Note: This tutorial is a revision and update of a previous version from 2018.
Algae are the oldest and most widely distributed form of plant life. They range in size from large ocean kelps to microscopic single cells, so small that a single drop of pond water may contain thousands of these simple plants.
Algae is an informal term for a large, diverse group of photosynthetic organisms that are not necessarily closely related, making them polyphyletic.
Microscopic algae in large quantities form the familiar layer on ponds and ditches. Other forms of algae are commonly found in streams, on rocks, and in moist environments.
Some algae, such as microscopic oscillatory species, live in and on soil.
Certain algae live in association with other organisms:
- Lichens consist of algae living in union with fungi
- Algae can live on surfaces such as the shells of snails
Major Groups of Algae
- Green algae
- Blue-green algae
- Diatoms
- Red algae
- Brown algae
Algae are not closely related in an evolutionary sense, and their phylogeny remains to be clearly defined.
Some groups share characteristics with protozoa and fungi, making them difficult to distinguish without the presence of chloroplasts and photosynthesis.
Is Algae in My Garden a Problem?
Short answer: Yes and no.
A small amount of algae in your garden or grow room is generally harmless and easy to manage.
Do not panic if you notice algae forming on the surface of a moist pot or growing medium. This commonly occurs:
- In seedlings or clones with underdeveloped root systems
- In plants that are overwatered
- When the medium stays constantly moist
In many cases, this type of algae growth resolves naturally within one to two weeks as roots develop and begin to absorb more moisture.
When Does Algae Become a Problem?
Problems arise when algae is allowed to grow under ideal conditions:
- Constant moisture
- Continuous light exposure
- Suitable environmental temperatures
Under these conditions, algae can spread rapidly and begin to cause issues.
Potential Problems Caused by Algae
- Attracts pests and unwanted bacteria
- Creates poor root zone conditions
- Contributes to excess moisture in the growing medium
- Leads to algae build-up in hydroponic systems
Growing and Dealing with Algae
Finding algae on or around your plants—especially on top of your medium—is not a major issue if addressed early.
Simple Algae Control Solutions
- Lower moisture levels around affected areas
- Reduce or block light exposure to the medium
- Reset your growing medium
How to Reset Your Growing Medium
To reset your medium:
- Prepare a hydrogen peroxide solution:
- Mix 1ml hydrogen peroxide per 10L of water
- Thoroughly flush the medium
- Follow with a low-strength A/B base nutrient solution
- Allow the plant time to recover after sterilisation
