The critical period of weed control in rice is approximately 1/3rd of the duration of the crop starting from 15th day to 45th day.

The common grass weeds in the rice fields of Kerala are: Oryza rufipogon (varinellu), Echinochloa crusgalli (kavada), E. colona (kavada), E. stagnina (kavada), Saccolepis interrupta (polla), Isachne miliacea (chovverippullu, naringa).

Grass weeds can be controlled by spraying Cyhalofop butyl 10 EC (800 ml/ha) or Fenoxaprop-ethyl 6.7 EC (875 ml/ha) at 15-18 days after sowing or transplanting.

The common sedges seen in rice fields are: Cyperus iria (manjakora, chengoal), C. difformis (thalekkattan), Fimbristylis miliacea (mungai). And the Broad leaved weeds include: Monochoria vaginalis (neelolppalam), Ludwigia perennis (neer-grampu), Limnocharis flava (nagappola), Ammania baccifera (nellicheera).

The sedges and broad leaved weeds can be controlled by spraying 2 4 D sodium salt 80 WSP(@ 1 kg /ha applied at 20-25 days after sowing/ transplanting), Chlorimuron ethyl 10%+ Metsulfuron methyl 10 % (20 WP)( @ 20 gram/ha with 0.2 % surfactant  applied at 20-25 days after sowing/ transplanting),  Carfentrazone 40 DF (@ 50 gram/ha applied at 20-25 days after sowing/ transplanting), Ethoxy sulfuron 15 WDG (@ 100 gram/ha applied at 20 days after sowing/ transplanting).

There are also broad spectrum herbicides that control both type of weeds. They include Azim sulfuron 50 DF(@ 70 gram/ha applied at 15-20 days after sowing/ transplanting),  Penoxsulam 24 SC(@ 104 ml/ha applied at 15-20 days after sowing/ transplanting),  Bispyribac sodium 10 SC(@ 250 ml/ha applied at 15-20 days after sowing/ transplanting). The broad spectrum herbicide will not control the weed Leptochloa chinensis.

Maintaining water to a height of 5 cm can control growth of most of the weeds.  Even as little as 1 to 2 cm of water reduces weed densities.

Apply herbicides at the recommended dose and time.  Drain the field before herbicide application. Re-flood after 48 hours to prevent further weed germination when post emergent herbicides are used.