• Mimosa pudica has short spines on stem.
• Oxalis latifolia produces bulbils in roots.
• Cyperus rotundus produces tubers.
• Panicum produces rhizomes in root.
• Common name of Aegeratum conyzoides is goat weed.
• The chief agents of weed seed dispersal are wind, water, animals and man.
• Weed seeds that disseminate through wind possess some structural modifications to help in their dissemination.
• Pappus is a parachute like structure produced by asteraceae members. Ex: Parthenium, Sonchus sps, Tridax procumbens, Aegeratum conyzoides
• Weed seeds are covered with special hairs (comose) in Calotrophis gigantea.
• Weed seeds enclosed in balloon like structure which is modification of calyx. Ex: Physalis minima Cordiospermum helicabum
• Seeds of Orobanche and Striga are so light in weight and easily disperse with windstorm, without any special arrangements.
• Many weedy fruits and seeds are eaten by birds and animals during grazing and above 10-15% of undigested weed seeds are passed through animal excreta in viable form. This mechanism of weed dispersal is known as endozoochory.
• Weed seeds are carried to long distances by clinging to the fur of the animals like sheep, goat etc. which are aided by special appendages such as hooks, sharp spines, sticky glands etc. Ex: Achyranthes aspera, Tribulus terrestris, Spear grass (heteropogon), Xanthium strumarium
• Satellite weeds are the weeds which resemble to that of crop plant. Ex: Echinocloa crusgalli Phalaris minor
PERSISTENCE OF WEEDS
• Persistence refers to the ability of weeds to invade an environment repeatedly even when it is completely removed from the place.
• Weed – seed production/plant
Amaranthus sps -1,96,000
Chenopodium album – 72,000
Striga lutia – 60,000-70,000
Echinocloa colonum – 40,000
Celosia argentia -11000
• One year seeding is seven years weeding.
• The viability of seed is the lifespan of a particular seed during which it is able to germinate.
• Cynodon dactylon seeds retain viability for 2 years.
• Tribulus terrestris seeds remain alive for 8 years.
• Cyperus rotundus seeds remain alive for 20 years.
• Convolvulus arvensis seeds remain alive for 50 years.
• The % of viability of weed seeds is very high and even the immature seeds are viable that is not so incase of crop plants
• Seed dormancy is a protective character that a seed possess for their sustainable survival on the earth.
• Majority of the weed seeds lying below 5 cm soil depth remain dormant and acts as source for future flushes of the weeds.
• Innate dormancy, enforced dormancy and induced dormancy are the three types of dormancies operating in weed seeds.
• Innate dormancy is a genetically controlled character. It is due to presence of hard seed coat. e.g., Tribulus terrestris Ipomea sps Xanthium sps.
• Enforced dormancy is due to placement of weed seeds deeper than 5cm in the soils. Weed seeds under this kind of dormancy germinate readily whenever they are brought to the top layers by tillage and provided congenial conditions for their germination.
• Induced dormancy results from sudden physiological changes takes place in the seed. Even the release of dormancy in weed seeds is erratic.
• Alternanthera echinata is highly drought resistant.
• Weeds reproduce both sexually and asexually.
• Weed – mode of reproduction
Cyperus rotundus – tubers & seeds
Cynodon dactylon – Seeds & vegetative propagules
Convolvulus arvensis – seeds & vegetative means
Panicum repens – seeds & vegetative means
• Weeds are persistent due to evasiveness. They are not easily destroyable by animals and man because of their bitterness, disagreeable odour, spiny nature e.g., Tribulus terrestris Chromolina odorata Eupatorium sps.
• Weeds possess self regeneration capacity. Weeds are self sown plants. They do not require any preparation of seedbed for their germination, fertilization, plant protection and irrigation.
• Weed seeds readily germinate on undisturbed land or soil under favourable environmental conditions.
• Ecology is the study of interactions between the plants and their environment.
• Weed ecology is the study of interactions between the weeds and their environment.
• Aldrich strongly suggests that weed management must deal with the interactions of all the factors with the weeds.
• Early weed science literature reveals that dominant weed population in many crops specially in small grain crops are annual broad leaved weeds.
• After the wide spread use of phenoxy acid group of herbicides (2,4D) there was a gradual shift from annual broad leaved weeds which are effectively controlled by phenoxy acid herbicides to annual grassy weeds.
• Monocropping and monocultural environment create ecological changes that determine what weeds will succeed.
• Tillage practices having influence on weed flora, change in the cultivation practices also leads to change in weed flora.
• Understanding weed crop ecology will lead to more effective weed prevention, management and control.
• Yellow nut sedge does well in subhumid and warm temperate regions, but it does not thrive well in temperate areas with prolonged frost.
• Purplenutsedge thrives well in humid tropics and subtropics.
• Water hyacinth an important aquatic weed in tropics and subtropics is not yet invaded the temperate waters.
• Soil pH is an important determinant of which weed plants grow in an area.
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