Ideal Characteristics For A Successful Weed

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 6.3. Characteristics of a successful weed.

  1. Germination requirements fulfilled by many environments
  2. Discontinuous germination (internally controlled) and great longevity of seed
  3. Rapid growth through the vegetative to the flowering phase
  4. Continuous seed production during the entire growing season
  5. Self-compatible breeding system but not completely autogamous or apomictic
  6. Cross-pollinated by unspecialized visitors or wind
  7. Very high seed output in favourable environmental circumstances
  8. Produces at least modest seed yield in a wide range of adverse environments
  9. Adapted for short- and long-distance seed dispersal
  10. Where the life cycle is perennial, the plant has vigorous vegetative reproduction or regeneration from stem or root fragments
  11. Perennials have brittle stems or roots that are not easily drawn out of the ground
  12. Abilities to compete with other organisms by specialized means such as rosette habit, choking growth or the production of allelochemicals

Table 6.4. Average output of seeds or fruits for ten common weeds that compete with crops.

Weed species

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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