Borago officinalis · borage family
Borage (Borago officinalis) is an annual in the borage family.
Also called Starflower.
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: 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: An annual that sets abundant seed which drops and germinates in place, so one planting reappears and spreads by self-sown volunteers each season without being replanted. The control follows from the same mechanism: seed that is removed before it drops cannot volunteer.
Evidence: confirmed against 2 sources in the research-based literature.
Univ. of Illinois Extension, 'Plant of the Week: Borage', extension.illinois.edu/blogs/hort-home-landscape/2016-06-24-plant-week-borage (read 2026-08-22): 'This annual self-seeds wherever it is planted, so in most years, an abundance of borage returns.'
NC State Extension Gardener Plant Toolbox, 'Borago officinalis', plants.ces.ncsu.edu/plants/borago-officinalis (read 2026-08-22): 'the plant is also self-seeding and will remain in your garden year after year.'
To keep borage going, leave a few seed heads to ripen and drop. To limit where it comes up, cut or deadhead the flower heads before the seed sets and drops, and pull volunteer seedlings while they are young and shallow-rooted.
Another 88 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. "Borage (Borago officinalis)." https://milpa.garden/plants/borago-officinalis/
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.