Horticulture Principles and Practices

MODULE 1
PLANTS AS PESTS
Weed
A plant that is growing
where it is not wanted.
7.1 WEEDS
A weed may be broadly defined as a plant out of place. A corn plant in a bed of roses is
as much a weed as a rose plant in a cornfield, no matter how attractive it may look. In
theory, any plant can be considered a weed at some point. However, the term is commonly
used to refer to certain undesirable plants, often with no economic use to growers.
Such plants vary widely in characteristics and include algae, mosses, ferns, and
flowering plants. Weeds tend to be more aggressive than cultivated plants and have characteristics
that enable them to survive under very harsh conditions. They can thrive on
marginal soils and have seeds that have longevity, dormancy, and other allied survival
characteristics associated with life in the wild.
7.1.1 ECONOMIC IMPORTANCE
Weeds are undesirable in cultivated fields or the landscape—or for that matter
anywhere. They should be controlled because they usually result in economic losses.
The following are some of the significant ways in which weeds show their undesirable
characteristics.
1. They compete with cultivated plants for growth factors (light, moisture, air, and
nutrients [Chapter 4]), thus diminishing the performance of crop plants. Weeds
often are better competitors than cultivated plants. They are more vigorous in
growth and produce many seeds for effective dispersal. They have long viability
and efficient dormancy mechanisms.
2. Weeds may harbor pests of cultivated plants and other dangerous animals.
For example, chickweed harbors whitefly, red spider mites, and cucumber
mosaic virus. Charlock may harbor clubroot organisms, which attack
brassica crops. Groundsel may harbor nematodes. Snakes and rodents hide
among weeds.
3. They are an eyesore in the landscape, diminishing the aesthetic value of lawns
and other ornamental displays.
4. When they arise, weeds are removed at a cost. Mechanical removal of weeds by
using a hoe or uprooting is tedious. Herbicides are commonly used in large operations
to control weeds—at a tremendous cost, not to mention the adverse environmental
impact.
5. Weeds increase the costs of crop production. Additional equipment and
machinery are needed to control weeds. When weed infestation is high and
weeds are allowed to flower, the harvested grain may be infested with weeds.
These unwanted weeds are cleaned at additional cost to the operation.
6. They reduce the quality and market value of horticultural products. Weeds
compete with crops for nutrients and cause desired crops to grow and develop
improperly. Yield is decreased, and along with it, income. Seeds infested with
weeds command a low market price. Impurities may be eliminated, adding to
production costs.
7. They may be poisonous plants that can harm people. Pollen from weeds causes
allergies in humans. Plant species such as ragwort and buttercup are toxic to
animals. If the fruits of weeds such as black nightshade contaminate the desired
crop seed and are eaten by accident, humans may be poisoned.
214 Chapter 7 Biological Enemies of Horticultural Plants