Some Effects of Weed Competition on the Growth of Onions

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Abstract
Summary In competition with annual weeds (160–230/m2) the photosynthetic capacity and consequent bulb size of onion plants were greatly reduced. Growth was depressed, leaf-blade production was delayed and fewer leaves were formed; the latter were smaller, contained less chlorophyll and died more rapidly than those of the weed-free crop. When weeds were removed 7½ weeks after 50% crop emergence, there was little subsequent increase in dry weight or bulb size and only one or two further small foliage leaves were produced. By this time the dry weight of weeds per unit area was twenty times that of the crop, and about half the applied nitrogen and a third of the potassium had been taken up by the weeds. The results indicate that the severe effects of weeds on onions are associated with an initial competitive disadvantage of slow crop growth, while the limited duration of the vegetative phase prevents recovery when the weeds are removed after competition has begun.

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