Fennel is allelopathic; a suppressive effect on neighbouring vegetables is plausible but poorly characterized (evidence is weed-directed, in vitro).
A good hunch · suboptimal if ignored → advise
How it works
Reported allelopathic exudates. Effect size poorly characterized.
The evidence
Evidence: confirmed against 3 sources in the research-based literature.
the 3 sources
Nourimand, Mohsenzadeh, Teixeira da Silva & Saharkhiz (2011), 'Allelopathic Potential of Fennel (Foeniculum vulgare Mill.)', Medicinal and Aromatic Plant Science and Biotechnology 5(1):54-57 (read 2026-07-13): fennel-seed ethanolic extract inhibited germination and seedling growth of FOUR WEEDS (perennial ryegrass, wild barley, oat, dandelion) in vitro at 2.5-10%; no crop or vegetable was tested.
Tanase, Istrate & Stoleru (2026), 'Allelopathic Interactions in Vegetable Production Systems', Horticulturae 12(4):438, doi:10.3390/horticulturae12040438 (read 2026-07-13): lists fennel among aromatic vegetables (with dill, parsley, carrot, onion) whose extracts can have 'both inhibitory and stimulatory effects on plant germination and growth, depending on the species, concentration, and application conditions'; no fennel-specific suppression of a neighbouring vegetable is reported.
Gap: no study tests fennel suppressing a neighbouring VEGETABLE in situ; documented activity is weed-directed and in vitro. The claim was reworded 2026-07-13 to say exactly that, and the maintainer read both papers 2026-07-25 and promoted it. What is verified here is the HEDGE - that the effect is plausible and poorly characterized - not a suppression effect. A field trial of fennel against a neighbouring vegetable would change this rule whichever way it came out.