LIMNOLOGY 621
weeds in water as shallow as ten inches and as deep as five
feet. Cutting results in immediate removal of nuisance sub-
merged plants.
Hand-pulling, hand-cutting, and mechanical cutting are
inexpensive, easy to implement around docks and swim-
ming areas, environmentally safe, and allow removal of
undesirable aquatic plants while leaving desirable plants.
On the other hand, these methods are labor-intensive, time-
consuming, and may need to be repeated several times each
summer. It may be difficult for the laborer to see and dig out
all plant roots. Some plants, such as water lilies, are difficult
to cut or pull. Visibility may become obscured by turbidity
generated by cutting activities. All plant fragments must be
removed from the lake to prevent them from rerooting or
drifting onshore. This is particularly important in the case
of invasive, non-native plants such as Eurasian watermilfoil.
Environmental impacts of hand-pulling and cutting include
short-term, localized increases in water turbidity and some
disruption of benthic infauna.
Mechanical harvesting is a short-term technique to tem-
porarily remove plants that interfere with recreational uses
or aesthetic enjoyment of a lake. Mechanical harvesters are
large machines which cut plants below the water surface and
collect the plants and plant fragments for disposal. Harvested
plants are removed from the water by a conveyor belt system
and stored on the harvester until disposal. A barge stationed
near the harvesting site for temporary plant storage in an effi-
cient disposal method. Alternatively, the harvester can carry
cut weeds to shore for disposal in landfills or for use as com-
post. Harvesting is usually performed in late spring, summer,
and early fall when submersed and floating-leafed plants have
reached or are close to the surface of the lake. Harvesters
can cut and collect several acres per day depending on plant
type, plant density, and the storage capacity of the equipment.
Depending on the equipment used, the plants are cut from 5
to 10 feet below the water’s surface in a swatch that is 6 to
20 feet wide. Harvesting is most appropriately used for large,
open areas with few surface obstructions.
Like mechanical cutting, harvesting results in imme-
diate open areas of water and can be targeted to specific
locations. The lake can continue to be used for recreational
purposes while harvesting is underway. Another advantage
of mechanical harvesting is that removing plants from the
water also eliminates a possible source of nutrients often
released during fill dieback and decay. Furthermore, harvest-
ing can reduce sediment accumulation by removing organic
matter that normally decays and adds to the bottom sedi-
ments. Harvested vegetation can often be easily composted
and used to enrich soil.
Mechanical harvesters share the previously mentioned dis-
advantages of creating plant fragments and requiring repeated
application. Several disadvantages are unique to mechani-
cal harvesting. Off-loading sites and disposal areas for cut
plants must be available. On heavily developed shorelines,
suitable off-loading sites may be few and require long trips
by the harvester. Some large harvesters are not easily maneu-
verable in shallow water or around docks or other obstruc-
tions. Furthermore, harvesting can be detrimental to non-targetplants, insects, and small fish; these are often removed from
the lake along with target plants.
The use of bottom barriers (sediment covers) is an
effective physical method of plant control. Barrier mate-
rial is placed like a blanket over the lake bottom to prevent
plants from growing. Applications of bottom barriers can be
made up to any depth, with divers often utilized for deeper
water placement. Readily available materials such as burlap,
plastics, synthetic rubber, polypropylene, perforated black
mylar, fiberglass screens, woven polyester, and nylon film
can all be used as bottom barriers. There are also commercial
bottom barriers which are specifically designed for aquatic
plant control. These include Texel, a heavy, felt-like poly-
ester material, and Aquascreen, a polyvinylchloride-coated
fiberglass mesh which resembles a window screen.
The ideal bottom barrier should be durable, reduce or
block light, prevent plants from growing into and under the
fabric, be easy to install and maintain, and should readily
allow gases produced by rotting weeds to escape without
“ballooning” the fabric upwards. Even the most porous
materials, such as window screen, will billow due to gas
buildup. Therefore, it is very important to anchor the bottom
barrier securely to the bottom of the lake. Unsecured barriers
can create safety hazards for swimmers and boaters.
The duration of plant control depends on the type of
material used, application techniques, sediment composi-
tion (bottom barriers are difficult to place on deep muck
sediments), the rate that plants can grow through or on
top of the bottom barrier, and the rate that new sediment is
deposited on the barrier. Installation of bottom barriers is
easiest in winter or early spring when plants have died back.
In summer, plants should be hand-pulled or cut first in order
to facilitate bottom barrier installation. Bottom barriers may
also be attached to frames rather than placed directly onto
the sediment. The frames may then be moved for control of
a larger area.
Bottom barriers can provide immediate removal of nui-
sance plant conditions upon placement. It is easy to install
bottom barriers in small, confined areas such as around
docks, moorages or beaches. Other advantages of bottom
barriers are that they are hidden from view, do not interfere
with shoreline use, and do not result in significant produc-
tion of plant fragments.
Disadvantages of bottom barriers include high cost of
some materials, suitability only for localized plant control,
possible regrowth of plants from above or below the bar-
rier, and the need for regular inspection and maintenance to
remove accumulations of sediment and any rooting plant frag-
ments. Bottom barriers can also cause localized decreases in
the populations of benthic infauna such as aquatic insects.
A fifth type of physical plant control is water level draw-
down. This involves exposing plants and root systems to
prolonged freezing and drying or hot, dry conditions to kill
the plants. Drawdown is usually performed during the winter
and is more common in management of aquatic macrophytes
in reservoirs and ponds than in natural lakes. Accurate iden-
tification of target plant species is important because aquatic
plants vary greatly in terms of susceptibility to drawdown.C012_002_r03.indd 621C012_002_r03.indd 621 11/18/2005 10:36:48 AM11/18/2005 10:36:48 AM