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Iowa State Extension, 'Growing Sweet Corn in the Home Garden', yardandgarden.extension.iastate.edu/how-to/growing-sweet-corn-home-garden (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Sweet corn pollinated by popcorn will be tough and starchy'; 'Supersweet and augmented supersweet cultivars should be isolated from standard, sugary enhanced and synergistic sweet corn types'; 'Isolation can be achieved by planting different types at least 250 feet apart'; 'A minimum of 14 days should separate the tasseling time of different corn types.'

Iowa State Extension, 'Growing and Harvesting Ornamental Corn', yardandgarden.extension.iastate.edu/how-to/growing-and-harvesting-ornamental-corn (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Isolate ornamental corn from sweet corn and field corn to prevent cross-pollination. Isolation can be achieved by planting the different types of corn at least 250 feet from one another'; 'A minimum of 14 days should separate the tasseling times of the different types of corn.'

Illinois Extension, 'Corn' (Home Vegetable Gardening), extension.illinois.edu/gardening/corn (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Supersweets (Sh2) should be isolated from any other type of corn tasseling at the same time to ensure sweetness and tenderness'; 'Supersweet varieties pollinated by standard sweet corn, popcorn or field corn do not develop a high sugar content and are starchy'; 'Their pollen is weak and easily supplanted by other types, which causes the kernel to revert to a form with the toughness and starchiness of field corn.'

Utah State University Extension, 'Classes of Sweet Corn', extension.usu.edu/vegetableguide/sweet-corn/varietal-selection (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'All sweet corn varieties should be isolated from field corn by 250 feet or 14 days' difference in pollen shed (tasselling dates)'; 'Isolate shrunken-2 varieties from standard and sugar enhanced types to minimize starchy kernels'; 'While isolation of sugar enhanced from standard types is not necessary, it does ensure full expression of the different sweetness characteristics.'

UMN Extension, 'Growing sweet corn in home gardens', extension.umn.edu/vegetables/growing-sweet-corn (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): Shrunken (sh) 'Isolate from all other sweet corn types. Cross-pollination between Shrunken (sh) and any other types will result in starchy, tough kernels'; Sugary (su) 'Isolate from Shrunken (sh) and Synergistic (sy) types' - the stricter line, recorded and not enacted.

UMN Extension, 'Growing popcorn in home gardens', extension.umn.edu/vegetables/growing-popcorn (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Because popcorn takes a month or longer than sweet corn to mature, it is best to plant the popcorn first'; 'Wait three weeks before planting the sweet corn. The popcorn should produce tassels and silks before the sweet corn.'

NC State Extension, 'Organic Sweet Corn Production', content.ces.ncsu.edu/organic-sweet-corn-production (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'If sweet corn is planted downwind of popcorn or field corn, kernels will be starchy instead of sweet.'

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 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.