“A plant is a weed if, in any specified geographical area, its populations grow entirely or predominantly in situations disturbed by man." Baker (1965)

"A herbaceous plant not valued for use or beauty, growing wild and rank, and regarded as cumbering the ground or hindering the growth of superior vegetation." Little et al. (1973)

"A weed is a plant that originated in a natural environment and, in response to imposed or natural environments, evolved, and continues to do so, as an interfering associate with our crops and activities." Aldrich (1984)

The Weed Science Society of America (Anonymous1983) defines a weed as "any plant that is objectionable or interferes with the activities or welfare of man."²

History of weeds:

Most of the most common and widespread weed species we now have came as a consequence of crop domestication, planting and cultivation. These agricultural processes began about 12,000 years ago. They occurred on different continents and involved different native species available for selection as crops. Since those early origins both crops and their weeds have spread throughout the world. These crop-weed groups are the most successful invasive species in human history.³Weeds are adapted to habitats disturbed by man. They may be useful in some respects and harmful in others. They may be useful to some people and hated and despised by others. There are weed races of most of our field crops and these interact genetically with cultivated races as well as truly wild races. This interaction probably results ultimately in better crops and more persistent weeds⁴

The following are some examples of weeds and their origins:

· United States—common and giant ragweed, common milkweed, fall panicum, common cocklebur, poison ivy, marestail (horseweed), nightshade, wild or common sunflower, and wild onion

· Europe—quackgrass, chickweed, Canada thistle, common lambsquarters, common purslane, wild garlic, and yellow foxtail

· Asia or Africa—Johnsongrass, wild carrot, giant foxtail, velvetleaf, kudzu, and witchweed⁵

Characteristics of weeds:

There are approximately 250,000 species of plants worldwide; of those, about 3 percent, or 8,000 species, behave as weeds. Of those 8,000, only 200 to 250 are major problems in worldwide cropping systems. A plant is considered a weed if it has certain characteristics that set it apart from other plant species. Weeds possess one or more of the following characteristics that allow them to survive and increase in nature:

1. abundant seed production

2. rapid population establishment

3. seed dormancy

4. long-term survival of buried seed

5. adaptation for spread

6. presence of vegetative reproductive structures ability to occupy sites disturbed by humans⁶

1. Abundant seed production

Weeds can produce tens or hundreds of thousands of seeds per plant, while most crop plants only produce several hundred seeds per plant. The following are some examples of approximate numbers of seeds produced per weed:

· giant foxtail—10,000

· common ragweed—15,000

· purslane—52,000

· lambsquarters—72,000

· pigweed—117,000

Since most weeds deposit their seeds back to the soil, seed numbers in thesoil increase rapidly from year to year if the weeds are not managed. Despite that many weed seeds are either not viable, eaten by animals or insects, or decompose within several months after they are deposited, hundreds of millions of viable weed seeds per acre can still be present and waiting to germinate.⁷

2.Rapid population establishment

Most weeds can germinate and become established relatively quickly. They also produce viable seeds even under environmental and soil conditions that are not favorable for most crop plants. Under ideal conditions, dense weed populations can thrive and easily outcompete a crop if left unchecked. Under poor conditions, certain weeds can adapt and produce some viable seeds in a relatively short time period (6 to 8 weeks).

3. Seed dormancy

Dormancy is basically a resting stage or a temporary state in which the weed seeds do not germinate because of certain factors. Dormancy is a survival mechanism that prevents germination when conditions for survival are poor. For example, seeds of summer annual weeds will generally not germinate in the fall, preventing them from being killed by cold winter conditions. The various factors that affect dormancy are temperature, moisture, oxygen, light, the presence of chemical inhibitors, tough seed coat, and immature embryos.⁸ There are several kinds of dormancy, but the most commonly used terms to describe dormancy are innate, induced, and enforced.

Innate or primary dormancy inhibits germination at the time seeds are shed from the plant. After the seed shatters from the parent plant, time is required for immature embryos to develop, natural inhibitors to leach out, or extremes of temperature to crack hard seed coats and allow germination to occur. These conditions cause innate dormancy, and, once lost, this type of dormancy cannot reoccur.

