The same species behaves differently by cultivar group, so the planner resolves size, timing and support against the group rather than the species.
Leaf Lettuce — rosette, spacing in row 20 cm8 in, mature height 20–25 cm8–10 in, mature spread 20–25 cm8–10 in, days to maturity 40–50
Butterhead (Bibb / Boston) — rosette, spacing in row 25 cm10 in, mature height 20–25 cm8–10 in, mature spread 25–30 cm10–12 in, days to maturity 50–60
Romaine (Cos) — rosette, spacing in row 25 cm10 in, mature height 25–35 cm10–14 in, mature spread 20–25 cm8–10 in, days to maturity 60–75
Crisphead (Iceberg) — rosette, spacing in row 30 cm12 in, mature height 25–30 cm10–12 in, mature spread 30–35 cm12–14 in, days to maturity 70–85
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.
days between succession sowings — estimated
ph range — estimated
looseleaf: mature height — estimated
looseleaf: mature spread — estimated
looseleaf: days to maturity — estimated
butterhead: mature height — estimated
butterhead: mature spread — estimated
butterhead: days to maturity — estimated
romaine: mature height — estimated
romaine: mature spread — estimated
romaine: days to maturity — estimated
crisphead: mature height — estimated
crisphead: mature spread — estimated
crisphead: days to maturity — estimated
Rules that apply to Lettuce
A good hunch
Do not plant lettuce or its family in ground that held it within 3 years, and know that lettuce drop's fungi live on most other vegetables too.
costly if ignored → advise
Mechanism: Bottom rot (Rhizoctonia solani) and lettuce drop (Sclerotinia sclerotiorum and S. minor) survive in soil as mycelium and sclerotia for years and build where lettuce repeats; a three-year break holds bottom rot down. Drop's fungi infect alliums, brassicas, nightshades, cucurbits and legumes as well, so a lettuce rotation slows them without starving them, and only cereals are safe ground.
Evidence: confirmed against 4 sources in the research-based literature.
NC State Extension, 'Lettuce Drop', content.ces.ncsu.edu/lettuce-drop (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Sclerotinia sclerotiorum and S. minor have a wide host range infecting allium (onion, garlic), brassica (cabbage, broccoli), solanaceae (pepper, potato), cucurbits (cucumber, squash), lettuce, and sweetpotato crops'; 'Cereals (maize, wheat, sorghum) are less susceptible to these pathogens'; 'the broad host range of both pathogens significantly limits the efficacy of crop rotation.'
Utah State University Extension, 'Drop (Sclerotinia Rot)', extension.usu.edu/planthealth/ipm/notes_ag/veg-drop-sclerotinia (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'The sclerotia can survive dormant in the soil for extended periods'; 'S. sclerotiorum affects a wide range of hosts, including root crops, legumes, and solanaceous crops however, drop is specific to lettuce'; rotate 'with nonsusceptible crops such as grains or corn to reduce disease build-up.'
UMass Vegetable Notes 2024 vol. 36:2, umass.edu/agriculture-food-environment/vegetable/newsletters/vegetable-notes/vegetable-notes-2024-vol-362 (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): bottom rot - 'Rhizoctonia survives in the soil, as mycelium or sclerotia'; lettuce drop - 'It produces sclerotia that can survive in the soil for many years'; 'Its host range includes many vegetables such as tomato and beans.'
remedy
Plant lettuce and its family in different ground, or wait until 3 years have passed since this ground last held them. Where drop has shown, the break that counts is one of grains or corn, since most vegetables carry it, and dead plants go in the bin, not the compost: the sclerotia survive it.
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.
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.
