A persistent seed bank of the bull thistle Cirsium vulgare

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Abstract
Seeds of Cirsium vulgare were collected from two populations in southwestern Ontario and stored at the site of collection at three depths in the soil. After intervals of 6 to 30 months, seed samples were extracted, the seeds classed as germinated, dead, or firm, and then the firm seeds were set to germinate in a growth cabinet. Both populations established a persistent seed bank when left at the 15 cm depth; however, most seeds do not get buried at great depths. After storage at the surface or 3 cm deep, seeds in an arable sandy soil did not persist beyond 6 months. In contrast, some seeds stored in shaded conditions in a clay loam soil maintained viability for 30 months at die surface and at 3 cm. In some cases, the germination rate of seeds was retarded after storage, especially for seeds that were extracted from soils subjected to wetting and drying before seed retrieval. A separate experiment confirmed that wetting and drying can delay the germination of some seeds of C. vulgare. Key words: seed bank, germination, seed wetting and drying, Cirsium vulgare, bull thistle.