The effect of weeds on the growth and development of narcissus.
Competition from weeds (Agropyron repens, Stellaria media, Poa annua and others) in the first growing season hastened foliage senescence and reduced bulb growth in narcissus cv. Carlton. It also reduced flower production, bulb size and total yield of bulbs in the second growing season even though the crops were free of weeds in the second season. Weeds had no adverse effect before the crop flowered but dense weed cover between the end of the flowering period and the onset of normal senescence adversely affected the crop. Weed removal in May or early June avoided crop loss but removal thereafter merely accelerated crop senescence. Overwintered weeds had most effect on the crop since their major growth phase paralleled that of the crop. Spring-germinating weeds grew fastest during the final stages of the crop cycle and therefore had less adverse effect, although the crop itself reduced the growth and development of spring-germinating weeds. The results are discussed in relation to ways of improving the efficiency of weed control.