Many hydroponic growers underestimate the serious damage that algae can cause to marijuana. In fact, most of them allow algae to form in the setup, thinking that it’s okay. But the truth is, it is a living organism that steals nutrients and oxygen from the plants.

Despite its harmless appearance, algae can grow greedy, leaving nothing for the crop. Aside from the common green, it can also come in other colors like black, red, and blue. In general, it tends to attach itself to hydroponic and aeroponic equipment. Sometimes, it travels into the tubing, creating problems with the setup.

Worse, algae can find its way into the roots of the cannabis plants. If left unchecked, it will rapidly multiply and cover the entire root system. In effect, the roots will experience choking and difficulty absorbing nutrients and oxygen. Of course, this condition is a death sentence for the plants.

So, if growers don’t act fast, algae will ruin hydroponic marijuana. Basically, once the first sign of algae shows up in the system, the plants are no longer getting enough essential elements. Therefore, we must be vigilant and immediately remove it from the setup. So, this article will learn how to spot an algae invasion as well as the techniques to get rid of it.

Controlling Algae in Hydroponic Marijuana

To produce thriving marijuana plants, we must control the presence of algae in the grow area. Hence, the following will discuss the basic methods and steps to prevent and remove it. But first, let’s get familiar with the signs that confirm an algae problem.

  • Recognizing Algae Infestation
Recognizing Algae Infestation
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As mentioned, algae will compete with the plants for necessary elements to survive. For this reason, we need to take care of it as soon as we notice it building up in the setup. To do this, monitor the setup for signs that the plants are not getting enough oxygen or nutrients.


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Essentially, the leaves will show symptoms of nutrient deficiencies such as yellowing and browning. Additionally, dark-colored spotting will appear as well as burnt edges.


Roots – Image powered by Learn.woahstork.com

Because of algae growth, the roots will clump together and turn dry and brown. In effect, the plant will start to wilt and eventually die.

However, an algae infestation of this level takes time to develop. So, if we act as soon as its growth becomes obvious, then we can easily prevent it from progressing.

  • Preventing Algae Formation

It is no surprise that hydroponic setups promote the growth of algae. The combination of rich a nutrient mixture, ample oxygen supply, and good lighting encourages it to thrive. But if we apply the following do’s and don’ts, we minimize the chances for its growth.

  • Keep Reservoir Clean
Keep Reservoir Clean
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As with any other infestation problem, the best way to prevent algae growth is to keep everything in the reservoir clean. So, this section will focus on how to do this to achieve an algae-free grow area.

  • Apply Pure Water
Apply Pure Water
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An essential rule is to start with pure and clean water and keep it as such. This means that using rain, river, or stream water is not allowed. Also, avoid utilizing stagnant water that has been uncovered for an extended period.

  • Change Water Regularly
Change Water Regularly
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As mentioned, clean water is crucial in stopping the growth of algae. So, when the water starts to smell bad, immediately replace it with a new one. Ideally, it should smell good like fresh alfalfa sprouts.

A good rule of thumb is to change the water after 10 days of its first filling. Then, we can increase it to every 14 days as the crop grows. However, when the plants mature and grow more than 12 inches, then, we must do the changing as often as 7 days.

  • Remove Dead Roots and Leaves
Remove Dead Roots and Leaves
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Since algae love nitrogen, never allow decaying roots or fallen leaves to decompose in the reservoir. As such, always monitor for unwanted dead leaves or rotting roots and immediately remove them.

  • Use Opaque Materials
Use Opaque Materials
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Aside from a nutrient-rich water, algae enjoy basking in any light source. However, we can’t totally remove light since it is vital for the marijuana to thrive. Instead, we reduce its brightness by using opaque materials for the hydroponic tubing, buckets, and other supplies.

For growers who use rockwool in their setups, make sure to cover the slabs with dark-colored fabric to keep light from breaking in. This way, the algae will have no chance to develop in the grow medium.

  • Use Materials that Slow Down Growth

While helpful in treating reservoirs already infested with algae, these products are best for prevention. Essentially, they won’t exterminate the algae that are already there but they can slow it down.

  • Barley Straw Mat
Barley Straw Mat
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Traditionally, barley straw mats are an organic way to control algae in lakes, ponds, and other bodies of water. Hence, they are friendly to the plants as well as to the beneficial microorganisms.

