1. Introduction - A WEED
- Characteristics of weeds
2. The impact of weeds
- on agriculture
- on livestock
- on human health
3. Benefits of weeds
4. Classification of weeds
5. Propagation method of weeds
6. Control of weeds
7. Weed management techniques
- Mechanical weeding
- Cultural method
- Biological Methods
- Chemical weed control (herbicides)
- Integrated Weed Management (IWM)
Introduction - A WEED
Worldwide, there are approximately 250,000 species of plants, of those, about 3% or 8000
species behave as weeds.
Weeds can be defined in a variety of ways: a plant growing where it isn’t wanted, a plant that
interferes with farming or grazing, a plant that was not intentionally sown, a plant that is
persistent and detrimental to the plants around it, among others. Basically, what it comes
down to is that weeds are plants that humans don’t want to have around for one reason or
some weeds are simply plants that are not wanted in the place where
they are growing, but are useful at other times.
For instance, a wild raspberry plant smothering a crop of strawberries planted by a farmer.
In other areas, a weed might be an invasive species of plant which threatens a country’s
natural flora. An example of this is broom, or Cytisus scoparius, which grows abundantly in
Scotland and is used in ornamental landscaping, sand dune stabilization and wasteland
reclamation. It was even used as a symbol of the Plantagenet kings. However, in Australia,
New Zealand, and parts of North America, it is considered a pest species and often eradicated.
Therefore no plant is a "weed" in nature. Human activities create weed problem and try to
control. Weeds are naturally strong competitors and those weeds that can best compete
always tend to dominate.
Typically, weeds possess certain characteristics that make them particularly interfering with
the human activities and facilitate the survival.
CHARACTERISTICS OF WEEDS
Although weeds potentially produce many propagules per plant, but actual productivity
is much lower in competition with the crop or at high weed densities. The crop-weed
interaction can reduce potential weed seed production dramatically, as much as 50
Adaptation for spread;
Weeds possess different dispersal mechanisms and adaptations which are varied as the
number of weed species. Dispersal can be as a result of human activity (irrigation) or as
a result of natural activity (wind). Weed seed dispersed by wind (e.g. dandelion,
thistles) usually has structural modifications making them very lightweight in the air.
Flooding and irrigation are good dispersal mechanisms as most seeds can float and can
live in the water for some time. Birds and animals can move seed great distances. Seed
contamination via weed mimicry (e.g. clover in alfalfa) is also a source of dispersing
weed seeds to new sites. Agricultural activities like planting contaminated crop seed,
using unclean harvest equipment and tillage equipment, and moving machinery
between fields are significant weed seed dispersal procedures.
Weed seed dormancy can be identified as another type of dispersal—dispersal through
time instead of space. When seed is dispersed, most does not immediately germinate.
It remains dormant in a sort of sleeping stage until conditions are right. The factors that
break dormancy are unpredictable and dependent on the species, the weather
conditions, even physiological factors within the seed itself. Over time seeds that do
not germinate go from dormant to non-viable (dead).
Rapid population establishment
For seeds that do germinate and live, weed seedling survival after emergence and
population establishment is very high. Rates of natural mortality due to disease,
herbivory and drought are low for established weeds in annual crops. So, if a weed
makes it to seedling stage, its rate of survival to maturity is 25-75 percent, up to even
90 percent. Mortality also decreases with increasing plant size and age.Despite starting
small, weed seedlings quickly catch up with crop seedlings—they like the same growing
conditions as the crop seed does. Weed seedlings have a very high relative growth rate
(amount of growth/biomass) and quickly establish a fine root network for nutrient
uptake. Smaller seeds have small reserves compared to crops, making them more
dependent on soil nutrients.
Ability to occupy sites disturbed by human activities.
Ability to compete well.
The impact of weeds
Weeds reduce farm and forest productivity, they invade crops, smother pastures and in some
cases can harm livestock.
