Blanket weed is an algae that is highly invasive. It can quickly cover a pond by breaking itself apart and colonizing new areas. Blanket weed requires consistent environmental conditions, making landscaped water features like koi ponds highly susceptible to infestation. The most common forms of blanket weed are Cladophora, Oedogonium and Spirogyra.
Blanket weed is destroyed when the essential nutrients for growth are removed from the water. Chemicals are available from most garden centers that limit nutrients in the water, effectively killing the weed.
Blanket weeds require sunlight to grow. Adding a shade tree reduces the photosynthesis capability of the weed. Adding dyes to the water reduces the amount of sunlight that reaches it.
Algicides are chemicals that kill off algae by preventing photosynthesis and reducing reproduction. They are not advertised to kill blanket weed, but work as an effective control. Brand names include Algizin P, Pool Harmony and Algistraw.
Barley straw is an effective control of blanket weed. Fatty acids in it begin to compound when it comes in contact with water. Humic and fulvic acids become abundant when the straw begins to dissolve in the water. These acids release hydrogen peroxide, which reduces the algae growth. Eventually, the blanket weed dies off. Barley straw extracts are available that speed up the process.