Emergence of Chenopodium album and Stellaria media of different origins under different climatic conditions A C GRUNDY*, N C B PETERS  , I A RASMUSSEN à , K M HARTMANN§, M SATTIN – , L ANDERSSON**, A MEAD*, A J MURDOCH  & F FORCELLA àà *Vegetation Management and Plant Establishment, Horticulture Research International, Wellesbourne, Warwick, UK,  Long Ashton Research Station, Department of Agricultural Sciences, University of Bristol, Long Ashton, Bristol, UK, àDanish Institute of Agricultural Sciences, Department of Crop Protection, Research Centre Flakkebjerg, Slagelse, Denmark, §Universitaet Erlangen-Nuernberg, Institut fuer Botanik I, Oekophysiologie, Erlangen, Germany, –Istituto di Biologia Agroambientale e Forestale, Sezione di Malerbologia – CNR Agripolis, Legnaro (Padova) Italy, **Department of Ecology and Crop Production Science, Swedish University of Agricultural Sciences, Uppsala, Sweden,   Seed Science Laboratory, Department of Agriculture, University of Reading, Earley Gate, Reading, UK, and ààUSDA-ARS, North Central Soil Conservation Research Laboratory, Morris, MN, USA Received 23 December 2002 Revised version accepted 16 February 2003 Summary The emergence behaviour of weed species in relation to cultural and meteorological events was studied. Dissim- ilarities between populations in dormancy and germina- tion ecology, between-year maturation conditions and seed quality and burial site climate all contribute to potentially unpredictable variability. Therefore, a weed emergence data set was produced for weed seeds of Stellaria media and Chenopodium album matured and collected from three populations (Italy, Sweden and UK). The seeds were collected in two consecutive seasons (1999 and 2000) and subsequently buried in the autumn of the same year of maturation in eight contrasting climatic locations throughout Europe and the USA. The experiment sought to explore and explain differences between the three populations in their emergence behaviour. Evidence was demonstrated of synchrony in the timing of the emergence of different populations of a species at a given burial site. The relative magnitudes of emergence from the three popu- lations at a given burial site in a given year were generally similar across all the burial sites in the study. The resulting data set was also used to construct a simple weed emergence model, which was tested for its application to the range of different burial environments and populations. The study demonstrated the possibility of using a simple thermal time-based model to describe part of the emergence behaviour across different burial sites, seed populations and seasons, and a simple winter chilling relationship to adjust for the magnitude of the flush of emergence at a given burial site. This study demonstrates the possibility of developing robust generic models for simple predictions of emergence timing across populations. Keywords: predictive modelling, weed populations, win- ter chilling, weed seed origins, dormancy, emergence, Chenopodium album, Stellaria media, climate. Introduction A better understanding of the emergence behaviour of weed species in relation to cultural and meteorological events presents a number of opportunities. For example, this information could be used to target the timing of cultivation and maximize the efficacy of control strat- egies, regardless of whether by chemical or physical methods (Vleeshouwers, 1997). In relatively short-term studies, identification of the factors that are important in determining the patterns of emergence for different weed species is difficult. However, when results from longer term studies are averaged over time, they often demon- strate that some weed species follow characteristic, and potentially predictable, patterns of annual emergence (Lawson et al., 1974; Baskin & Baskin, 1985). The well- known periodicity tables for common agricultural weeds (Ha˚kansson, 1982, 1983; Roberts, 1982) are derived from data averaged from such long-term emergence studies. They provide a general guide to the average Correspondence: A C Grundy, Plant Establishment and Vegetation Management, Horticulture Research International, Wellesbourne, Warwick CV35 9EF, UK. Tel: (+44) 1789 470382; Fax: (+44) 1789 470552; E-mail: andrea.grundy@hri.ac.uk Ó European Weed Research Society Weed Research 2003 43, 163–176