Centeno

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.

University of Maryland Extension, 'Cover Crops For Gardens', extension.umd.edu/resource/cover-crops-gardens (fetched in-container 2026-09-25, read 2026-09-25): 'Fall-sown cover crops are typically planted in a two-month window, starting in mid-August and ending no later than mid-October. The plants should have enough time to germinate and grow before the first hard frost and short days force them into dormancy.'

USDA SARE, 'Managing Cover Crops Profitably' (3rd ed.), Crimson Clover, sare.org/publications/managing-cover-crops-profitably/legume-cover-crops/crimson-clover/ (fetched in-container 2026-09-25, read 2026-09-25): 'Seed six to eight weeks before the average date of first frost'; 'Don’t plant too early or crimson clover will go to seed in the fall and not regrow in spring until the soil warms up enough to germinate seeds.'

USDA SARE, 'Managing Cover Crops Profitably', Hairy Vetch, sare.org/publications/managing-cover-crops-profitably/legume-cover-crops/hairy-vetch/ (fetched in-container 2026-09-25, read 2026-09-25): 'Plant vetch 30 to 45 days before killing frost for winter annual management'

Clemson Cooperative Extension HGIC, 'Cover Crops for the Fall', hgic.clemson.edu/cover-crops/ (fetched in-container 2026-09-25, read 2026-09-25): 'Legumes should be planted 6 to 8 weeks before a killing frost, and non-legumes should be planted 4 to 6 weeks before a killing frost.'

USDA SARE, 'Managing Cover Crops Profitably', Selecting the Best Cover Crops for Your Farm (fetched in-container 2026-09-25, read 2026-09-25): 'In many regions, seed winter covers at least six weeks before a hard frost. Winter cereals, especially rye, are an exception and can be planted a little later. If ground cover and N recycling needs are minimal, rye can be planted as late as the frost period for successful overwintering.'

University of Vermont Extension, NW Crops & Soils, 'Optimize Your Cereal Rye Cover Crop with the Right Sowing Dates and Seeding Rates', uvm.edu/extension/nwcrops (fetched in-container 2026-09-25, read 2026-09-25): 'the optimal window for sowing cereal rye is during the four-week window around the historic first frost date for that location.'; 'At all sites, sowing after that window decreased ground cover by 57% and biomass by 44%.'