Various experts have defined weed in different ways. However, whether as an agriculturist, ecologist or horticulturist, a weed refers to those plant species that grow where they are not wanted. Example of weed include Pistia stratiote commonly called water lettuce, Azolla Africana commonly called African Mosquito fern, Oryza longistaminata commonly called wild rice, Eichornia natans commonly known as water hyacinth, Chromolaena Odorata commonly known as awolowo,
Ipomoea involucrate commonly called morning glory, Bryophyllum Pinnatum commonly called resurrection plant, Andropogan gayanus commonly called elephant grass, momordica charantia commonly known as African cucumber, Cynodon dactylon commonly called Bahama grass, phyllanthus amarus with the local name Iyeke-ebezukpe, Mimosa pudica commonly called sensitive plant, sida acuta commonly known as broom weed, Talinum triangulare, commonly called water leaf and many others.
These weed spread fast, thereby becoming very invasive in nature, because they grow very fast and sooner than later outgrow other plants in its natural habitat. Many invasive weeds have caused a lot of damages to the ecosystem and have altered the natural habitat of many plant species.
The characteristics of weed makes it to be very invasive. For instance, many weed have seed that can easily be dispersed either by wind, water, animal, or man. The seed of most weeds can resist very harsh weather condition. Many weeds are bisexual, while others are hermaphrodite, hearing male and female reproductive organs. Again, many weeds can reproduce vegetatively. Ornamental weeds are imported into the country for horticultural purposes and its sales and uses have further spread it to different parts of the country. According to a botanist, Dr. Emmanuel Aigbokhan, these invasive weeds also have economic and ecological implications for mankind. According to him, some of these invasive plant species are ornamental plant which serves very useful purpose in horticulture, landscaping, gardens, parks and botanical garden. They are of high economic value as people purchase them to beautify their homes, recreational areas, hotels, schools, churches and environment. Some of these plants are also useful in controlling erosion.
Nevertheless, many of these invasive plants have also been found to be very dangerous to agricultural crops as they constitute parasitic weed to many crops such as rice, millet, yam, groundnut, cowpea, potatoes, plantain etc. thereby affecting agricultural yields and creating food shortage. According to Okezie Akobundu, without improving on the present weed control mechanisms, crops yield will continue to drop even when improved varieties or cultivars are grown.
There are various methods of weed control. This includes hand weeding, hoeing, tilling, mowing, shredding, root pulling and bulldozing. Other methods include the use of herbicides and biological control as well as the use of molecular tools, that is, plant genes transfer technology like male and female sterility, seed sterility and parthenocarpy to neutralize the invasiveness of weeds.
According to Dr Aigbokhan, there is need to regulate the importation, movement, transfer, sales and use of ornamental invasive plants in Nigeria, just like it is done in America and Europe countries, in order to protect and preserve the ecosystem. For instance, the Kudzu ornamental plant species which covers over 10 million acres of land in South-East of America have been banned.
According to Dr. Aigbokhan, the awareness about the damages caused by invasive weeds in Nigeria is very low, and stressed the need for increase public enlightenment. He equally stressed the need for necessary legislation against the importation and spread of invasive ornamental plants. There is also the need for government to provide enough research grants for the study and control of’ invasive weeds in Nigeria. It must he emphasized that it is cheaper and better to prevent the spread of ornamental invasive weeds than controlling it once it has spread. One good example is the spread of Eichormia natan commonly called water hyacinth which has taken over virtually all water body in Southern Nigeria. The effects of water hyacinth in our rivers are many. First, it kills and endanger fishes in the river, secondly, it makes movement of boats, ships and canoes in the river rather difficult, if not impossible. Thirdly, it deprives the people their source of livelihood, especially the fishermen. It pollutes the water thereby making it unsuitable for human consumption.
Government should pay more attention to research findings as it concerns invasive weeds and its control. For now, there is little or no effort on the part of the government to combat the ravaging effects of invasive plant species on the ecosystem. However, it is recommended by experts that physical and mechanical as well as biological methods of weed control should be employed. Apart from the eradication of unwanted invasive plants, the methods will help provide employment for unemployed Nigerians, they are engaged in the clearing, hoeing, tilling, mowing and bulldozing of water hyacinth in rivers and invasive weeds on the nation’s highways, parks, gardens, recreational centres, landscape and reserved or managed habitats. This will culminate in decent environment, create income and wealth, and ensure the wellbeing of the people.
