Environmental Engineering Reference
In-Depth Information
usually easier and less costly than control or eradication attempts that follow intro-
duction because plants are most persistent and diffi cult to control after they become
established (Ross and Lembi 1999). If invasive plants are allowed to develop a res-
ervoir of seeds or buds, they usually will be present in that location for many years,
even decades. Several measures can be used to prevent the introduction of weeds
into non-inhabited areas. These include the following: cleaning mechanical imple-
ments before moving to non-weed-infested areas; avoiding transportation and use
of soil or gravel from weed-infested areas; inspecting nursery stock or transplants
for seeds and vegetative propagules of weeds; avoiding planting exotic or invasive
plants around homesites; removing weeds from near irrigation ditches, fence rows,
rights-of-way, and other non-crop land; preventing reproduction of weeds; using
seed screens to fi lter irrigation water; and restricting livestock movement into non-
weed-infested areas (Radosevich
et al
. 2007).
In many countries there are legal means of preventing potential weed problems
at the national, regional, or state level. Weed laws are one example of a possible way
of restricting introductions of unwanted plants and other pests. For example, the
Federal Noxious Weed Act, enacted in the USA in 1975, prohibits entry of weeds
by providing crop inspection for weed seeds at ports of entry. h is law also allows
establishment of quarantines to isolate and prevent the dissemination of noxious
weeds within a defi ned area or region, and provides for the control or eradication
of weeds that are new or restricted in distribution (Radosevich
et al
. 2007). Other
more local weed laws can mandate that property owners or public agencies must
maintain a programme of weed prevention or control on their lands. h e success of
such laws will of course depend upon the level of funding available, the knowledge
of the authorities about weed control measures, and the cooperation of public and
private land owners in establishing weed suppression programmes. Although these
legal restrictions on weeds were developed for agriculture, they can provide models
for enactment of similar regulations for introductions of horticultural species that
have potential to escape into wildlands (Reichard 1997).
9.4.3.2 Competition
In agriculture, cultural practices that shift the balance of competition toward the
crop will usually disfavour weed occurrence and improve crop yields. Similarly,
in some habitats native or other desirable plants can be used to outcompete inva-
sive plants (Luken 1997; Hoshovsky and Randall 2000). Plant canopy cover of
desirable species can be manipulated to suppress weeds by rendering the site less
suitable for seed germination or sprouting of vegetative buds, or by eliminating
the conditions that are optimal for growth and development of particular species.
For example, following weed removal, grassland or rangeland areas can be reseeded
with native species in order to provide a dense native canopy cover that suppresses
reestablishment of invaders (Hoshovsky and Randall 2000). In other areas native
communities may reestablish without additional planting and resume dominance
of a site as long as weeds are not reintroduced. This approach usually requires
