While we often think of algae as being a bright green growth in a nutrient solution or on substrate surfaces, the huge number of algal species means their appearance can be quite diverse. Algae can range in color from lime green, to a bubbly black, to red, to brown. These organisms might appear as long stringy filaments flowing in the current or as dense fur on the reservoir and growing channel surfaces. On growing media, algae might appear as a green stain on the surface of a substrate, or as thick, gelatinous slime under irrigation drippers. Algae is often accompanied by a smell, particularly as it decomposes, which might be musty or earthy and moldy, and it is well known for blocking drippers, emitters and other hydroponic equipment.

What Are Algae?

Algae are simple, aquatic, plant-like organisms that do not have true roots, stems and leaves. Since they are a form of plant life, algae need the same basic conditions as higher plants to grow and reproduce—warmth, moisture, nutrients and light. While we can’t exclude the first three conditions for algae growth within a nutrient solution, light is the one thing that can be controlled, making the best form of algae control a lightproof covering over growing channels, reservoirs and covers around the base of plants growing in substrates.

Problems That Algae Cause

Apart from causing an unsightly mess when uncontrolled growth is present, algae create a number of other problems for the grower. It is not so much that algae mop up nutrients from the solution, but as they bloom, die and decompose, they remove dissolved oxygen from the hydroponic system. This increases the biological oxygen demand of the system and plant roots might suffocate from a lack of dissolved oxygen. Some species of decomposing algae release toxins as they break down, and provide an organic food source for plant pathogenic fungi such as pythium, which might then multiply to high levels in the system.

Could Some Algae Be Beneficial?

While the negative effects of algae growth in hydroponics are fairly well-known and generally accepted as being a nuisance potentially toxic for plant growth, there is another side to algae that is still under investigation. Certain types of algae can be beneficial. One example of this is the giant brown algae species, also known as seaweed, extracts of which have been developed for horticultural and hydroponic use as plant growth supplements and organic fertilizers. Some researchers have even found that certain algae species release growth promoters in plant cultivation systems. Among these plant growth regulators are auxins, cytokinins, gibberellins, abscisic acid and ethylene. Other researchers have found that certain algae can produce anti-fungal and anti-bacterial compounds that might assist with disease prevention in the root zone. These types of trials generally found that while overall heavy algae growth in hydroponics has negative effects on crop growth, there exists the potential to identify certain algae species that produce beneficial compounds and isolate these for use in soilless systems. In the future, we could see extracts from beneficial algae (apart from seaweed products) being a routine supplement in hydroponic nutrient solutions used for plant growth promotion and control of root diseases.

Apart from the potential of beneficial growth compounds extracted from some algae, common everyday algae species growing in a hydroponic nutrient solution do in fact give off oxygen during photosynthesis. In heavily populated solutions, small bubbles of oxygen might be seen being released from the algae under good light conditions. However, while oxygen is released into the nutrient solution by live algae, once algae bloom then die, they then consume dissolved oxygen and can add a large volume of organic carbon into the system, which, in solution culture, needs to be avoided.

Where Do Algae Come From?

Hydroponic gardeners often wonder how this green scourge finds its way into a previously clean system and nutrient solution. Algae are widespread in the natural environment and are an efficient and rapid colonizer anywhere there is moisture, light and nutrients for growth. Algae are common in many water sources such as rivers and streams, ponds, lakes and rainwater storage tanks. Treatment of city water supplies usually destroys most of the algae present, and while that might slow the development of an algae bloom, it only takes one algae spore in the water supply to start an outbreak. Algae spores are everywhere and are usually dispersed in soil or water, or on plant or organic material. They also travel on wind currents, in the air, mixed with dust or are transported via living carriers including insects, animals and humans, thus making them impossible to avoid.

Algae Control and Prevention

Algae can be difficult to control. Prevention via light exclusion from the nutrient solution, reservoirs, growing channels and surfaces of growing media is the main form of defense for most growers and is the most effective. For larger-scale growing beds or containers, a plastic film cover over the substrate and around the base of the plant might be required. Some growing mediums are designed to stay dry on top, which prevents excessive algae growth on the surface. Algae might still be a problem directly under drippers or emitters where light naturally hits the nutrient solution before it flows into the substrate. Complete control over all algae growth in most hydroponic systems can be difficult. Most growers tolerate small amounts of algae in their system, provided it does not become excessive, and this usually causes no problems. When algae growth becomes thick and widespread, the best option is to clean up the whole system after plant removal, and often this requires a good scrub of all surfaces to dislodge stubborn algae, as well as running hydrogen peroxide, chlorine and other sterilization agents through the system, then rinsing well with water before replanting.

There is a range of chemical algaecide products that can be added to the nutrient to kill algae; however, since algae is a form of plant life, there is the potential with many of these to damage young or sensitive root systems, so care must be taken with the dose. Algae will regrow quickly after applications of algaecide products, requiring more chemical to get good control. In a European study on algae control in hydroponic systems, a number of products tested for control of algae in nutrient solutions were either totally ineffective in killing algae, or they killed off much of the algae but were also phytotoxic to the plants and caused root damage. It was also found that dosages of 50 ppm of hydrogen peroxide was required to control algae, but that this dose was too phytotoxic for young plants, although older plants survived this dose rate. Therefore, careful and selective use of hydrogen peroxide could be used on older, more resistant plants, but since hydrogen peroxide is a biocide, rather than an algaecide, there is always a risk of root damage.

Use of UV light sterilization systems to control algae in the nutrient solution has mixed results. While UV treatment will destroy algae spores suspended in, and floating around, the nutrient solution or source water as they flow through the UV sterilizer, algae already growing out in the hydroponic system fixed to channel and reservoir surfaces will not come into contact with the UV light treatment chamber and hence will not be killed. UV doesn’t add any residual killing effect to the solution—it only destroys algae it shines on as the nutrient solution flows past the UV light equipment.

Other studies have found organic algae control methods, such as adding certain grapefruit seed extracts at the correct dose rate to the nutrient, will kill algae without harming the plants. This is a method used in drinking water, ponds and lakes, and it appears to work well. However, once the algae have been killed, they will start to decompose and ideally should be filtered out of the nutrient solution. Algae also tend to reoccur a certain time period after treatment.

There could be the potential in larger hydroponic reservoirs to use barley straw rafts as a means of algae control, which has been proven to work in ponds, lakes and other waterways for many species of algae. The aerobic decomposition of barley straw releases certain chemicals that prevent the growth of algae. The specific decomposition chemicals have not been identified, but it is suggested these might be oxidized polyphenolics or even hydrogen peroxide. This process is rather slow and probably not the most convenient for small indoor gardens. It also requires good levels of dissolved oxygen to always be present to allow only aerobic decomposition of the barley straw. There are liquid bottled barley straw extracts on the market that can be used to control algae in fish tanks and ponds and are worth experimenting with in hydroponic systems at low dosages. However, as with any products that rapidly kill algae, if large amounts of algae growth suddenly die off and begin decomposing in the nutrient solution, this lowers the amount of dissolved oxygen present for plant root uptake. Large amounts of dead algae are best manually removed or filtered out after treatment with control products to control the biological oxygen demand in the system.

The best form of algae control in hydroponics will always be prevention by the exclusion of light hitting the nutrient solution. Wherever possible, growers should aim for this rather than relying on quick-fix chemical controls.


Small amounts of algae colonizing a hydroponic system are not a threat to plant growth—however unsightly they might look. In the future, growers might even find themselves using different types of algae extract products to boost plant growth and health.