Special weeds
- Parasitic Weeds
a) Root Parasites (Obligate)
- Striga spp. (Witchweed); Hosts: sorghum, maize, pearl millet, sugarcane. Losses: up to 70–100% in sorghum/maize if severe.
- Orobanche spp. (Broomrape); Hosts: tobacco ( cernua), sunflower, tomato, lentil.
b) Stem Parasites;
- Cuscuta spp. (Dodder); Hosts: lucerne, niger, lentil.
- Semi-Parasitic (Hemi-parasites); Loranthus spp., Viscum spp. (mistletoes) Hosts: fruit trees (mango, guava, citrus).
- Aquatic Weeds
Grouped by growth habit:
- Free-floating → Eichhornia crassipes (water hyacinth), Pistia stratiotes, Salvinia molesta, Azolla.
- Rooted-floating → Nymphaea, Nelumbo, Trapa natans.
- Submerged (rooted) → Hydrilla verticillata, Vallisneria spiralis.
- Submerged (free-floating) → Ceratophyllum demersum.
- Emergent → Typha angustata, Sagittaria, Phragmites.
- Amphibious (in paddy fields) → Marsilea quadrifolia, Ludwigia parviflora.
🔹 Impact: choke canals, reduce dissolved oxygen, obstruct navigation, harbor mosquitoes/snails, increase evapotranspiration.
Management:
- Biological: Neochetina beetles on water hyacinth, grass carp for submerged weeds.
- Chemical: 2,4-D, glyphosate (on label), copper sulfate for algae.
- Mechanical: weed harvesters, screens in irrigation channels.
- Invasive / Noxious Weeds
- Parthenium hysterophorus → dermatitis, respiratory allergy, allelopathic.
- Lantana camara → invades pastures/forests, poisonous to livestock.
- Chromolaena odorata → “Siam weed,” dense thickets in NE India.
- Mikania micrantha → “mile-a-minute weed,” smothers tea, coffee, rubber.
- Prosopis juliflora → invades grazing lands.
- Ipomoea carnea → along canals, toxic.
- Eichhornia crassipes → aquatic invader.
- Perennial Problem Weeds
Vegetative propagules make them very persistent:
- Cyperus rotundus (nut sedge) → tubers.
- Cynodon dactylon (Bermuda grass) → stolons & rhizomes.
- Sorghum halepense (Johnson grass) → rhizomes.
- Imperata cylindrica (speargrass) → rhizomes.
- Convolvulus arvensis (bindweed) → rootstock.
Management: stale seedbed, deep ploughing, repeated tillage, solarization, herbicide timed to carbohydrate movement.
- Mimicry Weeds
Crop-weed resemblance makes control difficult:
- Wheat → Phalaris minor (resembles wheat seedlings).
- Rice → Weedy rice (Oryza spontanea) – looks like cultivated rice, red grains.
- Lentil → Vicia sativa (common vetch).
- Wheat/Oats → Avena ludoviciana.
- Problem-Situation Weeds
- Saline/alkali soils → Suaeda maritima, Salsola kali, Atriplex spp.
- Waterlogged/canal banks → Typha angustata, Ipomoea carnea, Phragmites.
- Arid regions → Cenchrus biflorus, Tribulus terrestris, Calotropis procera.
- Orchards → Oxalis corniculata, Mikania micrantha, Chromolaena odorata.
- Poisonous / Health-Hazardous Weeds
- Parthenium hysterophorus → dermatitis, asthma.
- Argemone mexicana → epidemic dropsy (toxic oil).
- Datura spp. → alkaloids, poisonous seeds.
- Calotropis procera → latex irritant, cardiac glycosides.
- Ipomoea carnea → neurotoxic in cattle.
- Lantana camara → liver toxicity in livestock.
- Crop-wise Problem Weeds
- Rice: Echinochloa crus-galli, Cyperus difformis, Fimbristylis miliacea, Ludwigia parviflora.
- Wheat: Phalaris minor, Chenopodium album, Avena ludoviciana.
- Sugarcane: Cyperus rotundus, Cynodon dactylon.
- Cotton: Trianthema portulacastrum, Commelina benghalensis.
- Pulses: Vicia sativa, Cuscuta spp., Chenopodium album.
