Algae on leaves

Evergreen trees and shrubs are vital components of the winter garden, but green, powdery deposits on their leaves can make them dull and unsightly. These are caused by algae, which are harmless to the plant but they do cut out precious light to the leaf surface.

Algae on rhododendron leaves. Credit: RHS Advisory.

Algae on rhododendron leaves. Credit: RHS Advisory.

Quick facts

Common name Algae
Areas affected Leaves of evergreen shrubs and hedges
Main causes Damp, shady conditions
Timing Noticed more in winter, but can be seen after any wet spell

What is the problem?

Algae on leaves are usually single-celled organisms that build up on the leaf surface of evergreen trees and shrubs. Leaves of deciduous trees and shrubs tend not to be affected as they drop each year before the wet winter months and are replaced with fresh in the spring.

Algae on leaves is problematic since it makes evergreens look unattractive and blocks light from reaching the leaf surface.

Appearance

On evergreens, algae can be seen as a green, powdery deposit. It can make leaves dull and unsightly.

Note: Algae on leaves should not be confused with a black deposit called sooty mould. This is a harmless fungus that grows on honeydew (a sticky, sugary excretion) from sap-sucking insects such as aphids and scale insects. The sooty mould can be gently washed off with a sponge and water but the pest causing it must be controlled to prevent recurrence.

Cause

Shaded, dense foliage is where algae builds up most on plants, particularly at the base of evergreen hedges, such as holly and yew, or on congested shrubs such as camellia and laurel. These are areas where water does not quickly dry, creating the damp, shady conditions favoured by algae.

Control

Non-chemical control

  • Clipping evergreen hedges so that they taper outwards towards the bottom can help improve light access to the base
  • Congested evergreen shrubs should be thinned out at the appropriate time of year to increase air circulation around the leaves. This will allow rain to dry more quickly on wetted foliage and reduce the opportunity for algae to build up
  • Feeding straggly plants in the spring to encourage vigour should also help

Chemical control

There are no chemical controls for algae on plants.


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