Control of perennial weeds by mechanical methods and anaerobic soil disinfection

Perennial weeds are difficult to control and ask for a specific approach. During the most recent years it has even become a great challenge to control these weeds in conventional farming systems, although in comparison effective perennial weed control in organic farming systems remains more difficult. Without the option of herbicides, perennial weed control asks for a combination of proper timing and adequate mechanical weed control. Other options such as anaerobic soil disinfection may be useful as well. A field trial on marine loam soil in Lelystad was setup testing three mechanical weed control strategies and two anaerobic soil disinfection strategies to control Cirsium arvense, Rorippa sylvestris, Sonchus arvensis, Calystegia sepium, Equestium arvense, Tussilago farfara, Elymus repens, Rumex obtusifolius, and Polygonum amphibium. A rod weeder mounted behind a rigid-tine cultivator was compared with a broadcast root knife operating at 5 cm depth and at 15 cm. The machines were brought into action whenever weather conditions were suitable. Anaerobic soil disinfection treatments comprised the incorporation into the plough layer (c. 25 cm) of the weed biomass with 40 tons/ha fresh grass biomass or incorporation of the weed biomass alone. The grass was added to improve soil oxygen use thus lengthening the period with anaerobic conditions. Both treatments were covered with plastic sheets immediately after incorporation. The soil remained covered between 28 July and 15 October 2010. In the untreated control plots, soil coverage reached 100% in the course of the season, regardless of weed species. Of the mechanical weed control methods the rod weeder showed the greatest reduction in soil coverage. Seven passes with the rod weeder between April 15 and August 12 reduced soil coverage from at least 90% for Rorippa sylvestris to 99% or more for Cirsium arvensis, Sonchus arvensis, Calystegia sepium, Equestium arvense, Tussilago farfara, and Polygonum amphihibium. Soil coverage was reduced less by treatments with the broadcast knife. Furthermore, soil coverage was reduced stronger by seven passes with the knife at 5 cm depth than after five passes with the knife at 15 cm, although cultivations at 15 cm were supposed to be more effective, needing fewer passes, than the root knife at 5 cm depth. The broadcast knife was most effective in controlling Cirsium arvensis, Sonchus arvensis, Equestium arvense, and Polygonum amphibium, resulting in at least 90% reduction of soil coverage at both treatment depths. At Tussilago farfara this level of control was only reached after seven passes at 5 cm depth, the deeper cultivation only reaching 50% reduction of soil coverage. Treatment of Calystegia sepium, Elymus repens and Rumex obtusifolius with the root knife showed a soil coverage reduction of between 45 and 75% at 5 cm depth and between 15 and 40% at a 15 cm working depth. Anaerobic soil disinfection without fresh grass addition reduced soil coverage with at least 96%, whereas addition of grass material increased this to at least 98.5%. Addition of fresh grass increased soil coverage reduction at all weed species compared with plastic sheets alone. This experiment was performed in Wageningen on a sandy soil with the same species (except for Tussilago farfara, which was replaced with Mentha arvensis and Stachys palustris) First year results indicate that the rod weeder performed best on a sandy soil as well (72 to 97% control, depending on the species), compared to the root knife treatments at 15 cm (3 to 96% control, depending on species) and 5 cm (69 to 99% control, depending on species), respectively. In the upcoming season analyses and the trials will continue, monitoring long term effects of the treatments in 2010