This article is from the Fishing bass FAQ, by Bass Rogue with numerous contributions by others.
(Note: the following discussion was offered in response to a question
about controlling weeds in a shallow 50-acre farm pond - ed.)
It depends on the weed or plant species. In your area, it may be Eurasian
water milfoil, a nasty introduction causing many problems in the northern
United States. Unfortunately, grass carp don't much like milfoil and they
might eliminate any native plants that may limit the weed's growth.
Drawdown is a potential solution, but recovery of the black bass fishery,
especially up north, could take years. Just remember, the plants will
likely recover before the fish community. If your pond was deeper, a
partial drawdown might be a workable solution because you could leave a
pool of water in the deep zone for fish survival. I don't believe five
feet is deep enough for this to work for you. Consider digging a deep
basin if you drain the pond. This will serve as a nice, short-term
reservoir for fish when you need to bring the water down in the future.
Some pond managers recommend fertilizing to control aquatic vegetation,
usually by adding large doses of phosphorus to induce algae blooms. The
idea is to limit light penetration and thus reduce plant growth. The
result is usually clouded or pea soup waters. Your call, and it may not
work well in a pond as shallow as yours. A potential added benefit is
increased productivity of the pond.
Along similar lines are products that stain the water some color or
another (usually blue). The dyes limit light penetration and thus plant
growth. These dyes are generally broken down by sunlight and have to be
replenished somewhat frequently. Harmless to fish, but they might effect
the lure color you use.
There is some speculation that bacteria sludge reduction products (used in
intensive fish production facilities and sewerage treatment plants) may
out-compete both algae and plants for nutrients, which may eliminate weed
problems. Approval for the use of such products in less controlled
situations is not completely resolved (I am talking about EPA regulations
and state and local restrictions), and to my knowledge there is only
casual observation available to support the success of such treatment.
Establishing non-weedy species, such as eel grass (Vallisneria americana),
can greatly reduce invasion by weedy species and maintain decent
cover/habitat for many fish species and their foods. Several commercial
outfits supply native plant propagules.
Legal aquatic herbicides can be very efficient in managing weeds in water
bodies. With proper use and herbicide selection, you can remove only
weedy species, or remove plants only from desired areas. Contrary to what
many seem to believe, EPA-approved aquatic herbicides are not harmful to
fish or other aquatic wildlife, except when grossly overused (just about
anything grossly overused seems to harm something). I recommend that you
hire a licensed applicator if you go this route.
Strips of pond liners or other benthic barriers can be installed to
prevent plants from rooting in desired areas. If you are in an area prone
to silt-filled runoff, keep in mind that sediments will eventually cover
any bottom barriers and plants will be able to root again.
Mechanical harvesters of all sizes are available for weed removal. These
work more or less like lawn mowers, cutting the plants. The cheaper ones
simply cut, leaving fragments to float and possibly take root elsewhere.
The more expensive ones remove fragments. Plan on mowing frequently.
Kind of depressing, I know. It should be easier. Talk to your extension
agent, there are likely methods worked out for your area that are the
least expensive and most efficient. Good luck.