Why controlling weeds is so difficult?

Why controlling weeds is so difficult?

Weeds are undesirable plants in a particular situation or in other words "A plant in the wrong place” and it can interfere with human activities. Taxonomically, the term "weed” has no botanical significance. However, weeds can cause serious economic impacts on horticultural production. Previous research has shown that in restricted areas of container production, they can cause reduction in growth of ornamental plants by 60%. There can be severe competition between ornamentals and weeds within containers for space, water, nutrients, air, and light within greenhouse conditions causing even deaths of ornamental crops. Weeds can also harbor pests and pathogens and can cause serious reduction in crop production, aesthetic sense, and market value of the crops.

There are over 250,000 plant species, among them only 1% (2500) are considered as weedy plant species, and 0.1% (250) are the most problematic weeds. However, these weedy species possess some special characters that help them to compete better than our desired crop plants. Followings are some major survival strategies of weedy species, and which makes them so difficult to control:

·       Weeds have rapid seedling growth and the ability to reproduce when young e.g. Redroot Pigweed can flower and reproduce when it is less than eight inches tall.

·       Weeds have quick maturation period or take only a short time in the vegetative phase e.g. Canada thistle can produce mature seeds in two weeks after flowering.

·       Weeds may have dual mode of reproduction. Most weeds are angiosperms & reproduce by seeds and vegetatively too.

·       Weeds have environmental plasticity. Many weeds are capable of tolerating and growing under a wide range of climatic and edaphic conditions.

·       Weeds are often self-compatible, but self-pollination is not obligatory.

·       If a weed is cross pollinated, this is accomplished by non- specified flower visitors or by wind.

·       Weeds resist detrimental environmental factors. Weed seeds resist decay for long periods in soil/medium and remain dormant for longer duration.

·       Weed seeds exhibit several kinds of dormancy and escape the rigors of environment and germinate when conditions are more favorable for their survival. Many weeds have no special environmental requirements for germination.

·       Some annual weeds produce more than one seed flush per year and seed is produced as long as growing conditions permit.

·       Each weed plant can produce a large number of seeds per plant and seed is produced over a wide range of environmental conditions.

·       Many weeds have specially adapted long and short-range seed dispersal mechanisms.

·       Roots and other vegetative parts of perennial weeds are vigorous with large food reserves, enabling them to withstand environmental stress.

·       In perennials if vegetative organs (like rhizomes, lower stem nodes, and roots) are severed they will quickly regenerate into whole plant.

·       Weeds have great competitive ability for nutrients, light and water and can compete by special means (e.g. Rosette formation, climbing growth and allelopathy)

·       Weeds are ubiquitous (present everywhere).

·       Weeds resist control, including herbicides.

·       They have the ability to occupy sites disturbed by humans.

·       There is a presence of high level of genetic diversity among the weed population.

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