Weed Ecology

Introduction

• Weed ecology = study of the interactions of weeds with their environment (biotic & abiotic factors) and their adaptations for survival, spread, and persistence.
• It explains why weeds are so successful and how they interfere with crops.
• Understanding weed ecology is essential for:
  • Predicting weed dynamics in fields.
  • Identifying critical periods of competition.
  • Designing Integrated Weed Management (IWM)

Definition; Weed Ecology: “A branch of ecology that studies the interaction of weeds with their environment and crops, including the biological and ecological factors that determine their growth, persistence, and distribution.”

Weed–Environment Interactions

a) Abiotic Factors

• Light – Essential for germination (phytochrome-mediated). Some weeds germinate only in presence of light (e.g., Amaranthus). Shade-tolerant weeds (Oxalis) adapt to low-light conditions.
• Temperature; Governs seed germination, emergence, and distribution. Summer annuals (Amaranthus, Echinochloa) thrive at higher temps. Winter annuals (Phalaris minor, Chenopodium) thrive at low temps.
• Rainfall / Water; Aquatic weeds (Eichhornia, Hydrilla) flourish in wet conditions. Xerophytic weeds (Prosopis, Cenchrus) adapt to drought.
• Wind; Facilitates seed dispersal over long distances (e.g., Saccharum, Typha).

2) Soil (Edaphic) Factors

• Soil pH; Basophilic weeds thrive in alkaline soils. Acidophilic weeds dominate acidic soils (Rumex).
• Soil fertility; Fertilizer application often favors weeds (Phalaris minor in wheat after high N).
• Soil moisture & aeration; Wet soils → aquatic weeds, sedges. Dry soils → hardy grasses.

3) Biotic Factors

• Crop competition – Crops with good canopy cover suppress weeds (e.g., soybean).
• Allelopathy – Sorghum & sunflower release chemicals suppressing weeds; Parthenium harms crops.
• Agricultural practices – Tillage, irrigation, and fertilizer use shift weed flora.
• Parasitism – Some weeds are obligate parasites (e.g., Orobanche, Striga).

Weed Adaptations (Survival Mechanisms)

• High seed production – Amaranthus retroflexus → 1 lakh seeds.
• Seed dormancy – ensures staggered germination.
• Seed dispersal – via wind, water, animals, machinery.
• Vegetative propagation – rhizomes, stolons, tubers (Cyperus rotundus).
• Phenotypic plasticity – Echinochloa colonum in upland & lowland.
• Persistence & regenerative capacity – regrow after damage.

Seed Dormancy in Weeds; Dormancy = adaptation delaying germination until favorable conditions occur.

Types of Dormancy

• Enforced dormancy – due to unfavorable environment (deep burial, lack of light, aeration).
• Innate dormancy – genetically controlled (hard seed coat – Setaria, Xanthium; immature embryos – Polygonum).
• Induced dormancy – arises in non-dormant seeds due to sudden stress (e.g., high temperature, CO₂, waterlogging). Example: Avena fatua (wild oat) shows all 3 types of dormancy.

Conditions Favorable for Weed Seed Germination

• Light exposure – many weeds require red light (phytochrome effect).
• Aerobic conditions – though some germinate in anaerobic conditions (paddy fields).
• Soil disturbance – ploughing exposes buried seeds to light → germination.
• Temperature & seasonality – summer annuals need higher temp, winter annuals prefer cooler.

Weed Life Cycle

• Annuals – 1 season (Amaranthus, Chenopodium).
• Biennials – 2 seasons (Daucus carota).
• Perennials – several years, spread vegetatively (Cynodon, Cyperus).

Weed–Crop Competition

• Competition for light, water, nutrients, and space.
• Yield loss: weeds cause 30–50% reduction in crop yield in India.
• Critical Period of Crop–Weed Competition (CPWC):
  • Rice: 15–45 DAS
  • Wheat: 20–45 DAS
  • Maize: 15–45 DAS

Persistence of Weeds

Persistence = the adaptive capacity of weeds to survive, reproduce, and thrive in diverse and adverse environments, even under continuous management pressure.

Factors Affecting Persistence

a) Climatic Factors

• Light: Many weeds germinate only in presence of light (due to phytochrome). Amaranthus retroflexus → light-requiring seed. Shade-tolerant weeds (Oxalis, Cynodon) persist under canopy.
• Temperature: Cool-season weeds: Phalaris minor, Chenopodium album. Warm-season weeds: Echinochloa crus-galli, Amaranthus viridis.
• Rainfall: Heavy rains favor aquatic weeds like Eichhornia crassipes (water hyacinth). Drought-tolerant weeds like Cenchrus ciliaris persist in drylands.
• Wind: Aids seed dispersal of light-seeded species (Saccharum spontaneum, Typha angustata).

Soil Factors

• Soil pH: Acidophilic weeds: Rumex dentatus. Basophilic weeds: Chenopodium murale.
• Soil fertility: Fertilizer-responsive weeds: Phalaris minor in wheat (thrives in high-N soils). Low fertility: hardy weeds like Cyperus rotundus.
• Soil moisture & aeration: Waterlogged → sedges (Cyperus difformis, Fimbristylis miliacea). Dry soils → xerophytes (Prosopis juliflora).

Biotic Factors

• Crop competition: Dense crops like soybean suppress weeds. Poor stand establishment → dominance of Amaranthus or Digera arvensis.
• Agricultural practices: Flooded rice → aquatic weeds (Echinochloa crus-galli). Zero tillage → perennial weeds like Cynodon dactylon.
• Parasitic weeds: Striga asiatica (witchweed) → sorghum & maize. Orobanche cernua → tobacco & sunflower.

Weed Seed Ecology & Dynamics

i Soil Seed Bank

• The reserve of viable weed seeds in the soil surface or deeper layers.
• Acts as a source of future infestations for many years.
• Example:
  • Chenopodium album seeds remain viable for 30–40 years.
  • Datura stramonium retains >90% germination after 40 years.

ii Weed Succession

• Gradual replacement of one weed community by another due to environmental changes or management practices.
• Examples: In rice → continuous puddling & flooding → dominance of Echinochloa crus-galli over broadleaf weeds. In wheat → excessive N fertilization + isoproturon herbicide use → shift from Chenopodium to Phalaris minor.

iii Weed Dynamics

• Refers to population shifts of weeds over time and space under influence of agronomic practices.
• Examples:
  • Herbicide resistance: Continuous use of isoproturon in wheat → resistant Phalaris minor.
  • Cropping system influence:
    • Rice–wheat system favors grassy weeds (Phalaris minor in wheat, Echinochloa in rice).
    • Cotton–soybean system favors broadleaf weeds like Xanthium strumarium.
  • Tillage: Deep tillage buries Cyperus rotundus tubers → but shallow tillage encourages regrowth.

Weed Allelopathy Weeds/crops release chemicals affecting others.

• Examples:
  • Sorghum, sunflower → suppress weeds.
  • Parthenium, Cyperus → inhibit crops.
• Basis for bioherbicide research.