Blanket Weed is a long thin filamentous alga, which like Ulothrix thrives in ornamental ponds. As can be seen from the microscopic observation, the cells of Spirogyra are characterised by helical shaped chloroplasts which give the weed the pronounced green commonly associated with Blanket Weed. Its growth is increased by bright sunlight and shallow fast running water so that it will be found in water-falls and unshaded pond areas. In ornamental ponds which tend be stocked with fish, the tendency to overfeed causes additional fish excrement which contains nutrients favoured by Blanket Weed particularly nitrates and dissolved ammonia. This means that apart from normal growth by photosynthesis the weed will grow even faster.

The spores of Spirogyra can be preserved in the dry state for many years and come back to life when put into water. Ponds can be free of Blanket Weed for years and suddenly become infested. Pulling weed out actually releases spores produced by conjugation (see photo – bottom left), which then produce even more weed.

The proliferation of Blanket Weed is due to the fact that it grows very quickly in sunlight and if left unchecked can cause serious oxygen depletion in ponds. Additionally it can also cause stress to larger fish that find it an obstruction.