| Summary: | Weed management represents one of the most serious and costly challenges in organic crop production systems. Agricultural waste/byproducts might present phytotoxicity that can be exploited to control weeds. Two experiments were designed to study the effects of four concentrations of olive vegetation water (OVW) and a control water treatment (with no OVW) on cheeseweed (<i>Malva </i><i>parviflora</i> L.) seed germination in petri dishes and pots. In a third experiment, two rates of four composts (crop residue mix (CR), olive pomace (OP), dairy/horse manure (DM), and an OP/DM mix) were mixed into a clay‒loam soil at 0.10 or 0.20 L L<sup>−1</sup>, to assess their effects on weed number and biomass, in addition to bell pepper (<i>Capsicum annuum</i> L.) growth. In the petri dish experiment, the three highest OVW concentrations completely prohibited germination during the five-week duration of the study. For the pot experiment, 25 mL application of OVW significantly delayed and reduced cheeseweed germination, with the reduction being proportional to the concentration of OVW. In the third experiment, composts reduced weed dry matter (composed mostly of purslane (<i>Portulaca</i><i> oleracea</i> L.)), with the CR compost being the most effective, reducing total weed biomass by 67% compared to the control. CR10 and DM10 tended to increase bell pepper yields, although none of the plant parameters was significantly affected by the compost treatments.
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