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Broadleaf weeds are the third major weed group in rice. The most damaging species in flooded rice are monochoria (Monochoria vaginalis), gooseweed (Sphenoclea zeylanica), water primrose (Ludwigia spp.), and water spinach (Ipomoea aquatica). They compete for light and nutrients and, in dense stands, can smother young rice.

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When rice is most vulnerable

First 30 to 45 days after transplanting. Broadleaf weeds in flooded conditions are generally easier to control than grasses if selective herbicide is applied on time.

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How broadleaf weeds show in satellite data

  • Uneven low-canopy zones with a distinctly different green tone from rice - sometimes visible in true-color imagery even before NDVI reveals stress.
  • Weed flowers (blue for monochoria, white for gooseweed) can add visible speckling in high-resolution imagery when weeds bloom above the rice canopy.
  • NDVI alone does not separate broadleaf weeds from healthy rice canopy - use imagery timing and ground scouting.
Broadleaf weeds are more visually obvious on the ground than from satellite. First-30-days scouting is the primary detection method; imagery is confirmatory.

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Field symptoms

  • Wide, often heart-shaped or lanceolate leaves - clearly different from rice
  • Monochoria: rosette of arrow-shaped leaves, blue-purple flowers, thrives in stagnant water
  • Gooseweed: erect, fleshy stems with white flower spikes
  • Water primrose: creeping stems, yellow flowers, floats on water
  • Water spinach: hollow stems, arrow-shaped leaves, prostrate growth on water

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Reduce the damage by

  • Level the field and maintain 3 to 5 cm standing water in the first 30 days - most broadleaf rice weeds are suppressed by continuous shallow flooding.
  • Hand-weed at 15 and 30 days after transplanting on small holdings - broadleaf weeds are easy to identify and remove.
  • Pre-emergence herbicide - pretilachlor within 3 to 5 days of transplanting. Butachlor is banned in Malaysia, including butachlor-plus-safener formulations - see Pesticide registration.
  • Post-emergence herbicide - 2,4-D, bensulfuron-methyl, or metsulfuron-methyl at 2 to 3 leaf stage of weeds. 2,4-D must not be used near sensitive neighboring crops or during windy conditions.
  • Prevent seed set - broadleaf weeds seed prolifically; do not allow flowering weeds to persist through harvest.

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Set an alert for early warning

Turn the diagnosis into a trigger. Broadleaf weeds are best caught by scheduled scouting rather than by an NDVI change.
Recommended settings for broadleaf weeds on rice:
  • New Imagery alert during the first 30 days to prompt weekly scouting.
  • NDVI trajectory review - a field trailing the ecosystem baseline at 15 to 30 days is a good scouting candidate.