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 AFM 91-19 / TM 5-629 / NAVFAC MO-314
24 May 1989
53
in a month exceeds the water storage capacity of
the leaves off below the water surface at fre-
the canal or pond. Nor will fertilizing control
quent intervals. Five or six cuttings a year may
weeds in cooler waters of the central and north-
be necessary.
ern states.
(3) For temporary control of submersed
b. To control rooted submersed weeds and
waterweeds in large canals and rivers, around
waterlilies in ponds with stable water levels,
boat docks, and in fishing and swimming areas
apply 100 to 200 lb/a of nitrogen, phosphorous,
of ponds and lakes, mow as deeply as possible
and potassium fertilizer with an analysis of
and remove dislodged weeds. In canals, the
8-8-2 beginning in late winter or early spring.
dislodged weeds should be trapped downstream
Apply every 2 weeks until a white disc or plate
and removed by draglining or other means to
placed 12-14 inches below the water surface is
prevent them from lodging against structures,
invisible. Then apply fertilizer as necessary to
clogging the canal, and causing overflows, canal
maintain the density of the algae. Do not
breaks, or washed-out structures. In ponds and
overfertilize. An excessive algal growth is unat-
lakes, weed debris that accumulates along the
tractive, can cause oxygen depletion and kill
shoreline can be removed by draglining, cabling,
fish, and may clog pump inlet screens with
conveying, harvester-type devices, or other me-
filamentous algae. Growth of filamentous algae
chanical means. Removing and disposing of
is a sign of overfertilizing.
mowed and harvested weeds is both difficult and
c. Physical shading-out of submersed weeds
costly.
can be achieved with floating Styrofoam planks
(4) One or two harvests per year may be
or balls, or plastic sheeting. It can also be
sufficient in the northern states, but two to four
achieved by using dyes such as nigrosine or
may be required in southern areas because of the
eosine black.
longer growing season.
(5) A small harvester consisting of a cutter
head and a conveyor belt to deposit cut weeds
Section D -- Mechanical Removal
on a deck may cost less than $10,000, but
sophisticated equipment costs more. It may be
7-8. Hand Pulling. Although the traditional
most cost-effective to accomplish this work by
hand and mechanical methods of controlling
contracting with an aquatic-harvester company.
aquatic weeds have been replaced to a large
b. In canals with flows of water greater than
extent by herbicides, the hand and mechanical
70 cubic feet per second, control of submersed
methods are still advantageous in many situa-
weeds by mechanical methods often is less costly
tions. Young plants of cattail, buttonbush, wil-
than chemical methods. In canals that supply
low, and certain other emersed or marsh species
water for sprinkler irrigation, however, mechani-
can be eliminated by hand pulling. Frequent
cal methods are undesirable because fragments
inspection of the channel, pond, or marsh, and
of dislodged weeds often clog sprinkler heads,
pulling when plants are young and few in
valves, screens, and other irrigation equipment.
number, however, are necessary for effective
Also, spread of infestations downstream may
and economical control.
result.
c. For removal of submersed waterweeds from
7-9. Mowing and Draglining:
irrigation and drainage canals, pull a heavy
a. Self-propelled and float-mounted sickle-bar
chain, drag, or disk upstream along the bottom
mowers, and harvesters that cut off submersed
of the canal with a tractor on each bank. Several
weeds below the water surface at depths of 6
trips are usually necessary to dislodge all weed
inches to 6 feet, are available. They are re-
growth. Trap the floating masses of weeds at
stricted to operating in open water areas that are
strategic places downstream, and remove them
unobstructed by stumps and trees, and they are
mechanically or by hand.
often limited by difficulty of access and the
topography of the shoreline.
Section E -- Biological Control
(1) To eliminate cattails, cut the stems off
below the water surface during the early heading
stage and again 1 to 2 months later when all
7-10. Herbivorous Fish:
regrowth is emersed. A third cutting is necessary
a. Several species of fish, notably the Chinese
if regrowth occurs a second time.
grass carp (white amur) and several species of
(2) Small patches of weeds such as waterli-
Tilapia, are presently used to control algae and
lies and watershield can be eliminated by cutting
rooted aquatic weeds. Tilapia are tropical and
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