Spiny amaranth (Amaranthus spinosus L), a serious competitor to crisphead lettuce (Lactuca sativa L)

Three field studies on high-organic-matter soils were conducted to determine the zone of influence of spiny amaranth on lettuce head quality. Spiny amaranth reduced lettuce head firmness at all distances from the weed, less than or equal to 105 cm. Lettuce ribbiness increased at 15 and 45 cm compared with the weed-free control. Untrimmed lettuce head weight was not affected by spiny amaranth presence beyond 45 cm, Trimmed lettuce head weight was reduced at all distances compared with the control, Stem diameter and core length were not affected by spiny amaranth competition, The presence of a single spiny amaranth plant significantly influenced some lettuce quality traits at less than or equal to 105 cm.