Environmental Engineering Reference
In-Depth Information
and die of desiccation. Tillage produces the greatest need desiccation if
performed when the soil surface is dry and repeated several times.
Manual weed control, i.e. hand pulling or hoeing, has not been
widely practised except on small residential lawns or other similar
areas. Besides being very labour intensive, hand pulling and hoeing are
inadequate when weeds are well established. Many broad-leaf and
grassy weeds produce underground reproductive parts such as bulbs,
tubers or rhizomes that remain in the soil and can quickly regenerate. On
the other hand, proper mowing practices are a very effective tool of weed
control. When weeds are mowed frequently, their reproductive organs
are cut off before maturation, and their existing food reserves are de-
pleted in the continuous regeneration of cut-off stems, so the plants
eventually die. Most tall broad-leaf weeds can be completely eliminated
by mowing, and flat-growing (prostrate) broad-leaf species and those
that form rosettes can be strongly suppressed. Many prostrate dicoty-
ledons have primary growing points located close to the soil surface. In
these plants, mowing removes the oldest portion of the leaves but does
not injure the growing point, so new growth can resume. Weeds tolerant
of mowing include selected species of the families
Portulacaceae
,
Euphor-
biaceae
,
Rubiaceae
and
Polygonaceae
. Many of these must be controlled
chemically, otherwise their eradication from turf is virtually impossible.
Chemical weed control
Cultural control can be effective in many situations, but under some
circumstances chemical control becomes necessary. Because the major
goal is the best control of unwanted weeds with the least harm to the
natural environment, chemicals and the processes used to apply them
should be chosen carefully. Information regarding proper selection of
herbicides is usually provided by local pest-control bureaux, agricultural
universities, research stations, etc. Basic information regarding proper
use of chemicals is printed on the pesticide label, discussed above. The
information on that label represents regulatory law. Certain modifica-
tions are allowed, but the turf manager must not cross boundaries of law.
Types of herbicides
Over many years of weed science research, hundreds of herbicides have
been discovered. They have been classified and grouped on the basis of
their properties, chemical structure, mechanisms of their action, solubil-
ity and other factors. Only selected classes and few chemicals having
wide application in turf-grass management will be mentioned here.
Herbicides are chemicals that kill or alter the normal growth of
weeds. On the basis of their selectivity, herbicides are usually divided
