Sunflower residue suppresses germination of some small-seeded species.
Promising · costly if ignored → warn
How it works
Allelochemicals in Helianthus residue and root exudate.
What to do about it
Keep small-seeded direct sowings out of last year's sunflower ground, or set transplants into it instead of sowing seed.
How much it matters: Modest; matters for direct-sown neighbours, not transplants.
The evidence
Evidence: confirmed against 1 source in the research-based literature.
the source
'The Allelopathic Activity of Aqueous Extracts of Helianthus annuus L. ... on Germination, Development, and Physiological Indices of Pisum sativum L.', PMC10180669, pmc.ncbi.nlm.nih.gov/articles/PMC10180669 (read 2026-07-11): 'If a sensitive test plant is exposed to allelopathic compounds, its seed germination and seedling growth are reduced'; 'Aqueous extracts of various organs of sunflower significantly suppress the growth of weeds ... and agricultural plants.' Effect was concentration-dependent (stimulatory at low concentration, inhibitory at 75%), consistent with the 'modest' effect_size. NOTE: this study used pea (large-seeded); the small-seeded/direct-sown scoping is the corpus's operationalization (small seeds have less reserve to overcome inhibition; transplants bypass the germination window), not a claim from this source.