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Lichens

A fruiticose or shrubby Lichen

A foliose or leaf-like Lichen

Lichens are not a disease and do not harm any plants on which they are found. Their inclusion here is to answer an often-asked question as to whether they are damaging or if they have killed a tree or shrub. After all they grow just as well on stone and concrete, and as they are slow-growing the branches where they are found are old and if it has died or is performing poorly, something else has weakened the plant which has sparse foliage allowing light to reach the lichen.
They are used by people finding their way by Natural Navigation by observing where they are growing. The fruiticose or shrubby lichens with grey-green hues tend to grow on the Northerly side of rocks and trees, whereas whiter or orange crustose forms grow on the Southerly side.

Lichens do not stay around if the air quality is poor and have only returned to our cities since 'Clean Air' legislation led to a reduction in the amount of pollution. Having said that some can survive in extreme environments from the cold of the Arctic to hot deserts and on toxic waste heaps.

There are a few that can be unwelcome in the garden. The Dog Lichen is a foliose form that grows in poorly growing grass and the crustose forms such as Lecanora rupicola form white patches on paving and tarmac which are extremely difficult to remove.

A crustose Lichen growing on a boulder.

Lichens are made up of a two organisms living in a symbiotic relationship, a fungus which cannot photosynthesize and a green alga or a cyanobacterium which can. The fungus provides support and increases the surface area to capture moisture and minerals for both partners to use. Although they can be separated and cultured in a laboratory, neither can survive as individuals in nature.

A green foliose form such as Melanelixia fuliginosa has a thallus which is very similar to Liverwort, but grows on trees and in much drier conditions.

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