Back to: Jss1 Agricultural Science (PVS)
WEEK: 9 & 10
Weeds are plants that are generally unwanted or considered undesirable in a particular location or environment. They are typically characterized by their ability to grow quickly and spread easily, often competing with other plants for resources such as sunlight, water, and nutrients. Weeds can be found in a variety of settings, including gardens, lawns, agricultural fields, and natural areas.
While some weeds may have certain benefits, such as providing food or habitat for wildlife, many are considered a nuisance and can negatively impact the environment or human activities. Weeds may also be difficult to control or eradicate, requiring various management strategies such as manual removal, chemical treatments, or cultural practices like crop rotation and mulching.
Classification of weeds can be done based on various factors such as their life span, structure, and shape of their leaves. Here are some of the common ways in which weeds are classified:
Classification Based on Life Span
Weeds can be classified based on their life span as annual, biennial or perennial.
- Annual weeds complete their life cycle in a single growing season. They grow from seed, and flower, and produce seeds all in one year, then die off. Examples of annual weeds include crabgrass and chickweed.
- Biennial weeds have a two-year life cycle. They grow vegetatively during the first year and produce flowers and seeds during the second year before dying. Examples of biennial weeds include common mullein and bull thistle.
- Perennial weeds live for more than two years. They can be either herbaceous or woody. Examples of perennial weeds include dandelion, Canada thistle, and poison ivy.
Classification Based on Structure
Weeds can also be classified based on their growth habits and structures.
- Monocotyledonous weeds: Monocots are plants that have a single seed leaf when they first sprout. Monocotyledonous weeds have long, narrow leaves with parallel veins, and their stems are usually hollow and round. Examples of monocot weeds include crabgrass, Bermuda grass, and annual bluegrass.
- Dicotyledonous weeds: Dicots are plants that have two seed leaves when they first sprout. Dicotyledonous weeds have broad leaves with branching veins, and their stems can be round or square. Examples of dicot weeds include dandelions, plantain, and chickweed.
- Sedges: Sedges are grass-like plants that are often mistaken for grasses. They have triangular stems and leaves that are arranged in sets of three. Sedges can be difficult to control because they grow in clumps and have a deep root system. Examples of sedges include yellow nutsedge and purple nutsedge.
- Prostrate weeds: These are plants that grow horizontally along the ground and can root at nodes, which are points where the stem comes into contact with the soil. These weeds have a sprawling growth habit and can quickly cover large areas, making them difficult to control. Examples of prostrate weeds include knotweed and prostrate spurge.
- Erect weeds: Erect weeds grow upright and can be either simple or branched. They have a more traditional plant shape and can grow to a considerable height. Examples of erect weeds include common milkweed and pigweed.
- Climbing weeds: These are plants that use tendrils or other structures to climb and attach themselves to other plants or structures. They are often characterized by their ability to grow quickly and cover large areas. Examples of climbing weeds include morning glory and bindweed. Climbing weeds can be particularly challenging to control because they can quickly overwhelm other plants and structures, making them difficult to remove.
Classification Based on Leaf Shape
Weeds can be classified based on different criteria, and one of them is the leaf shape. Here are some common classifications of weeds based on leaf shape:
- Linear: Linear leaves are long and narrow, with parallel edges. Examples of weeds with linear leaves include bluegrass, cheatgrass, and ryegrass.
- Lanceolate: Lanceolate leaves are long and narrow, with pointed tips and wider at the base. Examples of weeds with lanceolate leaves include dandelion, chicory, and plantain.
- Oblong: Oblong leaves are longer than they are wide, with parallel edges and rounded tips. Examples of weeds with oblong leaves include ground ivy, henbit, and wild violet.
- Ovate: Ovate leaves are egg-shaped, wider at the base and tapering to a point at the tip. Examples of weeds with ovate leaves include purslane, lambs quarters, and pigweed.
- Orbicular: Orbicular leaves are circular or nearly circular in shape. Examples of weeds with orbicular leaves include clover, wild strawberry, and bindweed.
- Cordate: Cordate leaves are heart-shaped, with a wide base and a pointed tip. Examples of weeds with cordate leaves include cleavers, burdock, and wild ginger.
- Palmate: Palmate leaves are divided into several lobes, resembling the fingers of a hand. Examples of weeds with palmate leaves include poison ivy, poison oak, and buttercup.
