Horticulture Principles and Practices

into the greenhouse or garden. Poor-quality sod often contains weed seedlings. Further,
improperly laid sod with gaps between strips creates opportunities for weeds to rapidly
infest a lawn.
A plant’s life cycle or growth pattern influences the control strategy employed in
weed control. Winter annuals are best controlled while still in the seedling stage during
the fall and early spring. Similarly, summer annuals should be controlled soon after germination.
Biennials are vegetative in the first year of growth; they should be controlled
in that year. Perennials are very difficult to control and hence should not be allowed to
become established. Once established, they have a persistent root system, making them
difficult to control. Perennials can be controlled effectively during the period of rapid
growth before flowering or during the regrowth period after cutting. Flowering species
should be prevented from setting seed.
Biological, chemical, and cultural control of weeds and other pests is discussed in
Chapter 8. Annual weeds may be effectively controlled by mulching. However, mulching
is ineffective in controlling perennial weeds. Controlling any type of weed is easier when
the plants are in the seedling stages. Application of herbicides during the seedling stage
is often effective in controlling weeds; however, it is ineffective when weeds are mature.
Annuals and biennials also respond less favorably to chemical control when plants are
entering the reproductive phase. In perennials, chemical control is effective at the bud
stage, just before flowering.
7.1.4 WEEDS AS INDICATORS OF SOIL FERTILITY
Weeds may be plants out of place, but they arise where conditions are most favorable for
their existence. The type of weed species found on a piece of land is often a fairly good
indicator of the soil characteristics (especially fertility, pH, and type). Examples of such
telltale signs are as follows:
1. An area of land on which a good population of, for example, goosegrass, thistles,
chickweed, and yarrow are found usually indicates that the soil is fertile and
nutritionally balanced.
2. When dandelion, poppy, bramble, shepherd’s purse, bulbous buttercup, and
stinging nettle occur in dense populations, the soil is likely to be light and dry.
3. Sedge, buttercup, primrose, thistle, dock, comfrey, and cuckooflower are found in
wet soils.
4. Acidic soils support acid-loving plants such as cinquefoil, cornflower, pansy,
daisy, foxglove, and black bindweed.
5. White mustard, bellflower, wild carrot, goat’s beard, pennycress, and horseshoe
vetch are found in alkaline soils.
6. Clay and heavy soils hold moisture and favor crops such as plantain, goosegrass,
annual meadowgrass, and creeping buttercup.
It should be emphasized that large populations of mixtures of several of the associated
species listed must occur for the diagnosis to be reliable.
7.2 PARASITIC PLANTS
Parasitic Organism
An organism that lives on
or in another species and
derives part or all of its
nourishment from the
living host.
There are more than 3,000 species of parasitic angiosperms. These plants have little or
no chlorophyll and thus are incapable of photosynthesizing to meet their nutritional
needs. Parasitic plants connect themselves to their host plants’ water- and foodconducting
tissues through specialized rootlike projections called haustoria (singular:
haustorium). The dodder (Cuscuta salina), a parasite with bright-yellow or orangecolored
strands, is a member of the family Convolvulaceae (morning glory family). It is
a stem parasite. Others in this category are Loranthes and Arienthobium. Root parasites
include the broomrapes (family Orobanchaceae). The Indian pipe (Monotropa uniflora)
216 Chapter 7 Biological Enemies of Horticultural Plants