It is not uncommon for most growers to find “green slime” growing on the surface of the growing medium. This green slime is predominately composed of blue-green and green algae. In order for algae to grow, they need water, light and mineral nutrients. These requirements are easily met in the greenhouse, especially in mist beds or crops that are grown cold.
Concerns about algae
Algae growth starts as a slight greening of the medium surface. Over time the green layer of algae grows and becomes thicker to the point where is looks black. This gives the illusion that the growing medium surface is wet. In response a grower holds back the watering, the algae dries up, rolls up from the edge of the container and forms a water impermeable crust. This crust may need to be broken up in order to get water into the growing medium.
Algae do not harm plants, but it attracts fungus gnat and shore flies and slows gas exchange into and out of the growing medium. Unfortunately, fungus gnats and shore flies not only feed on the algae and other fungal growth in the growing medium, but they can feed on plant roots and potentially transfer plant pathogens through their mouth parts.
So where does algae come from?
The cells and spores of algae are found everywhere. Irrigation water is the most common source, especially if it comes from a pond or shallow well. Water lines also can have a significant biofilm build up on the inside walls of the pipes, which is mostly composed of algae. Dust originating from the soil that gets blown into the greenhouse can contain algae spores. Some algae can be found in the growing medium too.
Controlling algae on the growing medium surface
To control algae growth on the growing medium surface or in other areas in the greenhouse, start by looking for potential sources of algae. If it is in the water lines, they will need to be cleaned out and then kept clean using one of the following: chlorine dioxide, copper ionization, hydrogen dioxide, etc. Water coming from pond or shallow wells may have large algae loads in which the water may need to be treated with the same materials.
Cultural adjustments can also help minimize algae growth. Algae require a wet environment to survive so simply holding water back and allowing the growing medium surface to dry out between waterings will reduce algae. Consider increasing air flow in the greenhouse to encourage faster dry down of the growing medium surface. Another suggestion is to use a high porosity or bark-based growing medium with lower water-holding capacities, as they will naturally dry out more rapidly. Unfortunately, when the weather is cloudy and cold or crops are grown cold, it may be very difficult to dry out the growing medium, so chemical controls, such as hydrogen dioxide or Physan 20, may be needed.