Broadly speaking there are 3 modes of propagation in weeds i) Sexual ii) Axsexual iii) Vegetative, which give rise to different types of propagules in different weed spp.
Sexual reproduction (propagation through seeds)
This refers to the fusion of two reproductive units or the gametes by a) Conjugation b) Fertilization. Majority of our weeds reproduce by distinct seed formation through fertilization and they are largely „monoecious‟. A few like Canada thistle (Cirsium arvense) and eel grass (Vallisneria spirallis) are „dioecious‟ which bear male and female flowers on different individuals. Obviously, only the female plants of such weeds set seeds. Seed production in weeds is prolific, particularly in annuals and biennials (table 1) but in perennial weeds seed production facility is limited like in eg.Cyperus and Cynodon spp which produce only 40-170 seeds / plant but with exceptions which produce thousands of seeds / year / plant by johnson grass (S.halepense) and tiger grass (S. spontaneum) as annual weeds
In an undisturbed field like, no tillage farming system weed seeds fall on the ground tend to remain on top 1 cm. soil layer. But in conventional farming system tillage tend to disturb their distribution and weeds present in the plow layer of soil.Some weeds go up to 30 cm or more through cracks and crevices. Commelina benghalensis places its seeds directly inside the soil since it produces aerial and underground flowers and seeds. In addition to prolific seed production, weed seeds are capable in retaining their viability for 2-25 years depending on weed species within the soil. Rumex spp showed 63-90% germination up to 2 to 7 years after these were buried upto 30 cm deep. While Echinochloa, Poa and Chenopodium spp.germinated even after 25 years. Weed seeds differ widely in their shape and size as well as their viability. Many of these germinate immediately after harvest, but Datura, Physalis and Setaria spp. which remain dormant for short or long periods before reviving their viability. In case of non dormant weed seeds like Eleusine indica and Bidens sps exhibited over 60% germination against Cynodon dactylon and Cyperus rotundus which showed only 0 and 6% germination respectively
Asexual reproduction
This occurs without union of reproductive cells. Here each reproductive cell is simply cut off from the parent plant and it grows into a new individual by fission or spore formation. The sexual conjugation and asexual reproduction is limited to weedy algae, horse tails (Equisetum sp) and ferns
Vegetative reproduction
In vegetative propagation a portion of the mother plant either stem or roots gets detached and grows into a separate individual capable of colonizing new areas of land or the weeds may produce some specific modified vegetative organs for this purpose both above and under ground. The vegetative reproduction in weeds may be as simple as budding (gemmation) like in water blooms (phytoplankton ‟s) or it may involve highly developed adventitious organs such as found in perennial vascular weeds Vegetative propagation is primarily a feature of perennial weeds and this has two advantages like purity of parental stock is maintained and quick multiplication. These modifications in the form of rhizomes root stocks, runners, tubers, bulbs, bulbils and bulblets, stems and roots.
Rhizomes and root stocks
It is a horizontally growing underground modified shoot bearing nodes, internodes, buds and scaly leaves. Cynodon dactylon uses rhizome under the ground, runners and stolons over the ground. When the rhizome tends to grow vertically downward, it s called a rootstock, such as seen in Johonson grass (Sorghum halepense). The two terms rhizome and rootstock are often used synonymously. In quack grass (Agropyron repens) rhizomes are sometimes called SOBOLES. The vegetative reproduction through rhizomes is a feature of perennial grasses, sedges, cattails and certain broad leaf weeds including some ferns
Runners
Aerial shoots coming from axils of lower leaves are called runners. The runners trial on soils surface in different directions and strike roots from their terminal buds at short distances. This is followed by the appearance of new shoots form their crown region. Daughter plants of such weeds repeat the process and form big patches. Stolons, suckers, offsets are different forms of runners.
Stolons, Suckers and Offsets These are different forms of runners.
When a runner, instead of trailing on the soil surface, rises in the form of an arch before hitting the soil, it is called a Stolon. Weeds belonging to the family rosaceae propagate by stolons.Suckers, in variance with runners, trial little below the soil surface as in hawkweed (Hieracium spp.). Runners of floating weeds like water hyacinth and water lettuce (Pistia lanceolata) are called Offset.
Tubers Swollen ends of its wiry rhizomes and suckers are called tubers. Eg. Nut sedge (Cyperus rotundus). A tuber possesses scaly leaves, inconspicuous nodes and internodes, and minute buds which give rise to new aerial shoots, rhizomes and roots.
Bulbs
When the crown region of a plant is compressed in the shape of a disc, it is called a bulb. Each bulb contains many fleshy leaves, axillary buds and flowering buds at its apex. Wild onion (Allium canadense) and wild garlic (Allium spp.) Propagate by bulbs.
Bulbils and Bulblets
Bulbils, also called aerial bulblets, are modifications of vegetative or flower buds. They are commonly found in woody sorrel (Oxalis corniculata), wild onion and wild garlic, sprout leaf (Bryophyllum pinnatum) and walking fern (Adiantum candatum). Hydrilla (Hydrilla verticillata) produce aerial buds called turions as additional means of vegetative propagation
Stems and Roots Fragments of stems and roots of many weeds can grow into full plants. Detached stems pieces of dodder (Cuscuta arvensis) and prickly pear (Opuntia spp.) And the creeping horizontal roots of canada thistle, perennial sow thistle and field bind weed for instance, are common means of propagation of these weeds.All submerged aquatic weeds are capable of propagating in water bodies through plant fragments and sticky glands (Boerhavia repens). Some annual weeds can also adopt specific vegetative propagation mechanism. Such weeds vigourate their crown buds to produce new plants, when parent plants are cut at the ground level. Carrot grass (P. hysterophorus) arrow-wood (pluchea lanceolate) Lantana (L. camara). Weeds propagated through vegetative propagation mechanism are difficult to manage since their propagates are located up to 100cm depth. Pluchea lanceolate may hit several meters deep.
Dissemination (dispersal) of weeds
The dispersal of seeds and vegetative propagules of weeds away from the parent plant is nature ‟s way of providing non-competitive sites or places to new individuals. If all seed falls near the mother plant, there will be a severe competition among themselves to their great disadvantage. But fortunately (to the weeds), it is not so weed dispersal has no geographical boundaries.
Dispersal of weeds
Weeds seeds / fruits produced by mother plant are dispersed off in three ways.1) A part of it may fall near mother plant. 2) A part of it may move out of the fields as contamination with crop harvest. 3) The remaining portion dispersed with agents like wind, water, transport system, animal, man and manure to short or long distance as follows. An effective dispersal of weed seeds (fruits) requires two essentials viz., 1) A successful agent and (2) An effective adaptation
Common agents of weed dispersal are wind, water, animals, birds, organic manures. agriculture implements and human beings. Wind:
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