What is Weeds – Definitions of Weeds, Its Nature and Characteristics
Nweke Jerry Anayo
Department of Crop Production and Landscape Management
Ebonyi State University Abakaliki
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Defining what a weed is, and appreciating the traits they possess, is a good start to understanding the nature of these plants. Evolution acts on individuals in a population, so understanding the nature of the weeds we have today requires an understanding of where and when particular plants species became weeds.
Weeds 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 weeds. Although some weeds have evolved elegant adaptations under the influence of man, many had weedy tendencies before man existed. Weeds are products of organic evolution; they exist in intermediate states and conditions. They are also genetically labile and phenotypically plastic. Weeds have been constant and intimate companions of man throughout his history and could tell us a lot more about man, where he has been and what he has done, if only we knew more about them.
J.R. Harlan, 1992.
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.
The definition of a weed
A weed is a plant that is growing where it is not wanted; it can have strong and healthy growth, and is able to overgrow valued plants by overcrowding, thus depleting soil nutrients and moisture that would otherwise be available to preferred plants.
Other definitions of weeds are
1: any plant that is objectionable or interferes with the activities or welfare of man (Anonymous, 1994)
2: a plant out of place, or growing where it is not wanted (Blatchley, 1912)
3: a plant growing where it is not desired (Buchholtz, 1967)
4: a very unsightly plant of wild growth, often found in land that has been cultivated (Thomas, 1956)
5: useless, unwanted, undesirable (Bailey and Bailey, 1941)
6: A generally unwanted organism that thrives in habitats disturbed by man (Harlan and deWet, 1965)
7: opportunistic species that follow human disturbance of the habitat (Pritchard, 1960)
18: a plant that grows spontaneously in a habitat greatly modified by human action (Harper, 1944).
Human desires, values, and most importantly economic needs are what drive a plant being defined as a weed. The qualities by which humans define plants as weeds include disturbance, aesthetics, utility or biological characteristics. All of these definitions are the consequence of interactions with humans. Many of these definitions are anthropomorphic, plant qualities as perceived by humans. As such they reveal the plants’ relationship to us, and tell us much of how we view nature.
THE NATURE OF WEEDS
The concept of a weed plant is inherently human in two different ways. In an immediate sense, they are the plants we define as weedy. In a historical sense, they are plants that arose as a consequence of agricultural crop domestication, an inevitable result of the singular human act of planting a seed (or propagule). Understanding the intimate relationship of humans and plants in both of these ways can provide a more comprehensive understanding of the nature of these plant species.
Weeds are defined as a plant out of place, thriving in habitats disturbed by humans, possessing competitive behavior, and capable of mass movement from one area to another. Human values related to disturbed and agricultural habitats, appearance, utility and biological traits dominate how we define a plant as weedy.
What is the relationship between human nature and the nature of weeds? The nature of weeds is an inevitable evolutionary consequence of agricultural natural selection under the influence of human nature. Human nature includes inherent intuitions about the natural world: taxonomy and the classification of plants we observe eat and utilize. Human cognition is finely tuned to discern both good and bad qualities about the specific plants with which we interact. We humans are very sensitive to plant behaviors whose form and powers‘ we do not appreciate, especially in our managed landscapes. The evolutionary consequences of these human traits are the major crop-weed groups of contemporary world agriculture. Unfortunately for us, separating weedy species from desirable species is often genetically impossible.
The nature of weeds can be understood at a deeper level than definitions and human nature by observing the biological and adaptative characteristics that lead to their success. Defining what a weed is, and appreciating the traits they possess, is a good start to understanding the nature of these plants. Evolution acts on individuals in a population, so understanding the nature of the weeds we have today requires an understanding of where and when particular plants species became weeds.
Most of the 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. The processes of plant domestication, planting and cultivation created new plant communities featuring the crop genotypes they desired. These domesticated species typically came from preexisting wild relatives selected for their crop qualities. The wild relatives interbred with their new crop derivatives and new variants joined these heterogeneous communities, forming metapopulations extending across the landscape. Some of these new weeds were in turn again domesticated, others not. Over 12 millennium this promiscuous inter-fertility and gene flow led to the world crop-weed groups we now have. The most common pattern for the origins of agricultural plants is the inter-fertile wild-crop-weed (w-c-w) plant complex in which both crop and weed were derived from the same wild progenitor species.
What plant species evolved under such close human scrutiny and management? The nature of weeds is ultimately revealed in the particular weed species that have survived to plague human-managed ecosystems to the present day. It is the properties and stability of these successful weeds that define the nature of weeds most precisely.
What exactly are these crop-weed groups? A comparison of the origins of specific crop species with the current weed flora infesting contemporary agriculture reveals the close genetic relationship of most of our major weed species and crops: crop-weed groups
CHARACTERISTICS OF WEEDS
- Self compatibility
Here no external agents are required for sexual reproduction. This situation will definitely lead to the production of genotype of the same genetic composition. This situation helps the plant where there is no agents of pollination and ensures its survival.
- Strong competitive ability
Weeds compete successfully with crops by being more aggressive in their growth habit. ( reproductive potential and vegetative spread etc.). They compete better in obtaining and utilizing the essentials of growth and development at the expense of the crops, some weeds also exhibit alleopathy e.g Euphorhia sp.