.
Cold-hardy annuals can be sown or transplanted before the last spring frost — hardy crops about three weeks before, half-hardy about two weeks before. Tender crops wait for the frost itself.
suboptimal if ignored → advise
Mechanism: Cold hardiness. Hardy crops tolerate hard frost and half-hardy crops tolerate light frost, so they can occupy the ground weeks before frost-tender crops are allowed in. The offset is measured from the last spring frost.
Evidence: confirmed against 4 sources in the research-based literature.
the 4 sources
University of Illinois Extension, 'Time Your Vegetable Plantings by Cold Hardiness' (2018-03-26), extension.illinois.edu/blogs/ilriverhort/2018-03-26-time-your-vegetable-plantings-cold-hardiness: very hardy vegetables 'withstand freezing temperatures and hard frosts without injury' (planted earliest in spring); frost-tolerant vegetables 'can withstand light frosts, but not freezing temperatures' (planted a couple of weeks later). LLM-summarised from a read of the page; it establishes the CATEGORIES but states no week offsets.
Colorado State Univ. Extension, GardenNotes #720, 'Vegetable Planting Guide', extension.colostate.edu/resource/vegetable-planting-guide (read 2026-07-25): hardy vegetables go in 'As early as 2-4 weeks before the date of the average last spring frost'; semi-hardy 'As early as 0-2 weeks before the date of the average last spring frost'; tender from seed 'around the date of the average last spring frost'; very tender 'Typically 2+ weeks after'. This is the source that states the offsets in weeks.
Where this rule sits in that range: hardy at 3 weeks is mid-range of CSU's 2-4. Half-hardy at 2 weeks is the EARLIEST EDGE of CSU's 0-2 - the most aggressive planting CSU sanctions, not the middle of it. The rule is inside published guidance at both ends, but it leans early on the half-hardy side and a gardener following it in a cold spring is taking that boundary.
Do not import an offset from another publication's hardiness scheme without checking the category names: CSU has no 'very hardy' tier, so a '4-6 weeks before frost' figure written against a four-tier scheme does not describe CSU's 'hardy'. Categories are not comparable across sources.
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.
Another 108 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.