  • Grapefruit Seed Extract
Grapefruit Seed Extract
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Many cities use grapefruit seed extract to treat their drinking water and ponds. So, it is a proven method as well as a natural way of managing the algae problem.

  • UVC Lights
UVC Lights
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Using UVC lights is a great way to stop algae in the filtration before it reaches the grow space. Basically, the irradiated lights will catch any harmful microorganism, reducing its potential to multiply. However, it won’t harm humans or plants. We can find the same types of lights in hot tubs and aquariums.

  • Treating A Setup with Algae
Treating A Setup with Algae
Treating A Setup with Algae – Image powered by Brightagrotech.com

If algae have already created a problem in the system, we still have hope to correct it. As soon as we notice algae in the hydroponic system, we must immediately clean it. Start by rinsing off the reservoir and the slabs, then filling it in with fresh and clean water.

Replacing all the clear materials with opaque variations also helps. If time or money is an issue, we can cover the transparent equipment with dark fabric.

Finally, create a mixture of hydrogen peroxide and water at 1:10 ratio. Then, add it to the reservoir in frequent but small proportions. This will help plants that suffer a decrease in oxygen levels. But only give it to the matured ones since young plants won’t be able to handle the solution.

Being Proactive is Key in Controlling Algae

When it comes to algae, immediate action often results in thriving and happy marijuana plants. As such, we only need to follow a certain set of principles in dealing with it. This includes keeping the reservoir clean and using opaque materials and retardants. Additionally, we must provide an oxygen boost for the plants.

So, making the right changes usually renders the environment less hospitable to algae. To keep it at bay, simply apply all the methods we mention in this article. More importantly, keep the plants healthy by providing the basic needs. Finally, always start with strong seeds since they are less vulnerable to diseases.

The founder of I Love Growing Marijuana, Robert Bergman, is a marijuana growing expert that enjoys sharing his knowledge with the world. He combines years of experience, ranging from small-scale grows to massive operations, with a passion for growing. His articles include tutorials on growing... [read more]


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  1. By Symptom Checker – Identify Marijuana Plant Problems ,20 Jul 2015
    […] Algae […]
  2. By Stop Diseases On Marijuana Plants Now! ,21 Aug 2015
    […] Not to beat a dead horse, but your best defense is always preventative. Do your best to minimize the potential for algae growth before it occurs by the taking the proper precautions beforehand. Because algae and cannabis have similar […]Read More
  3. By k capoferri ,31 Aug 2015
    is covering the base of the seedlings with black plastic the diameter of the net pot plus changing the reservoir solution every week enough to prevent algae?
  4. By Gary Muff ,12 Feb 2016
    I want. To know how I can get an accuall copy of the grow bible I would sure apprieciate it thanks.
  5. By Lynn ,12 Feb 2016
    I have been trying to enroll in your course, but the system keeps rejecting my email address as invalid. Please contact me.
  6. By Lamar Thomas ,26 Apr 2016
    This is April; when is the best time to stop Cloning and which is the best Fertilizer to use to get Started?
  7. By latewood.ILGM ,26 Apr 2016
    Lynn, Don't give up. Many have enrolled in the course. We moved Bergmans Lab to a dedicated URL a couple months back, and that may have been your issue. Try again, If you have any issue; Join the ILGM forum […]Read More
  8. By latewood.ILGM ,26 Apr 2016
    Lamar Thomas, I do not know why you would stop cloning. What do you mean? As far as Nutrients. There are so many nutrient lines out there. A lot of nutrient choice is based on how you plan to grow. […]Read More
  9. By Bacterial And Fungal Controls ,28 Oct 2016
    […] and Pellets Barley straw rafts are used in hydroponic cannabis systems to inhibit the growth of algae. Barley straw does not kill existing algae but does prevent the growth of new algae cells. While […]
  10. By Penny ,27 Apr 2017
    I am growing hydroponically and have algae growing on the rock wool. My plants are 3 weeks old. I read your directions about ridding my plants of algae. What do I do about the rock wool my young plants are […]Read More
  11. By Jeanne ,01 Jun 2017
    Came home to little gray parasols (umbrellas) growing. The soil was not too wet. Planted in organic medium

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