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Prevent Algae in the Greenhouse

June 20, 2019

Greenhouses provide an ideal environment for the growth of algae. An ample supply of nutrients, high light levels and plenty of moisture in a high humidity environment encourage algae to grow.

In some conditions, a “green slime” can be seen growing on the surface of the substrate. This green slime is predominately composed of blue-green and green algae. In this article we discuss what algae actually are and how to prevent algae in the greenhouse.

Algae are very diverse and found almost everywhere on the planet. They play an important role in many ecosystems, including providing the foundation for the aquatic food chains supporting all fisheries in the oceans and inland, as well as producing about 70 percent of all the air we breathe.

What are algae?

Well, there are many types of algae and they can be found all over the world. These water-dwelling microbes even play a main role in the aquatic food chain and the production of oxygen (about 70 percent!). But of course that doesn’t mean you should be happy to find algae in your greenhouse.

Algae are actuallty tiny plants. While there are larger varieties like kelp, the algae you might find in your greenhouse are single-cell organisms. They reproduce vegetatively by single cell division. Some more facts on algae:

  • Colonies can grow quickly, break up or fragment, and spread.
  • Algae grow in any area where there is moisture, light and nutrients.
  • Algae have a reproductive cycle that produce spores which are transmitted through water, air and mechanical movement.
  • It is very difficult to control algae from being present and to prevent their spread.

Are algae harmful?

Algae do not harm plants. However their prescense can slow the development of roots. That is because they can slow gas exchanges into and out of the substrate. They can also attract insects that not only feed on algae, but also on plant roots.

In addition algae growth on greenhouse glazing can restrict light transmission compromising plant growth.

Vectors of contamination

So algae spreads easily. To properly prevent algae in the greenhouse, we need to understand exactly how algae spreads. There are many ways algal growth is spread in greenhouse conditions.

First of all, it is important algae can grow on the surface of a substrate, but it can also grow on hard surfaces. They have tiny, microscopic algal spores that stay behind in used pots and trays to contaminate fresh substrate. For that reason it is important to properly clean used pots. Secondly these spores can stick to humans and equipment or travel by water. Because of that you can easily spread algae growth by walking from one contaminated greenhouse to another.

We also discussed that algae can be transmitted through water. For that reason irrigation ponds exposed to light can enhance algal growth. Irrigation lines, above all the ones made of white PVC allow algae to proliferate and spread as well.

Finally, there is yet another way algae manage to spread plant pathogens. They actually attract vector insects, like fungus grants what can spread these spores.

You probably understand that preventing algae to spread is almost impossible.

Means of control

So how do we actually prevent the growth an limit its spread? All is not lost, there certainly are several  preventative measures you can take.

Environmental control

Controlling the moisture in the growing area is one of the most important preventative measures. Good ventilation is required to lower the humidity in the air. Horizontal air flow helps to keep surfaces dry and prevents algal growth.


When there are algae present it is good to try to remove them, to prevent them from spreading. As a result of reducing the algae population, you also reduce the amount of spores produced.

  • Keep floors and walls free of weeds and debris, since they are nutrient sources for algae. Power wash floors and walls on a regular basis. After a thorough cleaning of surfaces, treat them with a chemical sterilizer that is also sporicidal. This helps to control greenhouse pests as well.
  • Gravel floors are excellent for drainage, but they can be breeding grounds for algae and fungus gnat larvae. Regular use of a granular algaecide on these floors will help keep inoculum and pest populations low. It is critical to keep clean under-bench areas and difficult to access places.
  • If you reuse pots and trays, clean them thoroughly and treat them with a chemical sterilizer or a heat process.

Water management

Plants need water and algae require moisture, but there are several ways to deteriorate the conditions for algal growth.

Avoid or reduce any standing water on floors, and if necessary, employ additional drainage to ensure excess water is removed. Optimise the amount and frequency of watering. Keep irrigation water clean and understand that the potential contaminants may be present in irrigation ponds. It is essential to limit the spread of algae onto the crop. Constant monitoring of the water quality (pH, salinity, bicarbonates, etc) can als help prevent algae in the greenhouse.

Chemical control

Greenhouse walls, walkways or bench foundations, those which do not come into contact with plants, can be treated with copper compounds or with a phytotoxic formula (quaternary ammonium compounds, hydrogen dioxide, hydrogen peroxide and peroxyacetic acid, etc.).

If a surface comes into contact with live plants or roots at any time, a chemical which is non-toxic to higher plants should be used. Any copper fungicide used on plants for disease control (copper hydroxide, copper oleate, cuprous oxide, copper oxychloride sulphate, copper oxychloride, copper 8-quinolinolate), may be used at the same concentration.

These compounds are available in a variety of formulations. You should always read and carefully follow label recommendations concerning dosage rates. Make sure to consult with your country’s agriculture office if more information is required.

Substrate type

healthy roots as an example of Prevented Algae in the GreenhouseTo prevent algae growth on the pot´s surface, it is crucial to use the right type of substrate for the crop and conditions in place (pot size and climate). Algae growth on top of the pot occurs if the substrate’s drainage time is too slow and water stays too long on the surface. It causes algae build-up and formation of crust.

It is then important to use a substrate with increased drainage properties and high porosity e.g. Kekkiä FLOW.


As a result of these actions you should see a decrease in algae growth in your greenhouse!


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