The relative sensitivity of various crops and weeds to celery root residue incorporated into soil was determined in greenhouse experiments. A range of concentrations was used to determine the percentage of celery residue needed to cause a 50% reduction in plant growth (I50). The most sensitive weed was spiny amaranth and the least sensitive was common purslane. The most sensitive crop was radish and the least sensitive was celery. Selectivity was calculated as crop I50/weed I50 and varied from a maximum of 13 for celery and spiny amaranth to a low of 0.4 for radish and common purslane. Selective weed suppression might be obtained when celery residue is present.

Tolerance of direct-seeded pepper under clear plastic mulch to herbicides applied pre- and post-emergence was investigated in 1991 and 1992. Weed control of herbicides applied preemergence was also studied. Paprika cultivar ‘Buketen’ and canning cultivars ‘Piquillo de Lodosa’ and ‘Pico de Mendavia’ were seeded in the field in Zaragoza, Spain, in a loamy soil. Percent emergence, plant mortality after the postemergence treatment, dry weight of plants, and fruit weight were recorded. Pepper was tolerant to diphenamid and napropamide applied preemergence but these herbicides did not control broadleaf weeds. Clomazone, when applied preemergence resulted in chlorotic seedlings and at 2 kg ai/ha reduced the final stand, but was selective when applied at the 6 to 8-leaf stage of the crop. Pepper was tolerant to diethatyl-ethyl when applied preemergence. However, diethatyl-ethyl applied postemergence caused severe phytotoxicity symptoms, and when mixed with diphenamid, fruit ripeness in 1992 was delayed. Weed control during the crop emergence period was better when diethatyl-ethyl was used mixed with diphenamid or napropamide.

The seed composition in the upper 15-cm soil horizon was determined and correlated with weed seedlings growing with fieldbeans (Phaseolus vulgaris L. ‘Valley’). The total seed reservoir averaged 250 seed/kg of soil, and 19 species were represented. Seed occurring with the most frequency were redroot pigweed (Amaranthus retroflexus L. ♯ AMARE), common lambsquarters (Chenopodium album L. ♯ CHEAL), and common purslane (Portulaca oleracea L. ♯ POROL). Seed from these plants accounted for over 85% of the seed found. The number of barnyardgrass [Echinochloa crus-galli (L.) Beauv. ♯ECHCG], buffalobur (Solanum rostratum Dunal ♯ SOLCU), common lambsquarters, common purslane, and common sunflower (Helianthus annuus L. ♯ HELAN) seed in the soil was correlated with the number of plants growing in the field with fieldbeans. A correlation occurred between redroot pigweed, yellow foxtail [Setaria lutescens (Weigel.) Hubb. ♯ SETLU], and barnyardgrass growing in corn (Zea mays L.) fields in the fall of the year and plants growing in the field with fieldbeans the following year.

Patterns of weed emergence in upland and lowland soils were observed for several months after tillage by counting all emerged species. Forty to 50% of weed emergence occurred within 6 weeks of tillage on both sites. Significant emergence occurred within 3 weeks on both sites, but very little emergence occurred within 1 week in lowland soil. The data confirm the strong influence of soil tillage on weed emergence. They also suggest an influence of radiant energy on weed emergence. Peaks of weed emergence 6 or more weeks after tillage often occurred coincident with or soon after peaks of radiant energy.

The influence of two weed management systems was determined on weed seed and seedling populations and on yields of cantaloupe (Cucumis melo L. var. reticulatus Naudin ‘Perlita′), bell pepper (Capsicum annuum L. var. grossum ‘Grande Rio 66′), cotton (Gossypium hirsutum L. ‘CP 3374′), onion (Allium cepa L. ‘1015Y′), and cabbage (Brassica oleracea, var. capitata L. 'Sanibel′) sequentially grown in two 3-yr cycles. Palmer amaranth (Amaranthus palmeri S. Wats. # AMAPA) did not exist initially, but hurricane-introduced seed populations increased to 1.1 billion/ha as seed populations of common purslane (Portulaca oleracea L. # POROL) decreased from 786 million/ha to 124 million/ha in the 6-yr period, without weeding or herbicide. Use of herbicides and handweeding reduced Palmer amaranth seed populations 98%, but 18 million/ha still remained after 6 yr. The use of herbicides and Palmer amaranth interference decreased the seed populations of common purslane by 84%, but handweeding was inefficient. Yields of all but the first crop of cantalouple were almost totally eliminated by season-long interference of Palmer amaranth. Savings with the utilization of herbicides rather than handweeding ranged from $62/ha for cotton to $4703/ha for bell pepper.

Field studies were conducted in 1990 and 1991 at Davis and Five Points, CA to evaluate weed control with napropamide and hand-weeding and the effect on yield and profitability in transplanted bell pepper. Weed cover 8 wk after transplanting averaged less than 10% in plots hand-weeded biweekly for 6 wk after transplanting. Weed cover at harvest was less than 30% in plots hand-weeded at 8 wk after transplanting. Napropamide treatment reduced weed cover at 8 wk after transplanting an average of 34% compared to untreated plots, but differences declined to 14% at harvest. Time required to hand-weed plots was greater if the interval between weeding was 4 instead of 2 wk. The time needed to hand-weed plots was reduced from 38% to 71% by napropamide treatment. When weeds were excluded by hand-weeding for the entire season, bell pepper yielded 25 810 kg/ha and was 4% to 18% higher than other treatments. Net returns were greater for plots that were hand-weeded the entire season or when napropamide was combined with hand-weeding at 4 and 8 wk after transplanting, compared to other treatments.

Field studies were conducted in Arizona and California to evaluate the performance of glyphosate-tolerant lettuce and to determine the critical time of weed removal. Glyphosate was applied as a single or as a sequential application at 840 g ae/ha. Single glyphosate applications were made to lettuce at the two-, four-, six-, and eight-leaf stages. Sequential applications were made to lettuce at the two- or four-leaf stage followed by (fb) a second application 14 d after the first. Weed control efficacy, weeding times, and lettuce yield were all measured. Overall, glyphosate applied postemergence (POST) provided better weed control than the commercial standards bensulide or pronamide applied preemergence. Single glyphosate applications at the four-leaf stage and sequential applications at the two-leaf stage fb a second application 14 d later provided excellent control of most weeds, including redroot pigweed. Estimates of the critical time of weed removal were 26 to 29 d after emergence. Glyphosate treatments caused no adverse effects on lettuce. Lettuce head fresh weights in the glyphosate treatments were equal to or higher than those in bensulide or pronamide treatments. For crops such as lettuce, with few effective herbicides, the development of glyphosate-tolerant lettuce offers the opportunity to develop effective POST weed control programs.