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 52
AFM 91-19 / TM 5-629 / NAVFAC MO-314
24 May 1989
Chapter 7
CONTROLLING AQUATIC WEEDS
Section A -- General Considerations
submersed waterweeds (but not hydrilla) after 3
or 4 days of drying.
7-1. Aquatic Environments. Aquatic weeds vary
7-4. Bottom Liners. Lining ditches and canals
greatly in their nature and in the type of aquatic
with concrete or asphalt usually prevents or
environment in which they grow. Examples of
reduces the growth of rooted submersed weeds.
different aquatic environments are: (1) banks
Filamentous algae may grow on concrete linings
above the waterline of canals and ponds;
and structures, however, and greatly reduce
(2) shallow edges of canals, lakes, and ponds in
waterflow capacity. Also, silt deposits in lined
which emersed plants grow rooted in bottom
canals can support obstructive growths of rooted
mud, extending their leaves and stems above the
submersed weeds in water as deep as 8 to 10
waters surface, (3) deeper areas of lakes and
feet. A few species can grow at depths of 25 to
ponds, and (4) flowing water in canals and
30 feet.
drainage ditches. The environment largely deter-
mines what weeds will be present and what
7-5. Shaping Banks and Reducing Obstructions.
methods will be most effective for their control.
Shaping ditch-banks to provide uniform crowns
and slopes, and maintaining roadways on one or
7-2. Growth Habits of Aquatic Weeds. A n
both banks, are essential for efficient chaining,
aquatic weed may be a grass or grass-like plant
dragging, mowing, or spraying operations to
(e.g., common reed, cattail, or sedge), a broad-
control aquatic weeds. Careful designing and
leaf (dicot), an emersed or floating plant with all
spacing of checks, weirs, turnouts, bridges, and
or most of its foliage above the water, or a
other structures along canals minimizes interfer-
submersed plant growing entirely under the wa-
ence with equipment used for mechanical or
ter surface. The submersed weeds vary from
chemical control of aquatic weeds. Similar con-
those rooted in bottom mud to the leafless,
sideration can be given to the construction of
rootless algae. The microscopic algae give water
pond and lake shorelines. Removing stumps,
a greenish color that varies in intensity from a
logs, and other obstructions from ponds, lake
barely discernible tinge to a pea-soup color and
margins, and access channels facilitates using
density. It may cause objectionable odors and
underwater mowers and weed harvesters.
tastes in drinking water. Filamentous algae pro-
duce threadlike or surface scums that interfere
Section C -- Management Practices
with fishing and obstruct underwater screens and
sprinkler systems. Some algae grow on sub-
7-6. Draining and Drying. To control cattails in
merged rocks and on the bottoms and walls of
ponds and marshy areas, remove the water,
concrete swimming pools, making them slippery
plow the ground, and let the area dry for a few
and hazardous. The growth habit of an aquatic
weeks. Then maintain a water depth of 3 feet or
weed greatly influences the choice of control
more for several months, if possible. To control
methods.
submersed weeds in canals with uniform flow
gradients, drain the canals and allow them to
Section B -- Construction Aids
dry for 3 to 5 days. Repeat this as necessary,
when drainage or irrigation water is not needed.
7-3. Depth and Gradient. Deepening the edges
of ponds, lakes, and reservoirs to 2 feet or more
7-7. Fertilizing and Shading:
will prevent or reduce the growth of emersed
a. In the southern states, mineral fertilizers
weeds such as cattails, bulrushes, and water
are frequently applied in ponds to stimulate the
primrose. Deepening is also likely to narrow the
growth of planktonic algae that shades the pond
border of submersed weeds so that they can be
bottom and prevents or reduces the growth of
managed more easily. Providing a uniform gra-
rooted submersed weeds. This is a very useful
dient in the bottom of a canal or pond, eliminat-
practice in ponds that are used as fisheries.
ing high and low spots, permits thorough drain-
Fertilizing does not control submersed weeds,
ing and marginally effective control of many
however, where the outflow or change of water
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