The biological attributes favouring success as a weed are summarized in Table 6.3. An open growth habit combined with the early induction of flowering means that small plants reproduce rapidly by seed and continue growing as further flowering takes place. This ensures continuing outputs of seeds. Plants growing in disturbed (cultivated) habitats tend to allocate greater proportions of their photosynthetic output to seed production than the more stable types, as demonstrated in Table 6.4.
Table 6.4. Average output of seeds or fruits for ten common weeds that compete with crops.
Average output per plant
Groundsel (Senecio vulgaris) 1,000-1,200 F
Common chickweed (Stellaria media) 2,200-2,700 S
Shepherd's purse (Capsella bursa-pastoris) 3,500-4,000 S
Greater plantain (Plantago major)3 13,000-15,000 S
Common poppy (Papaver rhoeas) 14,000-19,500 S
Prickly sowthistle (Sonchus asper) 21,500-25,000 F
Perforate St John's Wort (Hypericum perforatum)a 26,000-34,000 S
Canadian fleabane (Conyza canadensis)a 38,000-60,000 F
Hard rush (¡uncus inflexus)a 200,000-234,000 S
aPerennial. After E. Salisbury (1961).
The competitive ability of weeds results from a combination of high reproductive potential and rapid vegetative spread. This embraces physiological (efficient ion uptake and rapid root growth) and morphological factors (ability to climb, scramble over competitors for light; produces rosettes which spread out close to the ground and smother competitors; and large and vigorous habit). The Brassica crop, especially where it is transplanted, may even improve conditions for the establishment of weeds after their germination by offering protection from extremes of temperature and the drying effects of wind. This provides a competitive advantage for some weed species by promoting an early start to their growth.
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