The effects of weeds on agriculture,
Weeds reduce crop yield by competing for water, light, soil nutrients, and space.
Weeds contaminate crops, reducing crop quality
Interference with harvest
Serve as hosts for crop diseases or provide shelter for insects to
Limit the choice of crop rotation sequences and cultural
Production of chemical substances which are toxic to crop plants
(allelopathy), animals, or humans.
Effects of weeds on livestock
Burrs in wool contaminate fleeces.
Grain milled with weeds like Saffron Thistle or Amsinckia results in discoloured flour.
Animals that eat specific weeds, such as wild garlic, produce tainted milk and meat.
Spines on fruit of Caltrop and Spiny Emex can damage the feet of stock animals.
Paterson's Curse irritates the udders of dairy cows and can kill horses.
Poisonous weeds like Hemlock can be lethal to both stock and people.
The impact of weeds on human health
Weeds can also cause human health problems. Many common weeds such as Ragweed, Rye
Grass and Privet cause asthma and other respiratory problems, especially in children.
Some weeds can also cause skin irritation and some are poisonous.
Some water weeds such as Water Hyacinth (Eichhornia crassipes) and Cabomba (Cabomba
caroliniana) can affect the quality of our drinking water if infestations are not managed
within water supply dams.
In 1907, 700 cattle that were killed overnight by a poisonous weed
BENEFITS OF WEEDS
Weeds can provide ecological benefits such as,
Helping to conserve soil moisture and prevent erosion. A ground cover of weeds will
reduce the amount of bare soil exposed helping to conserve nutrients, particularly
nitrogen which could otherwise be leached away, especially on light soils.
Food and shelter can be provided for natural enemies of pests and even alternative
food sources for crop pests. The actual presence of weed cover may be a factor in
increasing effectiveness of biological control of pests and reducing pest damage.
Weeds can also be valuable indicators of growing conditions in a field, for example of
water levels, compaction and pH.
Weeds can be an important source of food for wildlife, especially birds. Bird
populations have been declining on farmland over the last few decades and leaving
weeds as a resource has been shown to help revive bird populations.
Fix nitrogen (if weed is a legume)
Add organic matter
Provide nectar for bees
CLASSIFICATION OF WEEDS
The classification of weeds is helpful for adopting weed management methods for particular
group of weeds instead of against an individual weed species. It is always economical and
practically feasible to manage the group of weeds as compared to manage the individual
weed species. Weeds can be classified on different basis.
A. Classification of Weeds According to Life Cycle:
Depending upon their life cycle weeds can be classified as
1.) Annual weeds
They complete their life cycle within one year or one season.
a) Kharif Annuals / Kharif Weeds:
They appear with the onset of monsoon (June, July) and complete their life
cycle when rainy season is over (Oct or Nov) E.g Cock’s comb, dudhi, math,
chimanchara , parthenium etc.
b) Rabi Annuals / Rabi Weeds:
They complete their life cycle during winter season ( Oct/Nov to Feb) E.g
Vasanvel ( Chenopodium album) , Ghol ( Portulaca oleracea), wild oat etc.
c) Summer Annuals / Summer Weeds:
They complete their life cycle during summer season ( Feb to May), Majority
of the Kharif seaosn weeds grow during summer season in irrigated farming
E. g Parthenium , Amaranthus spp. Euphorbia Spp. ( Dudhi) etc.
The short- lived annual weeds are called ephemerals E. g Niruri ( Phyllanthus
niruri). These weed completes its life cycle within a very short period of 2 to
2.) Biennial Weeds:
They take at least two years or two seasons to complete their life cycle. They
complete their vegetative growth in first year or season and produce flowers and
seeds in the next year or season. E.g Wild carrot- ( Daucas carota), wild onion – (
Asphodelus spp) , Jangli gobhi- (Launea spp).
3.) Perennial Weeds:
They continue or grow for more than two years or several years.
