Algae and Hydroponics

By Frank Rauscher
Published: December 1, 2015 | Last updated: April 26, 2021 11:38:11
Key Takeaways

Many fans of hydroponics regard this method of growing as a form of weed-free gardening, but that doesn’t mean hydro growers are in the clear where weeds are concerned—we must deal with the dreaded, smelly algae. Fortunately, algae can be prevented using a combination of good gardening practices and completely organic solutions. Here’s everything you ever want to know about algae and hydroponics.

Source: Constantin Opris/

To the hydroponic grower, algae is a weed, and as gardeners, we cannot just accept the presence of weeds, we must fight to control them before they become an epidemic in our gardens, ruining all the hard work we have done. In the case of algae, there isn’t a single magic bullet to keep this pesky green growth at bay, but a number of good practices to follow.


What Are Algae?

Algae are plant-like organisms that contain chlorophyll and trap light from the sun. They are a photosynthetically active life form. There are a number of organisms that make up algae, which are grouped according to their pigments, their cell walls, the types of carbohydrates they store for energy, and the types of whip-like structures (flagella) they use to move themselves around.

The different colors of the various algae species are due to their mixtures of photosynthetic pigments. Algae can be single-celled, or large, multi-celled organisms. For example, giant kelp is considered an alga, even though it has stems and leaves. Cyanobacteria, which are often called blue-green algae, are considered algae because these life forms produce blooms just like algae and can damage water quality if left uncontrolled, also just like algae.


Algae Formation in Hydroponics

Algae start from spores, which are typically microscopic and blown in the wind. This means they can be present in your house or greenhouse, and then find their way into your water system. This is similar to outdoor gardens, where most of the weeds are started from seeds that are either blown in or left there by birds or other visiting creatures.

The bottom line for hydroponic growers is that algae reproduce quickly and require only sunlight or synthetic light, another form of energy such as sugar, carbon dioxide and a few nutrients to grow and thrive. Temperature also plays a big role in the rate of algae growth, as do various aspects of water quality such as pH levels, electrical conductivity, salinity and turbidity.

The Problems Algae Causes in a Hydroponics System

As it becomes more pervasive within your water system and the root systems of your plants, algae deprives your plants of oxygen. The root systems of plants perform a process called respiration, and instead of photosynthesis, the roots consume oxygen and convert sugars produced by the leaf system, produced into energy for the plant.


If the roots do not get enough oxygen because all of it is going towards algae growth, plants become weak and production is likely to suffer. When a moderate algae invasion occurs, plants may lose their vigor at a rate that might go unnoticed by inexperienced growers.

Algae Detection

It is important to spot an algae outbreak in your system before it becomes invasive. Algae is not always green; it can also be brown, golden, red or black. You’ll find it clinging to all kinds of things in your system, in addition to just floating around or suspended in the water. If you have a good filter, you’ll see algae collecting there when you go to clean it.


Algae will cause the water in your system to smell earthy or even moldy, especially when it begins to grow in large colonies. When looking out for an algae problem, carefully examine all aspects of the system, as algae will attach to equipment, grow media and plant roots. As it attaches, it becomes increasingly difficult to control.

Algae Prevention

Being aware of the factors that cause or promote algae growth is your first tool in preventing or minimizing its growth. Here’s a brief look at some of the things known to cause algae.

Nutrients – Nutrients promote the growth of algae and cyanobacteria. Denying algae the nutrients it needs is an important way to keep your system algae-free. The two nutrients most responsible for supporting algae growth are nitrogen and phosphorus, both of which are also required for growing crops.

An excellent way to minimize these two nutrients from causing algae growth is to avoid putting them in your water reservoir for long periods of time, and instead putting them in the line when the watering cycle is running. There are many different types of in-line nutrient injectors, from those that simply allow you to mix nutrients with water as it enters your feed line, to those that regulate the flow rate of nutrients as they go into your feed line. Using this method, the nutrients are in your grow media, not your holding tank.

Lighting – Light and light intensity is another important factor in algae prevention. Keep your water reservoir as dark as possible. It is interesting to note that long, extended periods of high-intensity light will actually diminish the growth of blue-green algae, but intermittent, high-intensity light will provide the right conditions for optimal algae growth. Of course, intermittent high-intensity light is what you provide for your plants, so simply avoid providing it for your water tank as well.

Blue-green algae have a competitive advantage over other types of algae, as these types can adapt to various light conditions better. This is why these types will likely be the most frequent culprits lurking in your water system.