Induced dormancy is a temporary dormancy that occurs when a seed is exposed to hot or cold temperatures. It continues after temperatures change and prevents germination during the wrong time of year. The dormancy is broken by temperatures opposite of those that induced it. Summer heat induces dormancy in summer annual weeds such as yellow foxtail and pigweed, preventing germination in the fall. Cold temperatures in fall and winter break this dormancy (usually by mid-winter), and the seeds germinate in spring when conditions are right. In winter annual weeds, the process is reversed.

Enforced dormancy takes place when environmental conditions—cold temperatures, lack of moisture or oxygen, and occasionally a high salt concentration in the soil—are unfavorable. When limitations are removed, seeds germinate freely.. Seeds of different weed species have various temperature requirements for germination. Common chickweed can germinate under snow cover, while common purslane will not germinate until the soil temperature reaches 70 to 75°F. Crop seeds are generally planted at or near the optimum soil temperature needed for quick germination—a temperature that is also ideal for some weed seeds.⁹

Seeds require water for germination. Seeds in dry soils may remain dormant even when all other factors promoting germination are favorable¹⁰.

4. Long-term survival of buried seed

If conditions are adequate, buried weed seeds have the potential to remain viable for 40 years or more. Broadleaf weed seeds tend to last longer in the soil than grassy weed seed since theyusually have tougher seed coats. In most cases, the majority of seeds only exist in the soil for a few years due to germination, decomposition, predator feeding, or other factors. However, with the large number of seeds produced, a small percentage may remain viable forlong-term survival.¹¹

5. Adaptation for spread

Weeds have certain mechanisms for easy dispersal of seeds. Most seeds or seed pods have special structures that allow them to cling, fly, or float. Common cocklebur and burdock seed pods have hooks that attach to animal fur or feathers; curly dock seeds have bladder-like structures that allow them to float; and milkweed, dandelion, and thistle seeds have a feathery pappus that allows them to be carried by the wind. Other weeds, such as jewelweed or snapweed, have pods that “explode” when the seeds are mature, projecting them several feet from the parent plant. Weed seeds can be widely spread through crop seeds, grains, feed hay, and straw. These and other human activities probably account for the long-distance spreading of weeds¹².

6. Vegetative reproductive structures

Most perennial weeds possess special vegetative structures that allow them to reproduce asexually and survive. These perennial structures contain carbohydrates (food reserves, sugars), have numerous buds in which new plants can arise, and include the following:

· stolons—aboveground, horizontal stems that root at the nodes (e.g., crabgrass, bermudagrass, ground ivy)

· bulbs—modified leaf tissues for carbohydrate storage that are located at the base of the stem or below the soil line (e.g., wild garlic, onion)

· budding roots—modified roots that can store carbohydrates and grow both vertically and horizontally (e.g., hemp dogbane, Canada thistle).

Despite these vegetative reproductive structures, many perennials also reproduce by seed. Some depend heavily on reproduction by seed (e.g., dandelion), while for others it is less important (e.g., yellow nutsedge). Ability to Occupy Disturbed Sites Weeds are very opportunistic.. If a weed becomes established first, it has the competitive advantage over crop plants or desirable vegetation.^13.

7. Ability to occupy disturbed sites

Weeds are very opportunistic. When conditions are adequate, weed seeds germinate and colonize if left unchecked. When a site is disturbed, weeds are usually the first to emerge. If a weed becomes established first, it has the competitive advantage over crop plants or desirable vegetation.

Classification of weeds:-

Almost all plants are categorized by some sort of plant classification system and given a scientific name to identify them anywhere in the world. Weeds are also classified by various means. In general, they can be classified by their structure and appearance (for example, dicots [broadleaves] and monocots [grasses and sedges]), habitat, or physiology. A common categorization system groups them according to their life cycle (how long they live). The three major life cycle groups are:-

· annuals,

· biennials,

· perennials

Annuals

Annuals are generally divided further into summer annual and winter annual weeds. Summer annuals germinate in the spring, mature, produce seed, and die in one growing season. Large crabgrass, giant foxtail, smooth pigweed, common lambsquarters, common ragweed, velvetleaf, hairy galinsoga, and common purslane are examples of troublesome summer annuals. Winter annuals germinate in late summer or fall, mature, produce seed, and then die the following spring or summer. Examples of winter annuals include common chickweed, henbit, shepherdspurse,downy brome, and annual bluegrass. (Some annual bluegrass subspecies can occasionally function perennial.) ^14.

