Not all green is good!

Algae can cause several problems for greenhouse growers. First, algae on the substrate surface can form a “crust," making it more challenging to evenly rewet substrate during irrigations. Additionally, algae can diminish the marketability of your crops because plants with algae covering the substrate surface can be less appealing to consumers. Finally, algae on greenhouse floors can be hazardous to employees, creating a slick surface that could cause someone to lose their footing. This month’s Production Pointers aims to help you understand algae and how to manage it in your greenhouse.

Where do algae come from? Algae have multiple modes of reproductive, including both sexual and asexual methods. Regardless of how they reproduce, algae must have a source from which it propagates. And there are several sources in and around the greenhouse. Algae populations that are already established in the greenhouse are the most common sources of propagules. Common locations for algae are places where water frequently puddles such as underneath benches, in walkways and underneath gutters. Algae also grows on equipment such as mist nozzles (Fig. 1), benches (especially wooden ones), and leaky hoses or spigots that frequently stay wet for extended periods of time. There is no single “silver bullet” that can completely eliminate algae. But there are several steps that, if taken together, can reduce algae in your facility.

The first key step to reducing algae growth is to put together (and stick to!) a plan for improving greenhouse sanitation. You must reduce algae elsewhere in your propagation house since any algae present in a greenhouse can act as a source for additional algae populations to establish and grow. The first place to check for algae are those areas mentioned earlier: anywhere that water may pool or surfaces that may stay wet. Also, check the tools and equipment that are used during propagation. It is good practice to routinely use a water nozzle with good pressure, or preferably, a power washer to clean locations and equipment where algae are growing.

Once these areas and equipment are clean, you can use an algaecide such as a hydrogen dioxide and peroxyacetic acid product (ZeroTol 2.0 or OxiDate 2.0, BioSafe Systems) or a quaternary ammonium product (Physan 20, Maril Products; Green-Shield, Whitmire Micro-Gen) to kill any remaining algae (Fig. 2). There are different application methods for these products, so be sure closely follow the label instructions. Be mindful of any plant material in the greenhouse that may come into contact with some of these chemical treatments. Phytotoxicity can be a problem, and one you’d rather avoid. If you are trying to control algae on the surface of substrate plants that are growing in, you need to be mindful of which algaecide you apply. Although many algaecides are not labeled for application to plant material, Physan 20 (a quaternatry ammonium product) is labeled for this use. Physan 20 can be applied as a substrate drench or can be sprayed on the substrate surface. As previously mentioned, be sure to always follow label directions.

There is no single “silver bullet” that can completely eliminate algae. But there are several steps that, if taken together, can reduce algae in your facility.

Seeing algae throughout your greenhouse can be discouraging, as it poses a safety hazard and creates a less-sanitary environment. If allowed to grow in your facility unchecked, algae can cause problems for your employees and crops. Although there isn’t a silver bullet that can reduce or eliminate algae, careful sanitation of the greenhouse and equipment, modifying the environment and cultural practices, and chemical control are all tools that can be used to help combat algae.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu