Environmental Engineering Reference
In-Depth Information
into two main groups:
selective
and
non-selective
. Selective herbicides
control target weeds without harming desirable turf-grass species. Non-
selective herbicides kill all vegetation, including turf-grasses. Selectivity
of a herbicide is influenced by the way it is absorbed or translocated
within the plant as well as by morphological or physiological differences
between turf-grasses and weeds. The majority of herbicides for use on
turf are selective. Non-selective herbicides are generally used instead in
the renovation or establishment of new turf areas or as spot treatments of
isolated weed patches.
On the basis of the timing of their application, herbicides can be
classified as
pre-emergence
or
post-emergence
. Pre-emergence herbicides
are applied before germination of weed seeds, and post-emergence herbi-
cides are used to control weeds that have already emerged from the soil.
In temperate climates, pre-emergence herbicides are important and very
effective in controlling annual grasses and certain annual broad-leaf
weeds. They act by forming a 'chemical barrier' in the soil that prevents
seedlings from emerging and developing normally. The timing of
application is the most critical component of their effective use, but
they are a good choice when the majority of weed seeds germinate within
a relatively short period of time. Nearly all seeds in temperate climates
germinate in spring, when soil temperature reaches 13
C. Pre-
emergence herbicides remain effective in the soil for 2-3 months in the
temperate zone and can therefore provide excellent control if applied just
once a year. This pattern is not effective in tropical turf-grass manage-
ment, because in the tropics, the same weed species act as perennials,
germinate year round and persist in turf for many years. Under the
tropical conditions of high moisture, high temperature and overall high
microbial activity, herbicides stay active only for 4-6 weeks rather than 2-
3 months and must therefore be applied many times per year. Because
multiple applications are both expensive and environmentally unaccept-
able, use of pre-emergence herbicides in the tropics is quite marginal, and
the majority of weeds are controlled after emergence.
Post-emergence herbicides are applied directly to growing weeds. In
contrast to pre-emergence herbicides, this group does not provide any
residual control of weeds. Because seeds germinate continuously, com-
plete chemical weed control usually requires several applications per
year. In many cases, turf discoloration or injury results from application
of post-emergence herbicides that are not adequately selective. Some turf-
grass species, especially when well established and mature, are quite
resilient, but others are very susceptible to many post-emergence herbi-
cides. Turf-grass plants that are still in juvenile stages are much more
susceptible than mature plants. In general, regardless of turf-grass spe-
cies, application of post-emergence herbicides on newly established sites
should be delayed.
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