Where growing conditions are favourable, these algae, lichen, liverworts and moss usually appear in gardens of their own accord. These conditions include:
Algae: Algae will form on hard surfaces or plants, especially in shady, wet or poorly drained conditions. They proliferate in water such as ponds or water butts where light and nutrients are available.
Lichens: Lichens are favoured by humid, still conditions and clean air, so are often more common in rural areas and mature gardens. They are particularly adaptable as they are able to exist where nutrients, and sometimes water, are scarce. However, they grow only very slowly so, unlike moss and algae, are slow to colonise. They can be found most frequently on trunks and stems of trees and shrubs, on paths, patios, paving,and walls, and less commonly in lawns.
Lichens often grow more profusely on trees and shrubs which have been neglected - especially where the branches have become overcrowded or the plant is in poor health. However, they can also appear on vigorous new plants in humid areas and are fairly common in western districts, where they form a natural part of the garden ecology.
Liverworts: Research indicates that some liverworts, such as Marchantia polymorpha, multiply under conditions of high humidity, high soil moisture, low pH and little plant competition, especially where the soil is compacted.
Moss: Moss is favoured by wet, poorly drained, compacted or shady conditions. They occur naturally on the stem and trunk of trees and shrub, hard surfaces, borders and the top of compost in containers. On lawns their presence is encouraged by a lack of aeration, low fertility, over-acidity of the soil, very wet conditions, excessive shade and very close mowing. Rhytidiodelphus squarrosus is the species most usually encountered in turf or lawns, but Polytrichum juniperinum may also be found on acid heathland soils and on compact sandy soils. Polytrichum commune may be found in wetter conditions.