During the autumn of 1996, weed seeds were collected from around experimental fields at Horticulture Research International, Wellesbourne, Warwickshire, UK. For consistency, six species were chosen that had been included previously in the development of the original depth-of-emergence modelling studies (Grundy, Mead & Bond 1996; Grundy & Mead 1998). These species were Polygonum aviculare L., Chenopodium album L., Tripleurospermum inodorum (L.) Schultz-Bip, Veronica persica Poiret, Stellaria media (L.) Vill. and Veronica arvensis L. The species were also chosen for their horticultural significance and because they are representative of the range of seed weights typically found for broad-leaved horticultural weed species (Table 1). Plastic cylinders (length 300 mm, diameter 230 mm) were buried leaving 80 mm visible above the surrounding soil. Nylon mesh (Whaleys (Bradford) Ltd, UK) was placed at the base of each cylinder to prevent soil fauna from entering the cylinders and disrupting the seed positions or predating the seeds. Each cylinder was filled with sieved sterilized sandy silt loam (pH range 6–7, initial moisture content 3–5%, bulk density ranged between 1·0 and 1·5 g l−1, particles retained < 0·002 mm and screened to 8 mm) so that, after a period of natural settling, the soil surface within the cylinder was level with that of the surrounding soil. Ungerminated seeds were sown at each of six different depths including the surface (0, 1, 2, 4, 8, 16 cm). To allow the possible density–depth interaction to be assessed, a range of four densities were used, equivalent to 2407, 9628, 38 510 and 154 040 seeds m−2. This range of densities was comparable with those reported in surveys of weed seed densities (Roberts & Chancellor 1986). The different seed densities were achieved by scattering 100 seeds of a single species evenly within an area indicated by a circular template. The diameters of the templates were 28·75 mm, 57·5 mm, 115 mm and 230 mm, to give the equivalent density m−2.
Five replicates of each combination of species, depth and density were included in the trial, with each complete replicate occupying 24 plastic cylinders. Each cylinder contained one set of seeds of each species, each species being buried at a different depth. The allocation of species to depths within each replicate followed a 6 × 6 Latin square, with four cylinders in each replicate having the same allocation. The allocation of densities to depths was similarly achieved following an extended 4 × 4 Latin square (with two rows repeated), with six cylinders in each replicate having the same allocation. The final design was achieved by crossing these two designs to obtain each combination of species, depth and density once within each replicate set of 24 cylinders. By ensuring that each species appeared only once in each pot it was simple to determine the depth and density from which seedlings had emerged. When sowing the seeds, care was taken to avoid sowing seeds at high densities directly above each other, reducing the probability of emerging seedlings from lower depths disturbing the upper layer of seeds.
The soil columns in each experiment were left undisturbed for the duration of the recording period. The effects of rain, frost and wind on the weed seed distributions were considered negligible. Weekly weed emergence counts were made over a 2-year period with weed seedlings being removed by cutting at ground level once they had been recorded. The seeds were sown on the 2 December 1996 and the weekly recordings terminated on the 7 October 1998.