French Tarragon (Artemisia dracunculus) is a perennial in the daisy family.
Well establishedHardiness zone is the correct variable for perennials, and only for perennials.
suboptimal if ignored → advise
Mechanism: Zones encode average annual minimum winter temperature, which governs whether a woody perennial survives to spring. It is the wrong quantity for an annual, which never sees winter.
THE CEILING IS A SECOND AND DIFFERENT MECHANISM, and until 2026-07-26 this rule enforced it without naming it. A zone too WARM does not kill a perennial; it fails to chill it. Deciduous fruit crops need a winter dormancy period to break bud and set fruit, so in a zone milder than a plant is adapted to it may survive and never crop. Until ISSUES #13 was decided the band was enforced at both ends; now (option E) only the cold end gates — it alone follows from the definition of the zone system — and the warm end is carried as an ADVISORY, not a refusal.
AND THE WARM END IS A CULTIVAR'S ANSWER, NOT A SPECIES', which is the honest limit of what this rule can do. Chill requirement is published per variety and varies enormously within a species, so a species-level ceiling says "the modelled range does not reach your zone", never "no plant of this kind will crop for you". So the engine offers the plant above its band and advises choosing a cultivar rated for the zone rather than refusing it — the same move invariant 3 makes for soil, applied to chill.
Evidence: confirmed against 3 sources in the research-based literature.
the 3 sources
THE FLOOR IS TRUE BY DEFINITION and needs no citation: the USDA system defines a zone AS a band of average annual minimum winter temperature, so 'a plant colder-hardy than your zone survives your winter' is the definition restated. This rule was `verified_trivially` on that basis alone until 2026-07-26. THE CEILING WAS NEVER COVERED BY IT - a minimum-temperature definition says nothing about a plant failing for want of chill - and that gap is why ISSUES #13 exists. The pointer below closes it.
OSU Extension EC 1303, 'Growing blackberries in your home garden' (Bernadine Strik), catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/ec1303.pdf (read 2026-07-26). THE CEILING'S MECHANISM, STATED: 'Winter cold-hardy cultivars adapted to the colder regions of Oregon (such as zones 5 to 7), may not grow normally in regions of the Willamette Valley (zone 8) where winters are warmer and plants receive less winter chill.' A plant failing in a WARMER zone for want of chill - which is what this rule's upper bound enforces and what no source said until now. THE SAME PAGE MAKES IT A CULTIVAR PROPERTY: 'Even cultivars within the same type (trailing, erect or semierect) differ in fruit quality and flavor, appearance, tolerance to pests, cold hardiness, and plant longevity', and the remedy it gives is the one this rule's rejection text now repeats: 'Minimize risk by choosing types and cultivars that are adapted to your hardiness zone.' The page then publishes a table of sixteen cultivars EACH WITH ITS OWN ZONE RANGE, which is the strongest available evidence that a species-level band is an approximation. [read]
WHAT THIS DOES NOT SETTLE. The mechanism is now named, but the corpus still carries ONE band per species where the source carries one per cultivar, and 12 of 25 perennial zones have no citation at all. So the ceiling is explained without being precise. ISSUES #13 stays open on that, and the rejection text is deliberately phrased as 'the modelled range does not reach your zone' rather than as a prediction that nothing will crop.
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A good hunchDo not plant lettuce or its family in ground that held it within 3 years, and know that lettuce drop's fungi live on most other vegetables too.
costly if ignored → advise
Mechanism: Bottom rot (Rhizoctonia solani) and lettuce drop (Sclerotinia sclerotiorum and S. minor) survive in soil as mycelium and sclerotia for years and build where lettuce repeats; a three-year break holds bottom rot down. Drop's fungi infect alliums, brassicas, nightshades, cucurbits and legumes as well, so a lettuce rotation slows them without starving them, and only cereals are safe ground.