Biennials

Biennial weeds grow from seed anytime during the growing season. They normally produce a rosette of leaves close to the soil surface the first year, then flower, mature, and die during the second year. A true biennial never produces flowers or seeds the first year. There are relatively few biennial weeds. Some examples include wild carrot, common burdock, bull and musk thistle, and poison hemlock.

Perennials

Weeds live for more than two years and can be divided into two groups: simple and creeping. Simple perennials form a deep taproot and spread primarily by seed dispersal. Some examples of simple perennials include dandelion, broadleaf plantain, curly/broadleaf dock, and common pokeweed. Creeping perennials may be either herbaceous or woody and can spread by both vegetative structures as well as by seed. Some common herbaceous perennials include Canada thistle, common milkweed, hemp dogbane, creeping buttercup, slender speedwell, ground ivy, quackgrass, and yellow nutsedge. Some examples of woody perennials include poison ivy, multiflora rose, Japanese knotweed/ bamboo, brambles, wild grape, and Virginia creeper. Creeping perennials become established by seed or by vegetative parts. Since perennial weeds live indefinitely, their persistence and spread is not as dependent upon seed as the other two weed groups.

Other types of weeds:

· Grasses and broadleaf weeds.

· Woody and herbaceous weeds

· Parasitic weeds

· Crop associated and crop bound weeds.

· Exotic weeds.

· Facultative and obligate weed

· Noxious weeds

· Objectional weeds.¹⁵

Distribution

Weeds generally share similar adaptations that give them advantages and allow them to proliferate in disturbed environments whose soil or natural vegetative cover has been damaged. Naturally occurring disturbed environments include dunes and other windswept areas with shifting soils, alluvial flood plains, river banks and deltas, and areas that are often burned. Since human agricultural practices often mimic these natural environments where weedy species have evolved, weeds have adapted to grow and proliferate in human-disturbed areas such as agricultural fields, lawns, roadsides, and construction sites.²¹ The weedy nature of these species often gives them an advantage over more desirable crop species because they often grow quickly and reproduce quickly, have seeds that persist in the soil seed bank for many years, or have short life spans with multiple generations in the same growing season. Perennial weeds often have underground stems that spread out under the soil surface or, like ground ivy (Glechoma hederacea

), have creeping stems that root and spread out over the ground.^16.

Impact of Weeds

The impact of weeds on the environment

Major weed invasions change the natural diversity and balance of ecological communities. These changes threaten the survival of many plants and animals because the weeds compete with native plants for space, nutrients and sunlight. Almost all native vegetation communities have been invaded, or are vulnerable to invasion by exotic species that could result in changes to the structure, species composition, fire frequency and abundance of native communities.¹⁷ Nationally, invasive plants continue to invade the land with exotic species accounting for about 15% of flora. About one-quarter of them are either serious environmental weeds or have the potential to be serious weeds.

Weeds may out-compete native plants because:

They may not be affected by the pests or diseases that would normally control them in their natural habitats

The disturbed environment provides different conditions that better suit the invading weed.

As a result the weed may:

Grow faster than native plants and successfully compete for available nutrients, water, space and sunlight.

Reduce natural diversity by smothering native plants or preventing them from growing back after clearing, fire or other disturbance.

Replace the native plants that animals use for shelter, food and nesting.¹⁸

Weeds are often excellent at surviving and reproducing in disturbed environments and are often first species to colonies and dominate in these conditions. Weeds in the ocean spread over wide areas in a very short time. Introduced seaweeds came to Australia in the early 1980s in the ballast water of ships. They are now invading marine environments along the coast of south-eastern Australia.

The impact of weeds on agriculture

Weeds reduce farm and forest productivity, they invade crops, smother pastures and in some cases can harm livestock. They aggressively compete for water, nutrients and sunlight, resulting in reduced crop yield and poor crop quality. For example, prickle bushes such as Gorse, Blackberries, Prickly Acacia, Parkinsonia and Mesquite can invade vast areas of grazing land preventing productive use of that land.²⁷

Weeds contaminate produce, for example:

