Aldrich (1984)
defined weed as a plant that originated under natural environment and in
response to imposed and natural environments , evolved and continues to do so
as an interfering , associated with our crops and activities . Blatchley (1912)
defined it as a plant out of place, or growing where it is not wanted. Thomas
(1956) defined it as a useless, undesirable and often very unsightly plant of
wild growth, usually found in land which has been cultivated or in areas developed by man for specific
purposes other than cultivation . Baker (1965) defined it as a plant whose
populations, in a given
geographical area, grow entirely or predominantly in
situations markedly disturbed by man (without, of course, being deliberately
cultivated plant). Harper(1944) in his own opined that “weed is a plant that
grow spontaneously in a habitat that has been greatly modified by human
action.”
A weed is any plant that require some
form of action to reduce its effects on the economy, the environment, human
health and amenity. Weeds are also
known as invasive plants. Invasive weeds are among the most serious
threats to natural environment and primary production industries.
They have major
economic , environmental and social impacts causing damage to natural land
scapes, agricultural lands water ways and coastal areas . They displace native
species, contributes significantly to land degradation and reduce farm and
forest productivity. Weed inversions change the natural diversity and balance
of ecological communities; these changes threatens the survival of many plants
for space, nutrients and sunlight.
Weeds are either native species that are
colonizing ecosystem in which they did not previously exist, or they are exotic
plants introduced to a particular area which have formed naturalized as garden
plants, pasture, grasses or other horticultural plants.