B. Classification of Weeds according to the number of cotyledons which seed have:
Monocot plants have only one Cotyledon in seed. Dicot plants have 2 cotyledons in each seed.
C. Classification of weeds based on morphology.
Stem - Cylindrical in shape
Normally called as "Culm"
Leaf - Consist of two parts
- leaf sheaths
- leaf blade
Inflorescence-panicle or raceme grasses are monocots and show more tillering
Stem - solid
no nodes or internodes present cross section is angular shape.
3. Broad leaves
Stem - solid rounded in shape
Leaves - broad, only this broad leaves has top root system most of broad leaves are
D. Classification, based on growth habit.
E. Classification, based on woodiness.
Non woody weeds
F. Classification based on habitat.
G. Classification based on harmfulness.
Soft weeds - not causing many problems to crop plants.
Hard weeds – Compete with crop plants & reduces the yield drastically.
Propagation method of weeds
Weeds have two main modes of reproduction, by seed or vegetatively.
Most annuals and biennials weeds reproduce by seed and the seed production is often
quite prolific. For example, redroot pigweed can produce over 100,000 seeds/plant.
Perennials weeds can reproduce by seed as well as by vegetatively via rhizomes and stolons.
Control method of weed
Identification of weed plant life cycle and reproduction mode of problem weed species is
essential in determining control methods for weeds. For example, annuals can be contained
through tillage or mowing prior to seed production. On the other hand, tillage can increase
a perennial by breaking up the roots and creating new plants more quickly.
The best weed control is prevention. Prevention of entering of weeds to
landscape can be ensured by,
• Plant weed-free seed, sod, and nursery stock
• Avoid using plant species known to be invasive
• Use weed-free amendments, topdressing
• Uses mulch where appropriate
• Maintain healthy, competitive plants
• Blocking the pathway of entering weeds to agricultural lands; (through irrigation
Use clean equipment.
Completely remove weeds from agricultural lands.
3. Weed management
Maintain weed population in certain level. ( Below the economical harmful level)
Weed management techniques
I. Mechanical weeding
a. Tilling / Cultivating
Tilling or cultivating effectively controls 90% of annual and biennial weeds if done before
seed set. It also brings a new set of weed seeds to the soil surface ready to germinate. When
tilling for weed control, use only shallow cultivation. Deep tilling can damage crop roots.
Cultivating/tilling may actually propagate most perennial weeds.
b. Hand Pulling
Hand pulling is quick when pulled while the weeds are small, and it is effective for small
infestations. A few minutes on a weekly basis to keep the garden weed free will be more
effective than a long weed pulling session as the weeds get large. For many gardeners,
pulling weeds is a great way to vent stress. With hand pulling, most weed species require
that they be pulled out by the roots. The weed will readily regrow if just the tops are
removed. It is essential that weeds are removed before they go to seed, filling the seed
bank. Some weed species, like purslane, must be removed from the garden bed. It can
reroot if left in the garden.
c. Mowing Naturalized and Low Maintenance Areas
Mowing is a common weed management tool in natural areas and lower maintenance
sections of a yard, reducing the unsightly appearance of the yard and fire hazard
d. String Trimming (“Weed Whacking”)
Use of a string trimmer is a form of weed management by mowing. It can be effective in
preventing weeds from going to seed. However, it can sow seeds if done on weeds with
e. Flame (Propane Torch)
Flaming off weeds with a propane torch is a common practice in production agriculture and
has limited application in landscape maintenance due to fire hazards. During the flaming
process, heat from the flame is transferred to the plant tissues, increasing the thermal
energy of the plant cells and resulting in coagulation of cell proteins if the temperature is
above 50°C. Exposing plant tissue to a temperature of about 100°C for a split second (0.1
second) can result in cell membrane rupture, resulting in loss of water and plant death.
Thus, the weeds do not need to be burned up, but rather just scorched. Flaming works best
on very young weeds.
It is rather expensive and many not be cost effective in some production agriculture
situations. It presents a fire and explosion hazard; use with caution. Fire prevention
measures prohibit the use of flaming in many communities.
