Weed biology

1. Introduction

• Weed biology deals with the study of life processes of weeds—their growth, development, reproduction, survival, and dispersal.
• It explains why weeds are more competitive than crops and why they persist despite control measures.
• Understanding weed biology is essential for:
  • Predicting emergence patterns.
  • Identifying critical competition periods.
  • Designing effective Integrated Weed Management (IWM).

2. Growth and Development of Weeds

• Weeds have fast growth rates and establish quickly compared to crops.
• Phenotypic plasticity: ability to adjust morphology & physiology under varying environments. Example: Echinochloa colonum grows in upland (direct-seeded rice) and lowland (flooded rice).
• Photosynthesis: Many weeds are C₄ plants (efficient use of light, water, CO₂ → high biomass). Examples: Amaranthus spp., Echinochloa crus-galli, Cyperus rotundus.
• Rapid canopy formation shades crops and reduces photosynthesis.

3. Reproduction in Weeds

a) Sexual Reproduction (via seeds)

• Prolific seed producers: Amaranthus retroflexus → >100,000 seeds/plant. Chenopodium album → 72,000 seeds/plant.
• High genetic variability → adaptability & resistance evolution.
• Reproduction system:
  • Self-pollinated: Chenopodium, Amaranthus.
  • Cross-pollinated: Xanthium strumarium, Parthenium hysterophorus.

b) Asexual / Vegetative Reproduction

• Rhizomes: Cynodon dactylon (doob grass).
• Stolons: Cynodon, Oxalis.
• Tubers: Cyperus rotundus (nut sedge).
• Bulbs: Allium vineale (wild garlic).
• Roots: Convolvulus arvensis (field bindweed).
  👉 Makes perennial weeds very difficult to eradicate.

4. Weed Seed Biology

• Seed dormancy = survival strategy allowing weeds to germinate under favorable conditions.
  • Innate dormancy: genetically controlled (hard seed coat – Xanthium).
  • Enforced dormancy: unfavorable environment (deep burial).
  • Induced dormancy: stress-induced changes (Avena fatua).
• Seed longevity:
  • Chenopodium album → viable 30–40 years.
  • Nelumbo nucifera (lotus) → up to 1000 years.
  • Datura stramonium → >90% viability after 40 years.

5. Weed Seed Dispersal Mechanisms

• Anemochory (wind): Saccharum spontaneum, Typha angustata.
• Hydrochory (water): Eichhornia crassipes, Hydrilla.
• Zoochory (animals): Xanthium strumarium (hooked burs).
• Anthropochory (humans): seeds spread via machinery, vehicles, and contaminated crop seed.

6. Adaptations of Weeds (Biological Superiority)

• Prolific Seed Production; Weeds produce thousands to millions of seeds per plant (e.g., Amaranthus, Chenopodium).
• Seed Dormancy & Longevity; Seeds remain viable for decades (e.g., Datura, Chenopodium). Different types: innate, enforced, induced.
• Efficient Dispersal Mechanisms; Wind (Saccharum), water (Eichhornia), animals (Xanthium), humans (contaminated seed).
• Vegetative Propagation; Reproduce via rhizomes (Cynodon), tubers (Cyperus rotundus), stolons, bulbs.
• Rapid Growth & Early Maturity Faster canopy formation suppresses crops.
• Photosynthetic Efficiency; Many are C₄ plants → high biomass & competitive ability (Amaranthus, Echinochloa).
• Allelopathy; Release chemicals inhibiting crop growth (Parthenium, Sorghum).
• Regeneration Ability; Can regrow from underground parts after mechanical or chemical control.
• Phenotypic Plasticity; Adapt morphology/physiology to varied environments (e.g., Echinochloa in upland & lowland).

7. Weed Life Cycle

• Annuals – complete cycle in one season. Ex: Amaranthus viridis, Chenopodium album.
• Biennials – vegetative in 1st year, reproductive in 2nd. Ex: Daucus carota (wild carrot).
• Perennials – survive for many years; reproduce by seed + vegetative parts. Ex: Cynodon dactylon, Cyperus rotundus.

8. Weed–Crop Interference

• Weeds compete with crops for nutrients, light, water, and space.
• Critical Period of Crop–Weed Competition (CPC):
  • Rice → 15–45 DAS.
  • Wheat → 20–45 DAS.
  • Maize → 15–45 DAS.
• Weeds cause 30–50% yield loss in crops (sometimes up to 80%).

9. Importance of Weed Biology in Management

• Helps predict weed emergence, persistence, and spread.
• Guides herbicide application timing.
• Explains resistance development in weeds (e.g., Phalaris minor resistant to isoproturon).
• Forms basis for cultural, mechanical, chemical, and biological control.
• Crucial for designing eco-friendly integrated weed management strategies.

10. Key Examples to Remember

• Cyperus rotundus → most persistent perennial, tuber propagation.
• Amaranthus retroflexus → prolific seed producer, C₄ weed.
• Parthenium hysterophorus → invasive, allelopathic weed.
• Phalaris minor → herbicide-resistant grassy weed in wheat.
• Eichhornia crassipes → aquatic weed, spread by water.