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The Virtual WeedPatch
define.html
What is a weed?
Definitions of a Weed
1: anthropomorphic (plants relationship to us as humans); tells us something
about
how we view nature;
2: "a plant out of place" (WSSA, 1956);
3: "competitive and aggressive behavior" (Brenchley, 1920);
4: "persistance and resistance to control" (Gray, 1879);
5: "useless, unwanted, undesirable" (Bailey, 1941);
6: "a plant whose virtues have not yet been discovered" (Emerson,
1878);
7: "appearing without being sown or cultivated" (Brenchley, 1920);
8: "a plant that grows spontaneously in a habitat greatly modified
by human action" (Harper, 1944);
9: "unsightly" (Thomas, 1956)
The "Ideal Weed Concept"
Ideal characteristics of the worst weeds (Baker, 1965, 1974):
Seed bank:
-Germination requirements fulfilled in many environments.
-Discontinuous germination (internally controlled) and great longevity of
seed
Vegetative:
-Rapid growth through vegetative phase to flowering
-If a perennial, vigorous vegetative reproduction or regeneration from fragments
-If a perennial, brittlenenss so as not to be drawn from ground easily
-Ability to compete interspecifically by special means (rosette, choking
growth,
allelochemicals)
Reproductive:
-Continuous seed production for as long as growing conditions permit.
-Self-compatibility but not complete autogamy or apomixy
-Cross-pollination, when it occurs, by unspecialized visitors or wind
-Very high seed output in favorable environmental circumstances
-Production of some seed in wide range of environmental conditions; tolerance
and
plasticity
-Adaptations for short-distance and long-distance dispersal
Adaptive Characteristics of Agronomic Weeds
(Patterson, 1985):
Related to physiology, growth, and competitiveness
-High relative growth rates in seedling stage
-High rates of photosynthesis
-Rapid development of exploitative root systems
-Rapid partitioning of photosynthate into new leaf area production
-Rapid vegetative growth to reproductive phase
-Special "weapons" for interference
-Freedom from environmental constraints ("general purpose genotype");
high capacity
for acclimation to changing environment
Related to reproductive phase
-Breeding systems that provide some outcrossing but also allow self-fertilization
-Copious seed production under favorable conditions with some seed production
occurring over a range of favorable and stressful conditions
-Pollination by wind or generalized insect visitors
Related to cultural practices
-Morphological and physiological similarity to crop
-Timing of seed maturity to coincide with crop harvest
-Resistance or tolerance to chemical herbicides
-Resistance to mechanical control; regeneration from rhizomes or other vegetative
propagules
-Seed dormancy, longevity in soil; discontinuous germination over long periods
of time
References
Baker, H.G. 1974. The evolution of weeds. Ann. Rev. Ecol. Syst. 5:1-24.
Baker, H.G, 1965. Genetics of colonizing species.
Patterson