Burning of fields and ditch banks is a weed management tool in production agriculture.
Generally, a permit is required. Most communities prohibit burning of weeds inside city
Solarization is a method of heating the soil to kill roots, weed seeds, and soil borne insects
and diseases near the soil surface. In regions with hot summer temperatures, it is effective
in open areas will full sun. However, do not solarize the soil in the rooting area of trees,
shrubs, and other desired plants. Steps include the following:
1. Remove vegetation and cultivate the soil to a six inch depth.
2. Sprinkle irrigate the area.
3. Cover the area with 4 mil clear plastic. Bury the edges of the plastic all the way around
4. Leave in place for three weeks during the summer heat of July and August.
5. After removing the plastic, avoid deep cultivation what would bring up weed seeds,
insects, and disease pathogens from deeper soils.
Pros and Cons of Mechanical Method
Pros: Mechanical methods can be quick, inexpensive, environmentally friendly, and effective
on small weed seedlings.
Cons: Mechanical methods have limited effectiveness on many established perennials, and
could be detrimental at wrong time
2. Cultural method
Irrigation methods and frequency have a direct influence on weeds. Infrequent, deep
irrigation droughts out many shallow rooted weeds. Sprinkler irrigation (wetting the entire
soil surface) encourages weeds. Drip irrigation (keeping most of the soil surface dry)
discourages weeds. Keep non-irrigated areas dry to help suppress weeds.
b) Lawn Mowing
Many common garden weeds will not survive the frequent mowing of a lawn. However,
mowing the lawn too short (less than 2 inches for Kentucky bluegrass) encourage weeds as
it reduces vigor of the grass.
If maintained at adequate depths, mulching has many benefits including preventing weed
seed germination. For wood/bark chips, a depth of three inches is best for weed control.
Less is ineffective. Mulching may not effectively control established perennials growing from
d) Landscape Fabrics
In landscape management, landscape fabric with wood/bark chips or rock mulch above is
common. However, it prevents soil improvement by organic breakdown, decreasing plant
vigor. Weed seeds that germinate above the fabric layer will be difficult to pull and must be
removed with herbicides. Use of landscape fabric should be considered as a deferred
maintenance technique rather than a low maintenance technique.
e) Crop Competition
Competition with the crops and weeds for light, water, nutrients, and growing space is an
effective weed management tool. For example, mowing a cool season lawn (like Kentucky
bluegrass) gives the lawn a growth advantage, shading out many weeds like crabgrass.
Block planting in the vegetable garden and close spacings in a flowerbed, with plants filling
the bed space, helps suppress weeds.
Pros and cons of Cultural Methods for Weed Management
Pros: This is the best long-term control as the gardener increases the conditions for desired
plants to grow at the same time decrease the conditions for weeds.
Cons: Possibly more expensive and time-consuming; control may be slow.
3. Biological Methods
Biological methods include the use of carefully screened insects to attack portions of the
weed (i.e., stems, seeds, flowers, etc.). Development of biological methods with insects is
rather complex and must be used with caution. The introduced insects must survive and
become established in the new ecosystem. The insects need to reduce the weed population,
but cannot entirely eliminate it as the weeds as that would eliminate the insect’s food
supply. The insects must not attach beneficial plants. The insects must not become insect
pest. A great example of biological methods that failed is earwigs. They were intentionally
introduced into the United States as a biological control agent and have since become a
Biological methods also include the grazing of sheep, cows, horses, or goats. The purposeful
use of grazing animals to control weed patches can be extremely expensive.
Pros and cons of Biological Methods for Weed Management
Pros: Biological methods can be an inexpensive, long-term control solution. It can be
environmentally friendly and require little labor.
Cons: Biological methods are not always effective, may require a large population of weeds
to maintain insect populations (will not work in backyard setting), and does not eradicate
weeds. Insects can sometimes attack non-target plants.
