Are some weeds sleeping? Some concepts and reasons

Sleeper weeds, a relatively new concept, are defined as a sub-group of invasive plant species for which their population sizes are known to have increased significantly more than 50 years after they became naturalized. The invasions of the European herb Hieracium pilosella into New Zealand, the northeast Asian Fallopia japonica and the Sicilian Senecio squalidus into Britain are presented as well-documented examples of sleeper weeds. While the concept of sleeper weeds has gained some acceptance with weed managers, its application remains a retrospective activity at present. Genetic factors, environmental suitability and anthropogenic disturbance are discussed as they affect sleeper weed status. Accurate botanical records for invasive species over long periods of time are relatively rare and limit documentation of the phenomenon. Some mathematical and ecological problems remain with the concept and its wider application to weed invasion, including the adequacy of the species level for determining sleeper weed status in genetically diverse and widely distributed groups of invasive plants.