Ocimum basilicum · mint family
Basil (Ocimum basilicum) is an annual in the mint family.
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.
Mechanism: Ice nucleation in leaf tissue.
Evidence: settled without a trial - climatology.
Sow or set out after the last frost date, or start indoors and transplant once it has passed.
Mechanism: Fusarium oxysporum f. sp. basilici is a soilborne fungus specific to basil (not the tomato or watermelon Fusarium); once a planting is infested its resting spores persist in the soil for eight to twelve years. It usually arrives on contaminated seed rather than lying in wait everywhere, so it is a hazard only where infected seed has been sown - but where it is, plants of the mint family carry it without showing symptoms and can hold the ground infective from one year to the next.
Evidence: confirmed against 1 source in the research-based literature.
NC State Extension, Growing Small Farms, 'Fusarium Wilt of Basil', growingsmallfarms.ces.ncsu.edu/growingsmallfarms-fusariumbasil (fetched in-container 2026-09-03; read by the maintainer 2026-09-03): 'Once a field has become infested with the fusarium wilt pathogen, infective propagules may persist in the soil for 8-12 years. During that time, growers should avoid growing sweet basil or members of the mint family'; 'The disease is introduced into fields, hydroponic systems, and greenhouse culture primarily through contaminated seed'; 'If it is not possible to obtain tested seed, the seed should be soaked in cold water for four hours followed by a heat treatment of 20 minutes in 133-136 F water'; 'Mints will not exhibit symptoms of the disease but may carry over the inoculum from year to year'; 'Some seed companies also sell resistant varieties.' [read]
Start from seed tested clean for the fusarium fungus, or hot-water-treat your own (twenty minutes at 133-136 F); where the disease has appeared, grow a resistant variety where one is offered and keep basil - and any standing mint - off that ground for several years. A wait alone is only partial cover, since the spores last the better part of a decade and mints can carry them.
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.
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.
Mechanism: A frost-tender crop with a long days-to-maturity cannot mature if it is only direct-sown after the last frost — the frost-free window is too short. A head start under cover buys the weeks the season lacks; the seedling is set out only once frost has passed — the transplant date is still frost-gated.
Evidence: confirmed against 2 sources in the research-based literature.
University of Minnesota Extension, 'Starting seeds indoors', extension.umn.edu/planting-and-growing-guides/starting-seeds-indoors: seed-starting chart gives tomato '5-6 weeks' and pepper and eggplant '9 weeks' before the outdoor planting date (warm crops set out after the last frost). Read 2026-07-19 (LLM-summarised); read by the maintainer 2026-07-25, which is the read that promoted this rule. Note what the corpus's per-crop values do with the sources: tomato 6 sits at the top of Minnesota's '5-6 weeks' and inside Illinois's '6-8'; pepper and eggplant 8 sit BELOW Minnesota's '9 weeks' and at the top of Illinois's '6-8'. They are a reconciliation of two published ranges, not a figure either source prints.
University of Illinois Extension, 'When should I start my seeds?' (2023-02-10), extension.illinois.edu/blogs/good-growing/2023-02-10-when-should-i-start-my-seeds: peppers, tomatoes, and eggplant started '6-8 weeks before the last frost'. Read 2026-07-19.
Mechanism: Apical dominance and the leaf-to-seed switch. Pinching the terminal shoot releases the lateral buds below the cut, so the plant branches and grows bushier with more leaf-bearing stems; and because flowering redirects the plant from building leaves to setting seed, removing the buds before they open keeps it in leaf and the foliage tender.
Evidence: confirmed against 2 sources in the research-based literature.
Univ. of Illinois Extension, 'Basil' (Herbs), extension.illinois.edu/herbs/basil (read 2026-08-22): 'Pinching the tips of stems encourages a bushy plant and more leaf growth resulting in a round plant full of aromatic basil'; 'Basil flower buds should be removed by pinching as soon as you see them form. Leaving them on the plant will affect the flavor of the leaves.'
Penn State Extension, 'Basil, A Summer Favorite', extension.psu.edu/basil-a-summer-favorite (read 2026-08-22): 'Continue to pick the leaves regularly to encourage growth throughout the summer'; 'This keeps the plant growing strong and bushy, providing for a continual harvest through the warm days of fall'; 'After six weeks, watch out for early flowering. If flower buds do emerge, just cut or pinch them off'; 'The flavor will be adversely affected if the plant is allowed to flower.'
Once the plant has several sets of true leaves, pinch or harvest from the top just above a leaf pair on a regular cadence rather than stripping the lower leaves. Watch for flower spikes and pinch or cut them off as soon as they emerge.
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.
Milpa Gardens. "Basil (Ocimum basilicum)." https://milpa.garden/plants/ocimum-basilicum/
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.