Key words: organic farming, weeds, weed management, weed ecology, weed diversity, matrix population model, elasticity analysis, neighbourhood model, survey, crop row spacing, mechanical hoe, harrow, Polygonum convolvulus , Polygonum persicaria , Stellaria mediaExperiments, monitoring studies and modelling of weed population dynamics were carried out to investigate potential methods for reducing weed populations in farming systems where herbicides are not applied (organic farming). Six years of monitoring weed populations on five organic farms showed that farmers who took a long-term approach to weed management had lower weed populations. Farms with low weed densities also had a lower diversity of weed species. An experiment was carried out to investigate whether using the combination of a wide row spacing and aggressive weed control (mechanical hoeing) or a narrow row spacing and less aggressive weed control (harrowing) would decrease weed populations more. Using weed seed production as the criterion for comparing the row spacing/weed control combinations, it was concluded that a narrow row spacing with less aggressive control resulted in fewer weed seeds being produced. Because the experiment involved marking individual plants of the species Polygonum convolvulus , Polygonum persicaria , and Stellaria media , it was possible to investigate how individual plant biomass and survival are related to descriptors of the local environment. In this case the descriptors were distance to the nearest crop plant and the local row width. For all three species it was shown that the same form of predictor gave the best fit and included distance to the nearest crop plant and the crop row spacing where the plant is located. Survival in the wide spacing could be predicted using the distance to the nearest crop row, while in the other row spacings all plants had an equal chance of survival. Predictors of individual plant biomass and survival can be used in modelling the spatial dynamics of weed populations. Finally the effect of ...