The impact of uprooting and soil-covering on the effectiveness of weed harrowing

The impact of uprooting and covering plants on mortality and growth reduction was investigated in the laboratory using Lolium perenne L. and Lepidium sativum L. (harrowed 3–4 days after emergence) and Chenopodium quinoa Willd. (harrowed at emergence) as model weed species. Although the predominant initial effect of harrowing was to cover the plants, only 1–17% of the non-uprooted covered plants were killed because the depth at which they were buried by the harrow was shallow. Uprooting was more effective (47–61% mortality) but strongly dependent on soil moisture content. It accounted for 93 and 95% of L. sativum and C. quinoa mortality, but for only 60% of L. perenne mortality. In L. perenne , the species most sensitive to burying, a strong positive relationship was observed between the percentage of plants covered by harrowing and the fresh weight reduction of the total population 6 days after harrowing. The fresh weight reduction of the total L. sativum population was best related to the percentage of uprooted plants, but the percentage of covered plants also appeared to be a good predictor because of its correlation with uprooting. Most of the uprooted plants were also buried. The fresh weight reduction of the total C. quinoa population was not related to the covering effect of harrowing and only weakly related to the percentage of uprooted plants. The results indicate that the plant recovery process after harrowing needs further study and that field research methods should be refined so that they can better discern initial and final harrowing effects on weeds.