Lake guide

Duckweed on Lakes

Why tiny floating plants can cover a calm bay, what a mat does to oxygen, and what keeps it down.

The basics

What duckweed is

Duckweed is one of the smallest flowering plants in the world. Each plant is one to three flat green leaf-like bodies about the size of a pencil eraser, with a single root hanging below. It floats free, so wind and current push it into coves and along shorelines.

It is often mistaken for algae, but it is a true plant. If you scoop a handful and see separate tiny leaves, it is duckweed or its even smaller relative, watermeal. Both are native across Idaho, Oregon and Wyoming.

At a glance

What it isTiny floating plants, native to all three states
Where it growsCalm, sheltered, nutrient rich water
Main riskLow oxygen under a solid mat
Lasting fixFewer nutrients and more water movement

Identify it

Duckweed and similar floating species

Duckweed, watermeal and their relatives often grow together. Telling them apart matters, because watermeal is usually harder to control.

Causes

Why duckweed spreads

Duckweed takes its nutrients straight from the water, so it thrives where phosphorus and nitrogen run high and the surface stays calm. A few plants along a shallow shoreline is normal, and wind keeps them scattered. Trouble usually traces back to:

Lawn and garden fertilizer washing in from shore
Geese and ducks, which also carry plants between waters
Septic systems and runoff near the water
Sheltered coves, canals and dead end arms with little wind

Plants multiply by budding, so in warm weather a mat can double in a matter of days.

Duckweed covering the water surface — Duckweed piles up in still water along banks and structures.

Why it matters

Effects of full surface coverage

Floating plants release their oxygen to the air, not the water. A solid mat shades out the algae and submerged plants that oxygenate the lake, and it slows oxygen exchange at the surface.

Under a thick mat, oxygen can drop low enough to stress or kill fish, especially in hot, still weather. Treating a large mat all at once can cause the same problem, because decaying plants use oxygen too.

Fish gasping at the surface early in the morning is a sign of low oxygen. See our fish kill guide.

Diffused aeration bubbles rising at the lake surface — Diffused aeration moves water and adds oxygen from the bottom up.

Control

Control methods

Lasting control usually takes more than one of these.

Find the nutrient sourceWater testing shows how much phosphorus and nitrogen the lake carries and hints at where it comes from. See nutrients in lakes.
Get the water movingDiffused aeration or a fountain breaks up the calm surface duckweed needs and adds oxygen.
Remove what you canSkimming or netting helps in small coves and around docks, but rarely keeps pace on its own.
Treat in sectionsSeveral aquatic herbicides are labeled for duckweed. Applying them to a lake generally requires a licensed applicator. Working in sections protects oxygen.
Cut new inputsA planted shoreline buffer, less fertilizer near the water and managing geese all slow its return.
Check early each seasonDuckweed survives winter on the bottom and rises as water warms. Spring checks catch it while it is easy to manage.

Our aquatic plant and algae management starts by confirming the species and finding the nutrient source, then sets a plan for the season.

Questions

Common questions

Is duckweed bad for a lake?

A little is normal and even feeds waterfowl. It becomes a problem when it covers open water, blocks light and lowers oxygen.

Is duckweed a type of algae?

No. Duckweed is a flowering plant with leaves and roots. Algae is slimy or stringy, or turns the water green. Send us a photo if you are not sure which you have.

Why does it come back every year?

It survives winter on the bottom and returns as the water warms. Unless nutrients and still water are addressed, it regrows from what is left.

Will a fountain alone fix it?

Moving water helps, especially in small coves, but large mats usually need nutrient control, water movement and treatment together.

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