Alyssum

NC State Extension, 'Clubroot of Brassicas', content.ces.ncsu.edu/clubroot-of-brassicas (read 2026-07-11): 'Plasmodiophora brassicae is capable of surviving in the soil for more than 10 years by producing thick-walled, resting spores'; 'Infested fields should be rotated away from brassica for a minimum of 4-7 years, but this is not guaranteed to prevent disease'; 'Liming to raise soil pH to 7.[2]'.

Cornell Vegetables, 'Clubroot of Crucifers', vegetables.cornell.edu/pest-management/disease-factsheets/clubroot-of-crucifers (read 2026-07-11): 'the pathogen survives for many years even in the absence of host crops'; 'rotation of at least 7 years out of susceptible crucifers may be effective.'

UMass Extension, 'Brassicas, Clubroot', ag.umass.edu/vegetable/fact-sheets/brassicas-clubroot (read 2026-07-11): 'Long (5-7 years) rotations between brassica crops'; 'Maintain a high pH by regular applications of lime.'

NC State Extension, 'Clubroot of Brassicas', content.ces.ncsu.edu/clubroot-of-brassicas (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'The pathogen germinates in wet soils when the soil temperature is between 18 and 25 ºC (64 to 77 F), and the soil pH is acidic (pH less than 7.0)'; 'Alkaline soils and soils with high calcium content suppress the potential of the pathogen to initiate infection'; 'Liming to raise soil pH to 7.1-7.5 can slowly reduce disease severity, although this strategy is not completely reliable when there is a well-established infestation'; 'Calcitic lime is recommended over dolomitic lime unless the field has low levels of magnesium'; host list 'broccoli, Brussels sprouts, cabbage, Chinese cabbage, cauliflower, collard, kale, kohlrabi, mustard, oilseed rape (canola), radish, rutabaga, and turnip.'

UMass Extension, 'Brassicas, Clubroot', umass.edu/agriculture-food-environment/vegetable/fact-sheets/brassicas-clubroot (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'The disease is favored by soils with a pH less than 7.0'; 'Maintain a high pH by regular applications of lime'; 'Calcitic lime is preferred over dolomitic, except where magnesium levels are low'; 'High pH can lead to boron deficiency in coarse soils'; 'occurs on broccoli, Brussels sprouts, cabbage, cauliflower, turnip, rutabaga, and radish.'

UConn IPM, 'Clubroot of Crucifers', ipm.cahnr.uconn.edu/clubroot-of-crucifers (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Maintain soil pH at 7.0-7.2 where clubroot is a concern. Lime well ahead of planting and incorporate thoroughly'; 'Monitor boron where pH is raised, especially on coarse soils'; 'It affects crops such as cabbage, broccoli, cauliflower, Brussels sprouts, kale, collards, turnips, rutabagas, radishes, mustard greens, and other brassicas.'

Cornell Vegetables, 'Clubroot of Crucifers', vegetables.cornell.edu/pest-management/disease-factsheets/clubroot-of-crucifers (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): host range - most susceptible 'cabbage, Chinese cabbage, Brussels sprouts, some turnips, wormseed mustard, and some species of candytuft'; medium 'kohlrabi, kale, cauliflower, collards, broccoli, rutabaga, seakale, some turnips and radishes'; mildly 'rape (canola), black mustard, some turnip and radish varieties, and tumble mustard'; very resistant or occasionally immune 'wintercress; or yellow rocket, horseradish, shepherd's purse, wallflower, dames violet, peppergrass, garden cress, stock, and some radishes'. On pH: 'soil pH should be adjusted to pH 6.8 with ground limestone'; 'It is essential that hydrated lime be added each year unless the soil pH exceeds 7.5'; 'the use of nitrogen fertilizers will reduce soil pH (nullifying the beneficial effects of liming).'

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.

North Carolina Extension Gardener Handbook, ch. 13 'Propagation' / Anthony V. LeBude & Frank A. Blazich (NC State Extension, 2022), content.ces.ncsu.edu/extension-gardener-handbook/13-propagation (fetched in-container 2026-09-20, read 2026-09-20): 'When sowing light-requiring seeds, sow them on the soil surface'; the plants it lists as requiring light for germination are 'ageratum, begonia, browallia, impatiens, lettuce, and petunia'; and 'Extremely fine seeds such as carrot, petunia, and snapdragon should not be covered, but simply dusted on the surface of the germinating medium and watered with a fine mist.' The page states the practice, not the phytochrome mechanism.