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.
 

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 if the insect producing the honeydew remains active then the growth will return.

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 dry quickly, creating the damp, shady conditions favoured by algae.

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Cultural 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.