Duckweed (Lemna) as an Evaporation Cover

What it is

Duckweed (Lemna spp.) is a tiny free-floating aquatic plant that, given calm water and nutrients, spreads into a dense green mat across the surface. That mat acts as a living cover: it shades the water and forms a partial physical barrier, narrowing the vapour-pressure deficit that drives evaporation (see what is evaporation).

How well it works

In a controlled trial on dam water, a duckweed cover reduced evaporation by ~27% when living and ~23.5% as a dead (non-living) mat (Soltani et al. 2020). The dead-mat figure is notable: much of the benefit is the surface barrier and shade rather than the plant being alive.

That points to the key caveat for living covers — transpiration. Duckweed draws water up and releases vapour through its tissues, so part of the surface evaporation it suppresses is replaced by transpiration (the evaporation vs evapotranspiration distinction in what is evaporation). A living plant cover therefore rarely matches an equivalent area of inert shading.

Trade-offs

• Ecology. A full mat starves the water below of light and oxygen, which can kill fish and aquatic life. Duckweed is also an aggressive coloniser that is difficult to remove once established.
• Control and maintenance. It grows and dies back seasonally, drifts in wind, and needs active management or containment.
• Water quality. Decaying plant matter adds nutrients and organic load, which can worsen algae rather than help.

Where it fits

Duckweed is most defensible on small, calm, nutrient-rich ponds where a biological cover is acceptable and can be managed — and where harvesting the biomass adds value. For municipal, industrial or large agricultural storages that need a guaranteed, quantifiable reduction, engineered options scale far better; compare floating Azolla, palm fronds and the full methods comparison.