4. Chemical weed control (herbicides)
The use of herbicides is the use of chemicals that disrupt key physiological processes in
plants, leading to plant death. Among the various herbicides, many different modes of
action are found. Chemicals can be divided into many groups in according to the time of use,
place of application and target plants.
• Systemic or Translocated herbicides move internally in the plant. They must be applied
during period of active growth with adequate water. Systemic herbicides are especially good
for many perennials. Examples include glyphosate (Round-up), and 2,4-D.
• Contact herbicides only desiccate the portion of the plant that is contacted. Contact
herbicides are most effective on annuals. Examples include vinegar and diquat.
• Pre-emergent herbicides are applied to soil prior to weed seed germination, killing
germinating seeds. They will not kill growing weeds. Application timing is critical. For
example, to control crabgrass in lawns, pre-emergent herbicides need to be applied late
April to early May before the crabgrass germinates, about the time that common lilac
blooms. Most require soil incorporation by irrigation.
Some desired crops germinating from seeds may also be killed. For example, do not apply
pre-emergent herbicides prior to seeding or laying sod. Uniform application and strict
adherence to application rate are essential for attaining good weed control and for
preventing injury to landscape plants.
• Post-emergent herbicides are applied to foliage of actively growing plants. Example
include 2,4-D, and glyphosate (Round-up).
• Selective herbicides control a limited group of plants, like monocots versus dicots.
• Non-selective herbicides are effective on a broad range of plants.
Pros and cons of chemical Methods for Weed Management
Pros: Use of herbicides is generally effective (if the correct herbicide is used), cost-effective,
and provides quick control.
Cons: Use of herbicides can be environmentally problematic when incorrectly applied.
Proper use includes proper selection of the specific herbicide for the weeds and for the
growing crops in the area, timing of application, correct application rates, correct
application procedures, and application safety measure to protect the application and non-
target plants. Some require special licensing and may not be used in a home landscape or
Herbicides can be applied by followings ways,
Broadcast application refers to a uniform application over a treatment area.
Spot treat refers to application to a specific area, such as directly to individual
Foliar application refer to application to the leaves
Soil incorporation refers to tilling or watering the herbicide into the soil after
Common weedicides used in Agriculture
Selective, contact, foliar applicant
Dinitro phenol H2SO4
KCN Propanial 3,4 Dichloro propenaldehide.
Selective, translocation, foliar applicant
2-4 dichlorophenoxiacetic acid
Selective, translocation, root applicant
2-4 D 2-4 T MCPA
Non-selective, translocation, foliar applicant.
Amonium sulphate Chlorate compounds
Sodium arsenate Roundup
Non-selective, translocation, Root applicant
2- NCRC(Boron Compound)
Non- selective, contact, foliar applicant
3 Penta-borate Dinitrophenol
(NH4)2SO4 Arsenic compound Gramaxone
5. Integrated Weed Management (IWM)
Tntegrated weed management (IWM) is the control of weeds through a long-term
management approach, using combination of multiple management tools to reduce a weed
population to an acceptable level while preserving the quality of
existing environment, water, and other natural resources.
Combinations of biological, mechanical, and chemical
management practices are utilized in IWM programs to
efficiently suppress a weed population at the most
effective/desirable points during the weed lifecycle or growing
Collection of Common weeds in Sri lanka
1. Broad leaves
1. Ambul Ambiliya
4. Balu naguta
5. Bim pol
6. Bim thal
8. Eth adi
9. Gal kura
10. Gata thumba
12. Gira pala
14. Heen undupiyaliya
18. Kadu pahara
20. Kapum keeriya
21. Kata kaluwa
23. Kurunegala daisy
24. Maha Undupiyaliya
28. Pethi thora
29. Podi singhomarang
32. Wal aba
33. Wal kollu
35. Wel penela
37. Angili thana
41. Bela thana
42. Carpet grass
43. Crow foot grass (Putu thana)