The rules

55 rules, and every one of them has to say how it works. A claim that cannot name its mechanism in a sentence is not enforced here — it is written down, graded, and left unenforced. That is the whole difference between this and a list of gardening tips.

Advice, not enforcement

• A bulb ring suppresses turfgrass competition at a fruit tree trunk. Well established
• Allium deters borers. Contested
• Alliums interplanted with carrots reduce carrot rust fly damage. A good hunch
• Aromatic species interplanted with brassicas are reported to reduce cabbage-worm damage. The mechanism is unresolved and trial results are mixed. A good hunch
• Comfrey is a dynamic accumulator that mines subsoil minerals and makes them available. Contested
• Insectary plants near fruiting crops increase pollinator and predator visitation. A good hunch
• Living mulch suppresses weeds and conserves soil moisture. A good hunch
• Narcissus deters voles and rodents. A good hunch

Feeding the soil

• A lime or sulfur RATE requires a buffer-pH measurement. Direction is free; the amount is not, and it is the lab's to give, never ours. Promising
• A measured soil pH below a plant's sourced floor is worth saying out loud, with the shortfall and the direction of the fix. Above the ceiling too, but only for a plant that is documented to require acid soil — for everything else the ceiling is this corpus's own and fires nothing. A good hunch
• Do not apply fresh high-N fertilizer to root crops. Promising
• Fertilizer recommendations without a soil test are guesswork. Say so. Well established
• Ground the gardener has observed to stand water after rain is a site problem to fix before planting, not a plant problem to select around. Promising
• Legume green manure raises available N for the subsequent crop. Promising

Heat and cold

• Hardiness zone is the correct variable for perennials, and only for perennials. Well established
• Tomato fruit set fails when night temperatures persistently exceed ~21C (70F); high day temperatures (>29-32C) compound it. Well established

Plants that suppress each other

• Do not plant a juglone-sensitive species within the root zone of Juglans. The sensitive list is the hazard's own, and it is wider than the nightshades - it also carries apple and cabbage. Well established
• Fennel is allelopathic; a suppressive effect on neighbouring vegetables is plausible but poorly characterized (evidence is weed-directed, in vitro). A good hunch
• Sunflower residue suppresses germination of some small-seeded species. Promising

Pollination

• An insect-pollinated crop depends on an insect to move pollen between its flowers; no arrangement of plants substitutes for one. Well established
• Corn requires a block of at least 4 rows (roughly 12 plants) for adequate kernel set. Well established
• Self-incompatible species require 2+ genetically distinct plants. Well established

Rotation and soil-borne disease

• A guild deposits family history across its whole footprint. Promising
• Alliums and carrot family are unrelated to nightshades/cucurbits/brassicas and can break those families' rotation cycles — but they are not "low-pressure" and need their own rotation. A good hunch
• Do not establish Verticillium-susceptible perennials (raspberries, strawberries, stone fruit) in ground that recently hosted Verticillium dahliae — either nightshades, or a logged planting of a Verticillium host (strawberry, raspberry or other bramble, or stone fruit). Promising
• Do not establish Verticillium-susceptible perennials where the season log records that a Verticillium host (strawberry, raspberry or other bramble, or stone fruit) previously grew — a reservoir the coarse prior-nightshades family signal structurally misses. Promising
• Do not plant brassicas in ground that held brassicas within 4 years (extension recommends 4-7, and even that is not a cure). Promising
• Do not plant cucurbits in ground that held cucurbits within 2-3 years. Promising
• Do not plant nightshades in ground that held nightshades within 3 years. Promising

Space and light

• A bed whose shorter side runs well past ~1.2 m has interior ground that can't be reached from its edges. Promising
• A full-sun species must not be placed in ground the gardener has marked as shaded. Well established
• A plant's mature spread must fit its allocated area. Well established
• A planted bed keeps every plant within arm's reach of a path, edge, or walkable row. Promising
• A species with support.requires strong must have an assigned support that provides it. Well established
• Living supports have finite load capacity. Promising
• Perennial members must not be sited in ground that is annually turned or rotated. Well established
• Perennial woody guilds are not bed-scoped. Their ground entity is the drip-line ring. Well established
• Plants over 120cm mature height belong on the polar side of a bed. Well established
• Sun-requiring species must not sit in the computed shadow of a taller neighbour. Well established
• Vining cucurbits in beds under 2m escape the bed. Well established

Things that are not true

• Any co-planted set of vegetables is a guild. Well established
• Autumn olive is a good nitrogen fixer for a fruit tree guild. Well established
• Beans fix nitrogen that feeds the corn growing beside them, that season. A good hunch
• Marigolds interplanted among vegetables control root-knot nematodes. Refuted

Timing

• A species must reach maturity within the site's frost-free window. Well established
• A vine assigned to a living support must be sown after the support is established — corn ~6 in (~15 cm) tall, roughly 2-3 weeks — not at the same time. Well established
• Cold-hardy annuals can be sown or transplanted before the last spring frost — hardy crops about three weeks before, half-hardy about two weeks before. Tender crops wait for the frost itself. Promising
• Long-season warm crops are started indoors several weeks before the last frost and set out after it — tomato and tomatillo about six weeks ahead, pepper and eggplant about eight. Basil, six. Promising
• Onion cultivar group must match site latitude. Well established
• Succession sowing at 2-3 week intervals extends harvest for short-DTM crops. Promising
• Tender species must not be scheduled before the site's last frost date. Well established

Where the numbers come from

• An observed per-plot frost offset, fitted from 3+ seasons of logged outcomes on that ground, supersedes all modelled climatology for that plot. Well established
• Every derived climate value carries the tier, grade, station, and distance that produced it. Well established
• Frost dates can be downscaled by applying a standard lapse rate to elevation. Well established
• PRISM offset from a nearby station assumes the spatial pattern of mean minimum temperature predicts the spatial pattern of last-frost date. Promising