Introduction to Weeds in Sri Lanka

Weeds can be simply explained as plants that grow in an unwanted place. i.e. A rose plant that is grown in a vegetable cultivated field is considered as a weed. Weeds cause considerable economic losses to the farmer, if it is not manage properly.  Weeds compete with the cultivated crops for water, sunlight, nutrients and other factors required for plant growth. Weeds are more adaptive to growing environment than the crops. As a result, weeds grow better at the expense of the cultivated crops. So, it is really important to focus on managing the weed infestation. controlling of the weeds can be a costly operation if it not managed properly (Managing of weed infestation is economically more advantageous than controlling/eradicating them).

Some Definitions for Weed

• A plant that is not valued where it is growing and is usually of vigorous growth; especially:  one that tends to overgrow or choke out more desirable plants (Merriam-Webster).
• Any plant that grows wild, has no specific use or aesthetic value
• A plant that originated under a natural environment and in response to imposed and natural environments, evolved and continues to do so as an interfering associate with our crops and activities (Fundamentals of Weed Science)
• Any plant that grows out of place

Characteristics of weeds

• Harmful to humans, animals and crops
• Aggressive growth (wild growth) /Large population
• Persistence and resistance to control
• High Fecundity rate (Reproductive capacity)
• Seed dormancy
• May grow even in an undesirable  location (foot paths, on cracks, grounds)
• May have several propagation ways

Advantages of Weeds

• Weeds provide a vegetative cover
• Weeds play an important role in nutrient cycling and addition of organic matter to soil
• Many weeds are edible hence used as vegetables
• Many weeds have medicinal values
• Some weeds are sources of pesticides (Pyrythrum  by Chrysanthemum)
• Weeds provide food and cover for animals
• Weeds serve as hosts for beneficial insects
• Weeds serves as important genetic material for crop improvement through the years
• Some weeds help to beautify the landscape

Disadvantages of Weeds

• Weeds compete with cultivated crops
• Weeds reduce the quality of harvested products
• Disturbs field practices
• Block the irrigation canals/increase the cost
• Some weeds may poison animals
• Retention of herbicide residues in final products (Maximum residue limits in tea [MRL])
• A costly operation
• Weeds also serve as alternative hosts  for many plant diseases and animal pests Arthropods, nematodes, bacterial and fungal diseases in C. rotundus, Cynodon dactylon, Echinocloa colona, Elusine indica, Imperata cylindrica, Panicum maximum ,Amaranthus spinosus and etc.
• Some weeds become fire hazards  during dry seasons
• Presence of weeds reduces the value of a land.
• Presence of weeds also imposes  a limit on farm size

Methods of  Managing/Controlling Weeds

• Mechanical controlling method
• Chemical control method
• Biological control method

Mechanical controlling method

• Most often used methods are tillage, hand weeding and mowing
• This method is effective and capable of controlling weeds for longer period of time
• But, this method requires more labor and time 
• Have minimum impact on environmental pollution

Chemical control method

• Most commonly used method in conventional agriculture
• Effective and efficient method
• Requires less labor and time
• Largely contribute to environmental pollution

Classification of Herbicides/ Weedicides

• Based on Method of application
• Soil application
• Eg. Fluchloralin
• Foliar application
•  Eg.Glyphosate

• Based on Mode of action
• Selective herbicide
• Eg. 2,4-D
• Non-selective herbicide (Broad spectrum herbicides)
• Eg. Glyphosate

• Based on mobility
• Contact herbicide
• Bentazon
• Translocated herbicide
• Glyphosate

• Based on Time of application
• Pre-plant herbicides
• Eg. EPTC
• Pre-emergence herbicides
• Simazine
• Post-emergence herbicides
• Bromoxynil

Biological Control Method

• Bio-agents such as insects are used 
• Less effective and less efficient compared to chemical control method
• Requires less labor force
• Have minimum impact on environmental pollution
• Bio-agents usually feed on the weeds and suppress or kill the weeds