Concern about potential increases in weed populations without the use of herbicides has limited the uptake of organic farming. However, as both public demands for organic produce and the pro®le of organic farming have increased in recent years, so too has the range of weed control options. Progress in cultural methods of weed control has included the use of novel weedsuppressing cover crops, and the identi®cation of speci®c crop traits for weed suppression. Direct weed control has also seen developments, with new implements appearing on the market that could bene®t in the future from sophisticated machine guidance and weed detection technology. Advances in novel techniques such as steaming have also been made. Many weed control operations in organic systems present the grower with con¯icts, and both these and many of the most recent developments in organic weed control are reviewed. An increase in our understanding of weed biology and population dynamics underpins long-term improvements in sustainable weed control. The outcome of these studies will bene®t conventional and organic growers alike. Emphasis is given to the need for¯exibility and a combination of weed biology knowledge, cultural methods and direct weed control to maintain weed populations at manageable levels.

Summary Experiments to monitor weed seedling emergence from various soil depths were begun in 1989. Recently shed seeds were buried in either narrow bands at a range of depths in the soil or mixed evenly between the soil surface and each of these depths. Total emergence of Stellaria media (L.) Vill., Matricaria spp., Veronica persica Poiret, Veronica arvensis L. and Polygonum aviculare L., declined with increasing depth of burial. A similar pattern was observed for Chenopodium album L. with the exception of emergence from the surface layer. The reduction in emergence with increasing soil depth was greater for seeds in the narrow‐banded treatments than for those in broad layers. A model was developed using the data from the narrow‐banded treatments to predict emergence from distributions over broad layers. Predictions from the model closely agree with the data from the present broad‐layer treatments and those from a similar experiment made in the 1960s. By including the effects of depth of seed burial on seedling emergence, this model could be combined with models that determine the effects of cultivation on seed distribution, and therefore improve precision of predictions of seedling emergence from the seedbank.

The effects of soil temperature and soil moisture content on the rate of loss of N‐(1‐ethylpropyl)‐2,6‐dinitro‐3,4‐xylidine (I, AC 92,553) were measured under controlled conditions. The time for 50% disappearance in a sandy loam soil at 75% of field capacity was inversely related to temperature (98 days at 30°; 409 days at 10°). At 25°, the half‐life increased with decreasing soil moisture content (122 days at 75% of field capacity; 563 days at 12.5%). In seven soils with different properties there was a trend towards a slower rate of loss as the organic matter content of the soils increased and the half‐life varied from 72 to 172 days, first‐order kinetics being obeyed. The herbicide was lost rapidly from an inert surface and 97% loss was recorded after 28 days at 25°. Losses from soil surfaces occurred more slowly and were greater from wet compared with dry soil. In the field, it was more persistent when incorporated than when applied to the soil surface. More than 60% of I incorporated in April 1975 could be detected the following September, but when applied to the soil surface, only about 20% of the applied dose remained by this time. Residues measured by gasliquid chromatography using a thermionic nitrogen detector closely paralleled those measured by a bioassay based on the root growth of buckwheat.

In four field experiments in different years with summer cabbage drilled in May, naturally-occurring weed populations of 50-540 plants per m2 reduced the yield of marketable cabbage by 47-1007~ from that of weedfree crops. Species composition of the weed population and the rate of emergence and early growth relative to that of the crop were important factors in determining the extent of yield loss. If weeds were removed no later than about 3 wk after 50% crop emergence and the crop was subsequently kept clean there was no yield loss. If the crop was kept clean without soil disturbance for the first z wk or longer, weeds that established after

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