Advantages and Limitations
of Different Control Options
 Once
you have evaluated these criteria relative to your goals, you have a number
of management options. There are seven different general approaches: limiting
nutrient inputs, physical removal, shading, chemical treatments, biological
control agents, barley bales, and winter drawdown. The following is a brief
summary of these management options, including a description of the technique
and some of the advantages and disadvantages. This summary is not intended
to be comprehensive. |
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Nutrient Input Control:
Frequently
the main factor allowing excessive aquatic plant growth is an overabundance
of nutrients in surface water. Algae blooms are a good indicator of excess nutrients.
These nutrients may enter from tributaries and from groundwater. Over time,
these nutrients may also accumulate in the sediments within the pond.
Mgmt tool: Use best management practices (BMP's) for agriculture and
use streamside buffers to reduce nutrient loading transported in tributary streams.
Consider sewering to reduce inputs from septic systems and minimize the use
of lawn fertilizers and pesticides. For small ponds, dig a small catchpool within
the stream just upstream of the pond to trap sediments before they enter the
pond.
Limitations: This strategy requires considerable involvement from the
landowners and non-lakeshore community to in order to reduce nutrient loading.
Limiting nutrient inflow in streams or groundwater will not address nutrients
which have already accumulated in the lake sediments.
Physical Removal Methods:
Hand harvesting
Mgmt tool: Perhaps the simplest method is the manual removal of the plants
using a scythe, rake, hoe or other tool. This technique works well for small
patches of weeds, patches located in-between docks and other hard to get places,
and in very large lakes or high flow lakes where chemical treatment is impractical.
If a hoe is used, then the method is also fairly permanent because it allows
the removal of roots so that the plant is totally removed. This technique can
be inexpensive unless you have to pay diver wages. Hand removal also allows
the selective removal of certain plant species, leaving others intact. A pool
skimmer will work well for removing duckweed and true, floating plants..
Limitations: Cutting up some species allows them to disperse and get
established elsewhere. This method is labor intensive and therefore not practical
for large patches. It also works along shallow shorelines but is difficult where
water depths are greater than 3 ft. In such areas, it may require personnel
with diving abilities.
Machine harvesting
Mgmt
tool: For larger lakes and large beds of aquatic plants, a mechanical harvester
may be needed.
Limitations: Harvesters are expensive, specially designed boats which
generally require agency or government financing and operation. This technique
also results in large amounts of harvested weeds which require disposal. The
technique is not species-specific and also produces lots of small plant fragments.
For species such as Eurasian milfoil, these plant pieces are easily dispersed
and get established elsewhere in the lake. Harvesters do not remove roots so
regrowth and reharvesting is usually necessary within the same season.
Dredging
Mgmt tool: The most permanent form of management is to remove the entire
plant, including roots. This can be accomplished using underwater suction dredges
for submersed plants or by using a backhoe along the shoreline for emergent
species such as cattails.
Limitations: The equipment generally requires skilled operators and
is expensive to operate.
Mowing
Mgmt tool: Cattails (Typha spp.), giant reed grass (Phragmites)
and Japanese knotweed (Polygonum cuspidatum) are particularly invasive
emergent wetland species which dominate a pond shoreline. With persistence,
these species can be gradually eliminated by frequent mowing (3 times per year)
of the aboveground stems for several successive years. It is critical to cut
off the aboveground plant material before it has time to do grow large enough
to do significant photosynthesizing. The storage reserves in the below-ground
rhizomes then will gradually be depleted.
Limitations: Large mowing equipment is needed. The necessary cutting
is harder to do in high water years where the cattails will be in standing water.
Bottom Barriers:
Mgmt tool: Laying down dark burlap, plastic fabric, or geotextiles across
the bottom in early summer prevents light from penetrating and therefore inhibits
plants from growing. It is a useful technique for small patches and in-between
docks.
Limitations: The cloth is hard to handle and secure; divers are frequently
needed. Gas bubbles from the sediment can build up under the cloth and can cause
it to float to the surface. Small holes need to be punched into the plastic
at intervals of 2 feet. The cloths also tend to migrate down steep banks. Over
time, sediment may accumulate on top of the cloth and new plants will establish
themselves there.
Chemical Treatments:
Chemical shades
Mgmt tool: There are several different chemical dyes available for controlling
aquatic plant growth. They are usually inert, vegetable-based dyes, and work
by reducing the penetration of plant-preferred light wavelengths to the plants.
These are inexpensive and have no negative health impacts.
Limitations: Chemical shades are less useful in shallow pondshores (less
than 50 cm deep and for plants, such as curly-leaf pondweed or duckweeds, which
can get their foliage up to the surface to obtain light. Shades are also not
useful for water bodies with a high flushing rate. Most chemical shades require
a permit for their use.
Chemical herbicides
Mgmt tool: A multitude of different herbicides are commercially available
however they are almost all based on just seven different chemicals. They can
be fairly species-specific and are easy to apply. Pellet forms are good for
more localized use.
Limitations: Most herbicides require some delay before water can be
used for swimming, fishing or drinking. Herbicides, especially in liquid form,
do not work as well in systems with a high flushing rate. Rapid decomposition
of the treated plants may result in algal blooms or "pea-soup" conditions within
the lake. A few studies are linking herbicides to impacts on the health of amphibian
populations. Permits are needed.
Chart on chemical herbicides.
Biological Control Agents:
Sterile grass carp
Mgmt tool: Sterile grass carp, introduced from Asia, consume large quantities
of plants and are useful in the control of some species. Generally fish are
stocked at 5 to 10 fish per acre of pond, with fish of 10-12" in length. The
cost is relatively inexpensive, generally 15$ per fish. Only sterile grass carp
are used in order to ensure that they don't escape and invade natural aquatic
systems.
Limitations: The carp have plant preferences and if offered a range
of plant options, they may not eat the targeted species. The carp convert the
plant biomass into fish biomass and excrete large quantities of nutrients. These
nutrients may result in a bloom of microscopic algae. Carp are carefully regulated
by NYSDEC and a permit is needed for their release. Restocking is generally
needed after 5 yrs.
Insects
Mgmt tool: Several aquatic insects are being shown to be useful in the
control of E. milfoil and other invasive plant species. Densities of the insects
have to be fairly high for control to occur.
Limitations: Research is not yet complete regarding the success of this
method. Insects tend to be very host specific so accurate identification of
the plants is critical. Once introduced, the insect populations follow natural
cycles in density and their host plant will correspondingly vary in abundance
from year to year. Availability of the insects is currently limited and costs
can be high.
Winter Drawdown
Mgmt
tool: Consistent lowering of the water level to expose the shoreline each
winter will result in killing the seeds, rhizomes and other reproductive propagules
of many shoreline plant species.
Limitations: This method is not useful for floating plant species.
It requires intensive water level management techniques, such as a dam or standpipe.
List of Plants for which this technique works: alligator week (Alternanthera
philoxeroides), hydrilla (Hydrilla verticilata), cutgrass (Leersia
oryzoides), bushy pondgrass (Najas flexilis), smartweed (Polygonum
coccineum), leafy pondweek (Potamogeton epihydrous), and softstem
bulrush (Scirpus validus).
Barley Straw
Mgmt
tool: Several studies have shown that barley straw will reduce growth of
phytoplanton and filamentous algae in farm ponds. The method works best when
the straw is loosely placed in net bags and floated on the surface, and when
placed early in the season. The cost is minimal and there are no known health
effects.
Limitations: The scientific basis for the effectiveness of the barley
straw is not yet known. Therefore, the best methods of use are still uncertain.
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