Environmental Engineering Reference
In-Depth Information
the sites. The result was wasted money, wasted staff time, loss of habitat quality, and unnecessary
introduction of an herbicide. Complete die-off is unusual, and in many cases repeat applications
are required, especially for the hard-to-kill species. To succeed, a complete kill is needed on-site to
be assured that the weed does not interfere with the maturation of the restoration site.
The primary control strategy for perennial weeds is to kill them outright. Secondarily, one
should limit their spread by seed reproduction, if they reproduce by seed. It is always best to elim-
inate perennial weeds as soon as they are discovered because most of them rapidly spread into
disturbed ground or even into fully vegetated land.
Mulches
Organic mulches can be used around individual plantings to suppress the germination of weeds.
The mulch must be thick enough to exclude light from the soil, which sometimes necessitates
reapplying additional mulch. Mulches do not work well on sites with steep slopes or on sites that
flood periodically, or when using flood irrigation. Take care to ensure that the mulch is certified
“weed-free.” Noxious weeds have been spread to previously weed-free restoration sites through the
application of straw mulch grown in fields invaded by noxious weeds. Weed-free straw is generally
certified by the state department of agriculture in the state in which it is grown. Certified noxious
weed requirements are set forth by the North American Weed Management Association. Rice
straw is typically a good weed-free mulch and has been used frequently in riparian areas as well as
in semiarid regions of the United States with great success.
Application of Herbicides
Pre-emergent herbicides can be applied to a site to prevent weed seeds from germinating. Post-
emergent herbicides either kill the surface part of the plant (contact herbicide) or kill the plant
by affecting the plant's biochemical pathways (systemic herbicide). Some herbicides are selective,
meaning they kill only specific types of plants. Selection of the appropriate herbicide and the
prescription for herbicide application should be made by an experienced herbicide applicator or
pest control adviser.
The use of herbicides typically requires special permits depending on the ownership of your
project site. Work on federal lands requires a Federal Pesticide Use Permit; work on state lands
generally requires some sort of similar approval. All herbicide applications must be performed by
a pest control adviser or someone with a qualified applicator's license. All herbicide applications
should be documented and reported to the appropriate local and state agencies.
Weed Mats
Weed mats (sometimes referred to as fabric mulch) are pieces of landscape fabric placed around
individual woody plants to exclude light and to prevent weed seeds from reaching the soil. They
generally do a good job of weed suppression except when soil runs onto the mat, creating a germi-
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