Seed bank dynamics of Onopordum acanthium: emergence patterns and chemical attributes

Summary 1 Cypselas (fruits) of Scotch thistle (Onopordum acanthium) germinate unpredictably over a long period of time. We evaluated the effects of soil type, burial depth and collection date on the emergence patterns of two populations of O. acanthium, ESW and Quarry (Q). 2 From each of four separate collections made throughout 1996, five replicates of 200 cypselas each were placed on the surface or buried at depths of 3 or 15 cm in both sand and silt-loam soils (i.e. field conditions) and emergence was recorded biweekly between August 1996 and September 1999. 3 Emergence was intermittent over the 3 years, with a higher total percentage from silt loam (17%) than from sand (9%), and from 3 cm depth (18%) compared with the surface (8%). 4 After 3 years, all remaining viable, split (germinated but not emerged), decayed and unfilled cypselas were retrieved. Total germination over the 3 years (emerged plus split) was up to 77% at 3 cm in silt loam but always less than 10% at 15 cm in both soils for both populations. Although a higher percentage of cypselas germinated under controlled conditions when retrieved from loam compared with sand (89% vs. 66%), a significant number from all treatments did not germinate until scarified. 5 Cypselas that ripened under the warmest temperature (collection 3) had the highest emergence in the year of collection (1996), while more of those that ripened under cooler temperatures (collections 1 and 4) emerged in later years (1998 and 1999). Both initial and later emergence patterns for both populations varied greatly with collection date. This may be the first time that seedling emergence from the seed bank for different collections has been monitored over several years. 6 We compared waxes, lignins and soluble phenolics in the coats of cypselas retrieved from soil (early germinators) with those that germinated only after scarification. More surface wax was found in dormant cypselas. 7 Understanding these complex germination/emergence patterns may help develop control policies for O. acanthium.