1. Burrs in wool contaminate fleeces

2. Grain milled with Saffron Thistle or Amsinckia results in discoloured flour

3. Animals that eat specific weeds, such as wild garlic, produce tainted milk and meat

4. Spines on fruit of Caltrop and Spiny Emex can damage the feet of stock animals

5. Paterson's Curse irritates the udders of dairy cows and can kill horses

6. Hemlock can be lethal to both stock and people.

Weeds can also affect the operation of farm machinery. Farmers spend a large amount of time and money managing weeds. Despite control efforts, a recent Australian Bureau of Statistics survey of issues facing the agriculture industry found that weeds were the most commonly reported natural resource management issue affecting landowners. Weed-related issues affected 73% of Australian agricultural establishments during 2004-05. This compares to 46% of farmers reporting soil and land issues and 38% reporting water issues.¹⁹

The impact of weeds on human health

Weeds can also cause human health problems. Many common weeds such as Parthenium Weed, 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.

There are no common characteristics of a poison or harmful weed that would help distinguish them. But as a general rule, plants with a bitter taste, unusual smell, milky sap or red berries may be poisonous with some plants having poisonous roots and bulbs.

Benefits of Weeds

Despite the negative impacts of weeds, some plants usually thought of as weeds may actually provide some benefits, such as:

· Stabilizing and adding organic matter to soils

· Providing habitat and feed for wildlife

· Providing nectar for bees

· Offering aesthetic qualities

· Serving as a genetic reservoir for improved crops

· Providing products for human consumption and medicinal use

· Creating employment opportunities

Weeds have a controversial nature. But to the agriculturist, they are plants that need to be managed in an economical and practical way in order to produce food, feed, and fiber for humans and animals. In this context, the negative impacts of weeds indirectly affect all living beings.²⁰

S.NO	BOTANICAL NAME	COMMON NAME	FAMILY	MEDICINAL USES
1.	Abutilon indicum	Rakshi	Malvaceae	Antidiabetic
2.	Achyranthus aspera	Latkana	Amaranthaceae	In treatment of rheumatism
3.	Ageratum conyzoides	Gandhila	Compositae	In treatment of leprosy
4.	Boerhavia diffusa	Punernava	Nyctaginaceae	Anticancer
5.	Borreria hispida	Safed phooli	Rubiaceae	Astringent
6.	Caesulia axilaris	Balonda	Compositae	Diuretic
7.	Celosia argentea	Siliyari	Amaranthaceae	Antihypertensive
8.	Cucumis trigonus	Kolhi Kekedi	Cucurbitaceae	Antidiabetic
9.	Dactyloclenium aegypticum	Makra	Poaceae	Antiulcer
10.	Echinocloa colonum	Sawan	Gramineae	Used in treatment of piles.
11.	Eclipta alba	Bhengra	Compositae	Antiasthmaic
12.	Euphorbia hirt	Dudhi	Euphorbiaceae	Intreatment of conjunctivis
13.	Fimbristylis barbata	Chuhaka	Cyperaceae	Antidiarrhoeal
14.	*Kylinga brevifolius*	Bandar Phool	Cyperaceae	Hepatoprotective
15.	Leucas aspera	Gumma	Labiatae	Antimicrobial
16.	Ludwigia octovalvis	Laung Phool	Onagraceae	In treatment of migraine
17.	Mimosa pudica	Chhui Mui	Leguminoaseae	In treatment of leucoderma
18.	Oxalis latifolia	Khatti buti	Oxalidaceae	Antiscorbutic
19.	Parthenium hysterophorus	Gajar ghas	Compositae	Antitumour
20.	Phyllanthus niruri	Bhuin awla	Euphorbiaceae	In treatment of hepatitis B

S.NO	BOTANICAL NAME	COMMON NAME	FAMILY	MEDICINAL USES
1.	Ambrosia artemisiifolia	Ragweed	Asteraceae	Antimicrobial
2.	Chinopodium album	Bathua	Portulacaceae	Antiinflammation
3.	Portulaca oleraceae	Pigweed	Portulacaceae	Livertonic
4.	Cynodon dactylon	Doob	Gramineae	Antiemetic
5.	Verbascum Thapsus	Great mullein	Scrophulariaceae	Expectorant
6.	Saccharum spontaneum	Kansi	Gramineae	Emollient
7.	Ocimum basilicum	Van Tulsa	Labiateae	Carminative
8.	Physalis minima	Chirpoti	Solanaceae	Diuretic
9.	Solanum nigrum	Makoi	Solanaceae	Emollient
10.	Chloris barbata	Finger grass	Gramineae	Antidiabetics