Environmental Engineering Reference
In-Depth Information
9.4.2.3 Using machines for invasive plant control
Weeds and invasive plants can be controlled mechanically, which suppresses
plants by breaking, cutting, or tearing them from the soil, thus exposing tissues
to desiccation, and by smothering them with soil. Where it is possible, repeated
mechanical operations may deplete weed seeds or vegetative propagules in the
soil, providing that escaped or surviving plants are not allowed to reproduce.
Repeated mechanical operations may also be used to exhaust the carbohydrate
reserves of perennial weeds, thus suppressing them by reducing regrowth (Ross
and Lembi 1999; Radosevich
et al
. 2007). Seeds near the soil surface are usually
not injured by mechanical operations, while seedlings are quite vulnerable to such
disturbance. Annual, biennial, and simple perennial weeds are most susceptible
to mechanical control, since shoots are typically separated from roots or the entire
plant is uprooted. However, for creeping and woody perennial plants, typically
only the shoots are killed by mechanical disturbance and resprouting often occurs
(Fig. 9.5). Best results from these operations occur when the soil is dry and the
weather is hot since uprooted plants will desiccate and die following soil disturb-
ance. Even vegetative reproductive organs (rhizomes, stolons, etc.) of perennial
plants can be destroyed if they are uprooted and left on the soil surface under hot
dry conditions.
Due to the size and constraints of using equipment, mechanical control meth-
ods are typically performed on a larger scale than manual control, which can
damage native vegetation, disturb the soil, and leave debris that must be removed.
h us, mechanical methods of plant removal are generally site- and species-specifi c
and may even be prohibited where weeds occur in plant communities containing
native species (Whisenant 1999).
Mowing is also used to control weeds and invasive plants by cutting or shred-
ding their foliage. In some situations, mowing is used to decrease the amount of
plant biomass that is present but not kill the plants, for example, along power
line rights-of-way, roadsides, ditch banks, abandoned cropland, or vacant lots.
Mowing of herbaceous vegetation should occur before the plants set seed in order
to avoid weed seed dispersal. Since mowed plants, even annuals, may regenerate
new shoots, frequent mowing is required to prevent seed production (Ross and
Lembi 1999; DiTomaso 2000; Huhta
et al
. 2001).
Mowing can suppress some perennial weeds through carbohydrate starva-
tion, as described above. Similar to mechanically removing or disturbing plant
shoots, frequent mowing also stimulates new shoot development, which even-
tually depletes the plants' carbohydrate reserves if performed frequently enough.
Mowing every few weeks for at least one to two growing seasons is usually necessary
to suppress herbaceous perennial vegetation in this way. However, the plants being
controlled by mowing cannot be allowed to replenish their underground supply
of carbohydrates for this system of weed control to be eff ective (DiTomaso 2000;
Huhta
et al
. 2001). Such frequent, repeated mowing operations can damage desir-
able species as well as select for prostrate weed phenotypes with multiple shoots
