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Algae – a problem in hydroponic systems

VeggiTech / Precision Agriculture  / Algae – a problem in hydroponic systems

Algae – a problem in hydroponic systems

If there are favorable conditions, that is, the presence of nutrient, water and light, then algae can grow; because there are virtually always algae spores around, waiting for favorable conditions to germinate. The problem with algae, (apart from the appearance and smell problems it creates), is not so much that it uses up nutrients from the solution, but as it blooms, dies and decomposes, it removes dissolved oxygen from the hydroponic system. This increases the biological oxygen demand (BOD) on the system and the plant’s roots may suffocate from a lack of O2. Decomposing algae may also release toxins as it breaks down and provides a food source for plant pathogenic fungi which may then multiply to high levels in the system. Algae on plant root systems can suffocate the roots, making the plants prone to attack by opportunist pathogens such as Pythium.

Algae, is a form of plant life, it is a natural consequence of exposing water with nutrients dissolved in it to a light source. Where there is no light, algae cannot grow, so the most obvious solution to preventing algae growth is to stop light from reaching the nutrient solution where ever possible. Channels should have light proof covers, return gullies also need covers, large media beds can also be covered with either plastic film or a layer of substrate which is designed to act as a ‘dry mulch’ since algae cannot grow on dry surfaces. In aeroponic systems, the root chamber must be light proof and media based pot or container systems can have plastic or rigid collars made which cover the surface of the media. However, even in the best designed system, there is usually somewhere that light will fall on the nutrient – planting holes in NFT, return outlets in channels and tanks are common areas.

Control of algae, once in a hydroponic system can be difficult – most growers tolerate small amounts of algae in the system, provided it does not become excessive and this usually causes no problems. Where algae growth has become thick and widespread, often the best option is to clean up the whole system after crop removal and start again with a clean system. Some growers add algaecide products into the nutrient to kill off algae and there are a number of these products on the market. However, since any product which kills algae, a form of plant life, can also damage young or sensitive root systems. Care must be taken with the dose and damage has been known to occur. Algae will also re-grow, very quickly after applications of most algaecide products, requiring more and more of the chemical to get good control.

When the exposed surface of a hydroponic growing medium is wet enough with nutrient solution, algae will grow.  Poor aeration can lead to major problems and probably plant death, especially with crops vulnerable to crown rot, such as cucumber and gerbera. So, when media such as sawdust, sand, perlite, etc, have algae on their surface, it is a major warning. This often occurs because there is not enough height of media, perhaps combined with having too deep a reservoir (preferably no reservoir, or maximum depth 10mm, 3/8 inch). Avoiding this problem can sometimes be as simple as increasing the height of the medium (for all media, as the height increases, the water-holding capacity reduces and aeration increases).

Fungus gnats are small dark flies, with long legs and distinctive wings, which belong to the Sciaridae and Mycetophilidea families and are sometimes called ‘sciarids’ or ‘sciarid flies’. Adults are usually 0.06 to 0.125 inch long and are weak fliers, often seen sitting near plants and running on growing media or foliage. Females lay tiny eggs in moist media or potting mix. The larvae which hatch from these eggs, cause the damage to the plant root systems. Most fungus gnat larvae feed on fungi or dead plants in the media, but some species also feed on living tissue and will attack young seedlings or cuttings. Fungus gnats also spread plant disease pathogens in the media, so good control is important.

With rockwool there can be an algae layer on top of the starter cube. Once the plant has established onto the slab, moving the dripper from the cube onto the slab allows the top of the cube to dry out slightly—enough for the algae to also dry and hence, be less attractive to the flies. With organic media such as cocopeat, sawdust and especially compost mixes, the maggots also like the medium itself, so more care is needed.

There are also shore flies which live in or on algae growth or on very wet, decomposing organic matter and these are common in growing areas where conditions are damp. Shore fly larvae do not tend to feed on root systems, but they are frequently confused with fungus gnats since they often occur together. Fungus gnats are a more serious pest because they can cause so many crop problems by weakening the root system of a large number of plants.


Prevention of gnats includes screening of doors and vents and well as reducing media moisture and organic debris. Allowing the surface of the hydroponic media or substrate to dry out between irrigations will help prevent infestations, as will good drainage of growing beds or ebb and flow systems. Make sure all pruning, organic matter, dead leaves etch which have fallen into the surface of the media are removed regularly. Also make sure that any growing media is not already infected with gnat larvae when you purchase it and keep it enclosed in plastic until use. The first step in controlling gnats is monitoring for the presence or buildup of these pests. The adult flies are attracted to yellow sticky traps – but identification of fungus gnats and shore flies can be difficult as many species look similar. There are three types of control for gnats: chemical, growth regulators and biological agents.

Chemical products are often organophosphate based, and care needs to be taken when handling and applying these: products containing acephate or malathion are often used for chemical control, also carbaryl which also kills the larvae. Diazinon containing products have also been reported to be effective but they need to be applied to the media as high volume drenches. Pyrethrum or a pyrethroid can give some temporary control. It should be noted, however that most of these products are not registered for use in hydroponic system nutrients and media, so commercial growers always need to check with their department of Agriculture about which products can be legally used.

Of the insect growth regulator products – those containing Azadirachtin (from the Neem tree), kinoprene, diflubenzuron or cyromazine can be effective when applied to the growing media at regular intervals. Another biological agent is Gnatrol, a spore forming bacterium produced commercially, which can be applied to control the larvae in the media. Make sure the bacterial product you use is specific for gnats, not the ones used for caterpillar control which are ineffective against fungus gnats.

There are some insect predators now being used with much success to control gnats. There are two main gnat predators available commercially – one is the predatory soil mite, the other are species of nematodes. Hypoaspis miles is the species of soil mite which feeds on small soil inhabiting insects and is primarily a predator of fungus gnat larvae in the media. These predators will eat 1 to 5 larvae per day and can survive as a scavenger by feeding on algae and plant debris.

Plant roots grow in the dark and are adversely effected by light. When growing in media, light is naturally excluded, other than any exposed surfaces. When using pillow bags, such as with rockwool, cocopeat, and perlite, light is excluded by using a black bag. However, this would absorb heat and light, so the bags are made of co-extruded plastic, black inside and white outside to reflect the light and heat.

In channel systems, such as NFT, and aeroponics, it is possible to get light leakage directly onto the roots. With aeroponics it may be holes in the box, or an inspection door kept open deliberately or accidentally. With rigid NFT channels, sometimes a hole is left open to inspect the water flow and/or the roots. These are better covered with an easily removable cap.

To summarize the control measures of algae, justrestrict light (whether it’s natural or artificial). Flush your hydroponics system on a regular basis. Remove dead or decaying plants. Check to make sure the filter and pump are working properly. Use a ‘premium’ algae killing product. While all of the tips listed above are helpful for controlling algae in a hydroponics garden, the single most important thing to remember is that algae, like all plants, needs light to survive; therefore, you can solve your algae problem by restricting it from light. Tip: use 3% food grade Hydrogen Peroxide to oxygenate and disinfect the water.

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