Weed Management in Horticultural Crops

PERSISTENCE OF WEEDS (ADAPTATION)

Persistence is an adaptive potential of a weed that enables it to grow in any environment. In an agricultural situation, the cropping system with its (associated habitat) management practices, determines the persistence of weed species. It is largely influenced by climatic, edaphic (soil) and biotic factors, which affect its occurrence, abundance, range and distribution.

Factors Affecting Persistence

A. Climatic factors
Climate can effect variations in cuticle development, pubescence, vegetative growth, vigour, competitiveness etc. Climate thus has a profound effect on the persistence of weeds which can adapt to a wide variety of climates. The important climatic factors are light, temperature, rainfall, wind and humidity.

i) Light:
Light intensity, quality and duration are important in influencing the germination, growth, reproduction and distribution of weeds. Photoperiod governs flowering time, seed setting and maturation and on the evolution of various ecotypes within a weed species. Tolerance to shading is a major adaptation that enables weeds to persist.
ii) Temperature:
Temperature of atmosphere and soil affects the latitudinal and longitudinal distribution of weeds. Soil temperature affects seed germination and dormancy, which is a major survival mechanism of weeds.
iii) Rainfall:
Rainfall has a significant effect on weed persistence and distribution. More rainfall or less rainfall determines reproduction & survival.
iv) Wind:
Wind is a principal factor in the dissemination of weeds.

B. Soil factors:

Soil factors are soil water, aeration, temperature, pH and fertility level and cropping system. Some weed species are characteristically alkali plants, known as basophilic (pH 8.5) which can grow well in alkali soils and those grow in acidic soil is known as Acidophiles.
Basophiles Acidophiles Neutophiles
Alkaligrass – Puccinalia spp. Cynodon dactylon common weeds
Quack grass – Agrophyron repens Digitaria sanguinalis
Several weed species of compositae family grow well in saline soils. A shift in soil pH, towards acid side due to continuous use of Ammonium sulphate as a ‘N’ source could cause a shift in the weed spectrum.
Many weeds can grow well in soils of low fertility level however, can adapt well to soils of high fertility also. Weeds also has adaptation to moist soil, drought condition etc.

C. Biotic factors:

In a cropping situation, the major effects on weeds are those exerted by the crop as it competes for available resources. Once, a particular weed species is introduced, its persistence is determined by the degree of competition offered by the crop and also the agricultural practices associated with the growing of a crop may encourage or discourage specific weeds.
e.g. Ponding of water – Cynodon dies
Repeated cultivation – discourage nut sedge.
Crops that serve as hosts to parasitic weeds, (Sorghum – Striga sp) crop-induced stimulants are examples of other biotic factors.