Temperature – Blue-green algae blooms occur when the water temperature is warm (above 77°F), while other types of true algae thrive in temperatures around 57°F. This means that warming the water will only change the type of algae you need to be aware of. Keeping your source water close to 40°F can help reduce algae, especially cyanobacteria growth.

Water Flow – Most algae prefer water that is standing still, but some can adapt to more turbid environments. Many things occur in natural outdoor water systems like lakes and rivers that do not occur within indoor gardens or greenhouses. For example, stratification, where the water has various temperature layers, requires a reservoir size not found in hydroponics.

This is particularly relevant regarding bottom water that may become anaerobic. Likewise, if your system has some areas where the water could be prevented from moving and mixing with other water, such as a section of piping, be aware that this can present toxicity issues.

Algae Control

Allowing algae to have free rein of your water system is not acceptable. So, how can you reasonably and economically control algae growth? There are a number of ways you can slow down or prevent algae growth before it takes over your hydro system.

Nutrient Dosers – As mentioned earlier, the primary contributing factors to algae growth are nutrients and light, so keep nutrients out of the water for as long as possible. Placing nutrients into the water reservoir and allowing them to sit there until later—often much later—before being pumped to the grow media does not benefit your plants, but it does promote algae.

Remember, in-line nutrient injectors are available, which allow you to keep nutrients (especially nitrogen and phosphorus) out of your water reservoir, while still giving your crops the essential nutrients they require.

Filters – Having a good-quality filter and changing or cleaning the filter membrane frequently is the best way to control an algae problem. Algae reproduces quickly, so the more it is allowed to stay and grow in the system, the more rapidly a severe algae problem can develop.

Thoroughly clean your entire system before starting a new crop and then clean your filter routinely. If you are using a membrane-type filter that helps visibly show how much algae you are trapping, this will help you determine how often routine cleaning needs to take place.

Cleaning – When rinsing your water tank, use a mixture of 1:10 hydrogen peroxide and water solution, which provides extra oxygen while killing off many of the algae spores. The presence of hydrogen peroxide can damage new plant roots, so be careful with any residual solution that remains after cleaning.

UV Light – Ultraviolet light can be an effective treatment for an algae outbreak. It can also affect the nutrients in the water, especially chelated micronutrients, if you are not using an in-line injector, so it is best not to overdo or improperly use this method. UV light will disinfect the solution that flows into or through its treatment basin, but does not get the algae that may have attached to the surfaces of your water system. The best practice here is to use UV rays to treat the water prior to adding nutrients. The cleaner the water, the better your UV filter/system will work.

Light Limitation – Keep as much light as possible out of your reservoirs. Using black tanks will help. Always have a lid and keep it shut. This not only keeps out the light, but also greatly reduces the number of spores that can blow into the tank.

Be mindful of where you place power cords and the pump feed hoses. If these items are simply draped under the lid, you have created a way for a substantial amount of light and spores to access the tank. Instead, create holes near the top on the side of the tank for the cord and tubing to exit, and stuff something around the cord and tubing to prevent even more light from entering.

Algae can also start to grow on your medium, so consider keeping it covered with landscape fabric or other organic products that keep the grow medium in the dark, but still allow oxygen to reach the root system.

Chemical Control – There are a number of chemical and organic control options available that can get rid of algae. From grapefruit seed extract, to diluted hydrogen peroxide and more, these products are added to a hydro system to help fight algae growth.

On the organic side, when microbial equilibrium occurs within a water system, various bacteria compete with each other and provide another form of control. There are a number of organic additives out there specifically designed to help minimize algae growth in this way. Ask the guys at the hydro store for what works for them.

When growing hydroponically, algae is the primary weed to look out for. It is the most undesirable plant in our garden. Hydro growers need to stay abreast of how to control and minimize this pesky organism so they can produce the greatest yields possible from each and every crop.


Share This Article

  • Facebook
  • LinkedIn
  • Twitter

Written by Frank Rauscher | Writer, Owner of Garden Galaxy

Profile Picture of Frank Rauscher
During his many years of service in horticulture, product development and sales, Frank has performed innumerable visits to landscapes to facilitate a correction for struggling plants or assist with new design. He also writes for Southwest Trees and Turf and The Green Pages, is the owner of Garden Galaxy and manages several websites. He has four children and eight grandchildren.

Related Articles

Go back to top
Maximum Yield Logo

You must be 19 years of age or older to enter this site.

Please confirm your date of birth:

This feature requires cookies to be enabled