Horticulture Principles and Practices

8. Weeds may clog drains, waterways, and other water bodies such as ponds. A pond
or lake infested with weeds soon becomes shallow, thus reducing its effective use
(e.g., for irrigation). Waterways choked with weeds encourage flooding.
7.1.2 BIOLOGY, CLASSIFICATION, AND DISTRIBUTION
Weed species have wild characteristics for adapting to marginal conditions and have
effective mechanisms for self-perpetuation in the environment. Many weed seeds can
persist in the soil for long periods of time in a dormant state until appropriate conditions
for germination and growth occur. Many weeds such as groundsel establish rapidly and
thereby can suppress crop seeds that germinate later. Some weeds germinate during
particular seasons (e.g., orache in spring), whereas others, such as annual meadow grass,
germinate throughout the year.
Soil moisture, pH, and nutritional status affect the distribution of weeds. Some weed
species adapt to acidic soils, and others prefer poorly drained soils. Weeds, like other plants,
are adapted to various environments. Some are tropical, whereas others prefer temperate
conditions. Within the same environment, certain weeds are more of a problem in fields of
cultivated crops than others. Weeds also differ in growth pattern and means of propagation.
Some weed species have underground vegetative structures (e.g., swollen roots and
rhizomes) used for propagation. These weeds include bindweed, dandelion, bracken, and
plantain. Unfortunately, some cultural practices designed to control weeds in actuality
help them to establish more rapidly. Cultivation cuts these underground structures into
pieces and spreads them around. Grasses and other species with rhizomes are among the
most difficult weeds to control.
Types of Weeds Based on Life Cycle
Weeds may be classified into three groups based upon their life cycles:
1. Annuals. Annual weeds complete their life cycles in one year. Summer annuals
germinate in spring and grow through summer. They produce seed and then die
in fall or winter. Examples of summer annual weeds are lamb’s-quarter,
cocklebur, foxtail, and crabgrass, which infest lawns and vegetable fields.
Winter annuals germinate in fall, live through winter, and produce seed in spring.
The seeds remain dormant in summer, germinating in fall. Examples include
chickweed, shepherd’s purse, and hairy chess, which appear in plots of winter
vegetables and fall nursery plants. Annual weeds are relatively easy to control, in
spite of the fact that their short life cycle means they produce seeds more often.
2. Biennials. A few biennial weeds occur in crop cultivation and the landscape.
Biennial weeds germinate in the spring of one year and remain vegetative until
the next spring, when they flower. Weeds with biennial habits include hogweed
and ragwort.
3. Perennials. Perennial weeds are very difficult to eradicate once established. They
may be started from seed initially, but once mature, they also propagate by vegetative
means. Many noxious perennial weeds are grasses (e.g., Bermuda grass, nutgrass, and
quackgrass). Nongrass perennial weeds include plantain and dandelion. In fact,
Bermuda grass, guineagrass, barnyardgrass, johnsongrass, goosegrass, and purple
nutsedge are among the most difficult weeds to control.
7.1.3 CONTROLLING WEEDS
Since weeds may cause economic loss, they should be controlled when they occur. When
weeds appear after the crop is mature or ripened, controlling them may not be economical.
However, one of the preventive measures against weeds is to exclude weed seeds
from crop seeds at harvest. Planting impure seeds is a means of spreading weeds. Poorquality
compost and unsterilized soil are ways by which weed seeds can be introduced
7.1 Weeds 215