The effects of alternatives to soil residual herbicides on weed control, yield and quality of hops

Several non-chemical methods of weed control in hops (Humulus lupulus) were evaluated in a three-year study. The results show that the use of cultivation, grass and winter rye cover crops, straw and plastic mulches all gave satisfactory weed control in both alleyways and hop rows. Data are presented on the effects of the main treatment groups, on the yield, alpha-acid content and alpha yield of hops. The effects on other hop cone quality criteria are also reported. Cultivation to control weeds had to be timely, and by the end of the study, numbers of weeds germinating in the spring were increasing. The use of a winter rye cover crop was successful in reducing over-wintering weed populations in cultivated plots. Grass reduced yield but raised the alpha-acid content of the cones, resulting in no overall loss in yield of alpha-acid. Yield reductions were overcome by adding extra nitrogen as an early spring top dressing. but this tended to reduce alpha-acid content to give similar alpha yields. Analysis of soil mineral nitrogen contents of soils to 90 cm depth in spring and autumn indicated that extra spring nitrogen was readily taken up by grass swards, helping to overcome its competitive effect on the hop plants. Winter rye reduced soil mineral nitrogen levels although this failed to reach statistical significance (P>0.05) but this nitrogen was returned to the soil when the cover was cultivated out in spring. Straw mulches tended to raise alpha-acid levels enough to raise alpha yield one year by 5% compared with conventional residual herbicide, although this failed to reach statistical significance (P>0.05). The implications of the results to growers of hops are discussed.