July 2018

Probably the most useful part of Aquatic Plants and

Their Controlis Table 1 on page 7. This table helps

target and rate various herbicides that are available for

use in an aquatic environment. The table lists eight (8)

different herbicides and then rates them (poor, fair, good

or excellent) for controlling specific plants. Those plants

include common lagoon problems such as duckweed,

cattails, willows and cottonwoods among others. I

should mention that one of the chemicals listed is

copper sulfate. KDHE strongly discourages, possibly

even prohibits, the use of copper sulfate in lagoons for

several good reasons. Therefore, copper sulfate should

not be considered an option when controlling targeted

plants around a wastewater lagoon.

I also encourage reading Table 2 on page 8 that

reviews water use restrictions for various herbicides.

Restrictions listed summarize effects on humans

(drinking, swimming, fish consumption), livestock

watering (dairy and meat), irrigation and agricultural

spraying. While lagoons typically have long detention

times that would meet most restrictions (measured in

days after treatment before use of treated water), it is

still an important consideration for which systems must

account. Downstream water users should have

assurance that water quality has not been adversely

affected by the use of chemicals on a discharging


Another problem is the use of chemicals to correct a

seasonal turnover. Again, I would like to emphasize not

to overreact during turnovers, which typically occur in

early spring or late fall. Turnovers occur at times in the

spring and fall when air temperatures vary widely.

Cattails should never be allowed to overtake a lagoon cell

as in this photo. They can cause many problems including

excessive seepage due to their extensive root system.

Cattails are also a preferred food source of muskrats.

Common aquatic plants that show up in

lagoons and approved control options:


Cattails:one of the best ways to control the growth of cattails,

especially in the middle of a lagoon, is to maintain minimum

water depths of three feet. Most lagoon cells that have at least

three feet of water do not generally have cattail problems. The

water depth is usually sufficient to prevent sunlight penetration

that encourages cattail growth. However, cattails around the

water’s edge are very common and herbicides are an effective

means to control them. The most effective herbicides to use are

Glyphosate (Rodeo and others) and Imazapyr (Habitat). Both

manufacturers recommend adding a surfactant with the

herbicide so the solution adheres to the plants. Both herbicides

are also effective at controlling woody brush and trees such as

willows and cottonwoods.


Duckweed:duckweed is pretty easily identified, as it is one of

the few true floating plants found in sewage lagoons. Their root

hairs extend down into the water to absorb nutrients to survive

and multiply. My recommendation to any operator when dealing

with duckweed is to not overreact. Persons who have operated a

lagoon for several years know, based on past summers, that the

duckweed will form a thick blanket that blocks sunlight and

affects treatment, then control is needed. But in most cases,

duckweed never forms a thick blanket and is blown to a corner

of the lagoon on a windy day, causing no problems. If treatment

is needed, recommended herbicides include Fluridone (Sonar AS

and Avast), Diquat (Reward and Weedtrine D) and Imazapyr

(Habitat). While each herbicide works differently to control

duckweed, most operators report good control. It is also a good

idea when treating a heavy duckweed blanket to not treat the

entire area in one application. Instead, only treat a third to half

of the surface area at a time, and then wait five to seven days for

the next application. Otherwise, the die-off of the entire

duckweed blanket will cause dissolved oxygen levels to drop

dramatically, adversely affecting facultative bacteria breaking

down organic matter.


Filamentous Algae(often referred to as “horsehair” algae

Pithophora roettlerithat forms floating clumps): while not a real

common problem on sewage lagoons, filamentous algae can, like

duckweed, block sunlight and adversely affect treatment.

Filamentous algae typically form dense, free-floating mats. See

the referenced photo. This type of algae comes on quickly and

can cover much of the surface of a cell in just a few days. Of

course, the most effective chemical to use when controlling

filamentous algae is copper sulfate, which KDHE prohibits.But

there may be other solutions for controlling filamentous algae. I

am currently assisting a small community with an aerated three-

cell discharging lagoon that has these type algae on the second

cell. KDHE has recommended, and we are trying, barley straw to

control the algae biologically. The straw is suspended on the

surface of the water; as it decomposes it produces hydrogen

peroxide. Studies indicate that hydrogen peroxide is then toxic to

algae. Hopefully this will be an effective solution for this small

community. The use of barley straw may also translate to other

discharging lagoons needing to reduce excessive algae in their

effluent that can cause the lagoon to exceed their Total

Suspended Solids (TSS) permit limit. Unfortunately, studies have

shown that the treated water must be well oxygenated for the

process to be successful. My next article in The Lifelinemay

discuss this treatment option in more detail, especially if