Content

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 another.

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

Efficient reproduction 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 percent.

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.

However,

Seed dormancy

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 overwinter

Limit the choice of crop rotation sequences and cultural practices

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 Contribute forage Increase biodiversity Provide nectar for bees

aesthetic qualities

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.

d) Ephemerals: 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 4 weeks.

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.

1. Grasses Stem - Cylindrical in shape Solid nodes Hollow internodes Normally called as "Culm"

Leaf - Consist of two parts - leaf sheaths - leaf blade

Inflorescence-panicle or raceme grasses are monocots and show more tillering ability.

2. Sedges 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

decocts.

D. Classification, based on growth habit.

Vine

Shrub

Tree

weeds

Monocot

Grasses

Sedges

DycotBroad leaves

& other

E. Classification, based on woodiness.

Woody weeds

Non woody weeds

F. Classification based on habitat.

Terrestrial

Epiphytic

Aquatic

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.

1. Prevention

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

channels)

Use clean equipment.

2. Eradication

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

seeds.

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.

f. Burning

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

limits.

g. Solarization

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

the plot.

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

a) Irrigation 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. c) Mulching 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 root. 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 pest. 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

garden setting.

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

weeds.

Foliar application refer to application to the leaves

Soil incorporation refers to tilling or watering the herbicide into the soil after

application.

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

MCPA

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

S2O32- NCRC(Boron Compound)

Non- selective, contact, foliar applicant

ClO-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

season.

Collection of Common weeds in Sri lanka

1. Broad leaves 1. Ambul Ambiliya 2. Andanahiriya 3. Aswenna 4. Balu naguta 5. Bim pol 6. Bim thal 7. Epala 8. Eth adi 9. Gal kura 10. Gata thumba 11. Gatakola 12. Gira pala 13. Gotukola 14. Heen undupiyaliya 15. Hulanthala 16. Iramusu 17. Irisiyakatu 18. Kadu pahara 19. Kakilla 20. Kapum keeriya 21. Kata kaluwa 22. Kuppameniya 23. Kurunegala daisy 24. Maha Undupiyaliya 25. Meemana 26. Monarakudumbiya 27. Niyangala 28. Pethi thora 29. Podi singhomarang 30. Polpala 31. Sudana 32. Wal aba 33. Wal kollu 34. Wathupalu 35. Wel penela 36. Welkarambu

2. Grasses

37. Angili thana 38. Atora 39. Bajiri 40. Batadella 41. Bela thana 42. Carpet grass 43. Crow foot grass (Putu thana) 44. Gojara

45. Iluk 46. Kangaroo grass 47. Love grass 48. Mayura thana 49. Rila thana 50. Roughstalk blue grass 51. Sevana thana 52. Signal grass 53. Thuththiri 54. Wal amu 55. Welmaruk 56. Mana / Wild napier

3. Sedges

57. Halpan 58. Heen kudamatta 59. Kalanduru 60. Kokmota 61. Maha kudamatta 62. Mottu thana 63. Thunessa

4. Aquatic weed plant

64. Diya habarala 65. Hydrilla