This algal growth may not be considered as a 'weed' in the traditional sense, but is included here as it appears from time-to-time in the garden. It can be found on paths, on roofs and sometimes in poorly-growing lawns.

The genus Nostoc contains Blue-green algae which are composed of colonies of Cyanobacteria arranged in strings or filaments called trichomes which are surrounded by a thin sheath. They can be found in water and on land and are able to withstand extreme environments such as the freezing Arctic or the hot pools near an active volcano. They also have the ability to lie dormant for long periods when conditions are unfavourable and come back to life when rehydrated.

The Cyanobacteria can fix nitrogen from the atmosphere and are able to use energy from the sun, but not using the same pigments as the photosynthesis carried out by the chloroplasts in plant cells. Their photosynthesizing pigments are free in the cytoplasm and not contained within an organelle such as a chloroplast.
Geologocal studies have found that oxygen-producing cyanobacteria were present about 2.8 billion years ago and are probably responsible for the creation of the life-supporting atmosphere of the Earth. The chloroplasts in plants and similar organelles in algae, have evolved from Cyanobacteria and became enveloped in the cells at some point in their evolutionary history.
Some of the pigments they contain can absorb UV-A/B which help them to survive in exposed locations.

The pictures on the right show two forms of terrestrial blue-green algae. The upper one is arranged in rounded beads which contain a clear gel-like core when hydrated. The lower one, Nostoc commune, is made up of folded rubbery sheets when hydrated, which dry to a crispy sheet. It was once thought to be related to seaweed because of its physical appearence, but this is not the case.

It has been used in Asia as a herbal medicine for centuries and recent studies have found properties of pharmacological interest with compounds which can kill cancer cells and HIV. Other benefits include controlling hypertension and lowering LDL cholesterol levels.

When dry they can look unsightly, but when wet they can be very slippery. Lift away when found and improve drainage.

See also the monographs on Dog Lichen, Liverwort and Moss which may be found in similar locations.

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