Evidence: confirmed against 4 sources in the research-based literature.
the 4 sources
Cornell Vegetables, 'Do Rotations Matter within Disease Management Programs?', vegetables.cornell.edu/pest-management/disease-factsheets/do-rotations-matter-within-disease-management-programs (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): its rotation table gives lettuce bottom rot (Rhizoctonia) 3 years and drop (Sclerotinia) 3 years.
NC State Extension, 'Lettuce Drop', content.ces.ncsu.edu/lettuce-drop (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Sclerotinia sclerotiorum and S. minor have a wide host range infecting allium (onion, garlic), brassica (cabbage, broccoli), solanaceae (pepper, potato), cucurbits (cucumber, squash), lettuce, and sweetpotato crops'; 'Cereals (maize, wheat, sorghum) are less susceptible to these pathogens'; 'the broad host range of both pathogens significantly limits the efficacy of crop rotation.'
Utah State University Extension, 'Drop (Sclerotinia Rot)', extension.usu.edu/planthealth/ipm/notes_ag/veg-drop-sclerotinia (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'The sclerotia can survive dormant in the soil for extended periods'; 'S. sclerotiorum affects a wide range of hosts, including root crops, legumes, and solanaceous crops however, drop is specific to lettuce'; rotate 'with nonsusceptible crops such as grains or corn to reduce disease build-up.'
UMass Vegetable Notes 2024 vol. 36:2, umass.edu/agriculture-food-environment/vegetable/newsletters/vegetable-notes/vegetable-notes-2024-vol-362 (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): bottom rot - 'Rhizoctonia survives in the soil, as mycelium or sclerotia'; lettuce drop - 'It produces sclerotia that can survive in the soil for many years'; 'Its host range includes many vegetables such as tomato and beans.'
remedy
Plant lettuce and its family in different ground, or wait until 3 years have passed since this ground last held them. Where drop has shown, the break that counts is one of grains or corn, since most vegetables carry it, and dead plants go in the bin, not the compost: the sclerotia survive it.
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PromisingCulinary French tarragon has sterile flowers and sets no viable seed, so it is grown only from division, cuttings, or a bought started plant - never from a seed packet. Seed sold as "tarragon" is Russian tarragon, a taller, coarser plant whose flavour is far inferior. Sow that seed and you get the near-flavourless plant, not the culinary herb.
suboptimal if ignored → advise
Mechanism: French tarragon's flowers are sterile and it seldom sets viable seed, so the only way to keep the true culinary plant is to propagate it vegetatively - by division or cuttings from an existing plant. The "tarragon" seed on the market is a different, seed-fertile type, Russian tarragon, which lacks the odour and flavour that make French tarragon worth growing.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
Univ. of Illinois Extension, 'Tarragon: French', extension.illinois.edu/herbs/tarragon-french (read 2026-08-22): 'True French tarragon is only available as plants grown from cuttings or root divisions'; 'Because French tarragon produces flowers that are sterile, it cannot be grown from seeds'; 'Seeds that are sold as tarragon at seed racks or in catalogs are seeds of Russian tarragon'; 'This is a much taller, coarser plant and its culinary use is considered to be inferior because it lacks the odor and flavor characteristics of French tarragon.'
Utah State Univ. Extension, 'How to Grow French Tarragon in Your Garden', extension.usu.edu/yardandgarden/research/french-tarragon-in-the-garden (read 2026-08-22): 'French tarragon should be propagated through cuttings or root divisions from an existing plant. It does not produce viable seed, and growing from seed typically results in the less flavorful Russian tarragon'; 'Russian tarragon, while not classified as being a different species, has flavor vastly inferior to French tarragon.'
remedy
Buy a started French tarragon plant, or propagate by division or cuttings taken from a known French tarragon plant. Do not sow a seed packet labelled "tarragon" expecting the culinary French herb.
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Another 110 rules in the corpus apply by situation rather than by species — rotation intervals, frost windows, sun, spacing and soil. Read the full evidence page.
