Algae occur naturally wherever you have nutrient-rich water in a situation exposed to light. Although it is usually green in colour it can also be observed as brown, reddish brown or black. Algae can be problematic in hydroponic systems where it may inhabit the channels, pipes, pumps, drippers and emitters and spread into the media. It can, in effect, smother the whole system and seal it off like a carpet. Algae do the following:

● Use up nutrients that should be available for plant use.
● When they die and decompose they use up dissolved oxygen which is needed by plant roots.
● Release toxins during and upon decomposition.
● Attract parasitic insects e.g. gnats and root nematodes.
● Provide a food source for gnat and other insect larvae.
● Attract fungal pathogens by providing them with a food source e.g. Pythium which can then colonise the system.

Without light algae cannot grow. Although it is almost impossible to exclude all light from a hydroponics system, by covering channels and any other part of the system which carries nutrients and is exposed to light you can reduce the problem. The most common method used is to add light proof covers and plastic film or a dry layer of substrate to the growing beds. If the problem is extensive then it is best to dismantle the system, clean it thoroughly, and start again using the cover suggestions above.

Other approaches for algae control include:

● The use of algaecides: although sometimes used in hydroponics, it is not recommended; algaecides can damage plant roots systems along with killing the algae. Algae also regrow very rapidly after treatment.
● diluted hydrogen peroxide: this is sometimes used but is not without problems. It can only be used on older plants with care and at the correct dosage because young plants are too susceptible to root damage.
● Natural control methods: grapefruit seed extract is one such method that has proven effective without harming plants.
● Straw mats: another method is to place a barley straw mat at the bottom of the reservoir which retards algal growth without harming plants.

Prevention is always better than a cure.

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