Common Example of Weed
- Amaranthus (Amaranthus hybridus)
- Black nightshade (Solanum nigrum)
- Bitterleaf (Vernonia amygdalina)
- Broadleaf weeds (Chenopodium album)
- Cassia (Cassia obtusifolia)
- Chicory (Cichorium intybus)
- Commelina (Commelina benghalensis)
- Cowpea (Vigna unguiculata)
- Crabgrass (Digitaria sanguinalis)
- Dandelion (Taraxacum officinale)
- Euphorbia (Euphorbia hirta)
- Guinea grass (Panicum maximum)
- Imperata (Imperata cylindrica)
- Indian heliotrope (Heliotropium indicum)
- Jojo (Chromolaena odorata)
- Koster’s curse (Clidemia hirta)
- Lambsquarters (Chenopodium album)
- Mexican poppy (Argemone mexicana)
- Morning glory (Ipomoea purpurea)
- Nutgrass (Cyperus rotundus)
- Parthenium (Parthenium hysterophorus)
- Pigweed (Amaranthus viridis)
- Plantain (Plantago major)
- Purple nutsedge (Cyperus rotundus)
- Siam weed (Chromolaena odorata)
- Sicklepod (Senna obtusifolia)
- Signal grass (Brachiaria decumbens)
- Spurge (Euphorbia heterophylla)
- Star grass (Cynodon dactylon)
- Waterleaf (Talinum triangulare
Methods of Weed Control
Weed control is the management of undesirable plants that grow in an area where they are not wanted. Here are some of the common methods of weed control:
Cultural control methods involve altering the environment to make it less favourable for weeds to grow. Examples of cultural control methods include:
- Crop rotation: planting different crops in different seasons to reduce weed growth.
- Mulching: covering the soil with organic material to suppress weed growth and conserve moisture.
- Hand pulling: manually removing weeds by hand.
- Mowing: cutting down weeds to prevent seed production and spread.
Mechanical control methods involve physically removing weeds from the area. Examples of mechanical control methods include:
- Tilling: turning over the soil to uproot weeds.
- Hoeing: chopping off the tops of weeds using a hoe or other sharp tool.
- Cultivating: breaking up the soil surface to remove weeds.
Chemical control methods involve using herbicides to kill or suppress weeds. Herbicides can be either selective or non-selective.
- Selective herbicides are designed to target specific types of weeds while leaving other plants unharmed. Examples of selective herbicides include 2,4-D and dicamba.
- Non-selective herbicides kill all plants they come in contact with. Examples of non-selective herbicides include glyphosate and paraquat.
Biological control methods involve using natural enemies, such as insects or diseases, to control weed populations.
- Insects: some insects, such as weevils, are natural predators of certain weed species and can be introduced to control weed growth.
- Pathogens: diseases caused by fungi, bacteria or viruses can also be introduced to control weeds.
Effect of Weed control methods on vegetation and soil
Weed control methods can have both positive and negative effects on vegetation and soil. Here are some of the common effects:
- Effects on vegetation: Weed control methods can have a significant impact on the vegetation in the area.
- Chemical weed control methods such as herbicides can be effective at killing weeds, but they can also harm non-target plants. This can result in a decrease in plant diversity and a loss of habitat for wildlife.
- Mechanical weed control methods such as mowing or hand pulling can also impact vegetation. Overmowing or excessive disturbance of the soil can damage plant roots and lead to soil erosion. However, targeted hand-pulling or spot treatment can be effective at removing weeds without harming other plants.
- Effects on soil: Weed control methods can also have an impact on the soil and its properties.
- Chemical weed control methods can leach into the soil and affect soil microorganisms and nutrients. This can lead to a decrease in soil fertility and damage to beneficial organisms.
- Mechanical weed control methods can also affect the soil. Tilling or ploughing can disturb the soil structure, leading to erosion and compaction. However, targeted hand weeding or hoeing can help to remove weeds without causing excessive soil disturbance.
- Effects on beneficial organisms: Weed control methods can impact beneficial organisms such as pollinators, predators, and soil microorganisms.
- Chemical weed control methods can be harmful to beneficial insects such as bees and butterflies, which are important pollinators. They can also affect soil microorganisms such as bacteria and fungi, which are essential for nutrient cycling and soil health.
- Mechanical weed control methods can also have an impact on beneficial organisms. For example, mowing can damage insect habitats, while tillage can disrupt soil food webs.
- Effects on water quality: Weed control methods can impact water quality through runoff and leaching of herbicides and nutrients.
- Chemical weed control methods can lead to herbicide runoff and leaching, which can contaminate the surface and groundwater. This can have negative effects on aquatic ecosystems and human health.
- Mechanical weed control methods can also affect water quality by increasing soil erosion, which can lead to sedimentation and nutrient runoff.
- Effects on human health: Weed control methods can impact human health through exposure to herbicides and other chemicals.
- Chemical weed control methods can expose humans to harmful chemicals, which can have negative effects on health. Herbicide drift can also affect nearby residents and farm workers.
- Mechanical weed control methods can also have an impact on human health. For example, manual weeding can lead to repetitive stress injuries, while mowing can cause hearing loss.