Hydroponic growing has become a standard production method over the last 80 years. Its rise in popularity doubles and triples annually. The major crops grown hydroponically in the U.S. are tomatoes, peppers, lettuce, cucumber, herbs, strawberries and ornamentals. Hydroponics gives you an extended season and a boost in yields, leading to an increase in profitability. Since both the water and nutrients are recycled, this is an ecologically responsible way to produce food. Among the many benefits lurk potential pitfalls. There can be a large capital expense associated with the initial set-up. The hydroponic grower needs to be nutrient savvy and have a technical understanding of the system. These challenges are further compounded by the increase in disease pressure and algae through the recirculating water. Prevention and sanitation are the best defense a hydroponic grower has against potential production snags.
Sanitizing a hydroponic system vs. a conventional greenhouse
Since hydroponics uses water as the medium, the potential for contamination from water-borne pathogens increases. On the upside, foliar disease usually decreases. It’s difficult to prevent and decontaminate a hydroponic system. The most common pathogens are Pythium and Fusarium. These notoriously persistent diseases readily travel through water. They produce an abundance of zoospores, as well as chlamydospores or oospores allowing for long-term survival. Many times, these reproductive structures will colonize dead plant debris, which protects them from attempts to treat the water. The level of prevention and sanitation in a hydroponic greenhouse needs to balance the increase in disease pressure.
Algae is a nuisance in hydroponic growing that plagues the system throughout production. They thrive in the oxygenated, fertilized water. Not only is it unsightly and messy, but it harbors pests and depletes the dissolved oxygen levels in the water. This uninterrupted flow of liquid nutrient feeds algae and can create an impenetrable layer on greenhouse surfaces such as walkways, plastic, end walls, benches and on the inside of irrigation/mist lines. These conditions can be hazardous for workers and influence crop quality and yield. Algae grow in layers, so it’s important to be diligent when cleaning. Sometimes physically scrubbing is best to ensure that products are penetrating each layer of algae. A good way to decrease algae build-up and spread is to maintain your sanitation practices throughout the growing cycle, not just at the end of each crop cycle. Standardize practices, such as spraying racks, empty benches, walkways, and sanitizing pots and trays to maintain a clean environment during the production cycle. Be sure to check labels and make sure the products can be used while a crop is in production and doesn’t have to be rinsed.
In a hydroponic system, all contact surfaces and equipment that encounter flood water or plant material must be washed first and then disinfected. Washing with detergent THEN rinsing with water will emulsify organic matter and rinse it away. Plain water can’t do that. The photo of a steamed plug tray gives us a visual of what can be left behind when only rinsing is used. The tray in this photo was rinsed and then steamed, but much of the organic matter still remains on the surface. This resulted in an infection in the next crop. Incorporating the washing step will eliminate what water cannot. This is true of all surfaces, including water tanks, floors, benches and the irrigation system. Dead plant tissue harbors pathogens and decreases the efficacy of sanitizing agents, whether they’re added to the irrigation water or used in between crop cycles.
Keeping biofilm at bay
An important objective with cleaning and sanitizing is to keep biofilm from creating an irreversible layer on surfaces. This is a gluey mass of bacteria, algae and other free-floating microorganisms that form into a colony. The mass is protected with large molecules that are like the polysaccharide layer found in algae. Part of its survival strategy is creating a protective barrier using this slimy matrix. It’s tenacious and difficult to completely eradicate. We’ve heard of biofilm causing trouble in irrigation lines, but when surfaces aren’t thoroughly cleaned, bacteria will continue to build up. Biofilm adheres to materials, such as stainless steel, plastic, copper, rubber and lead.
Disinfection seals the pathogen’s fate by physically destroying it. There are many sanitation products available. These include activated peroxides (products with peracetic acid/hydrogen peroxide combination such as SaniDate® 5.0), bleach, chlorine dioxide, quaternary ammonium, and ozone and heat/UV. Most of these treatments are oxidizers, which increases the need to eliminate as much organic matter as possible. Some are more powerful than others because of their stability in the environment. Activated peroxides are stable, high-level disinfectants. They’re more resistant to environmental factors, making them less prone to quick degradation.
It’s worth doing your homework on sanitizers prior to making a choice. Important considerations are the stability/power of the product, PPE requirements, REI restrictions and whether a rinse is needed after the application. Irrigation lines, whether underground, soil level or overhead have a literal hidden dark side … in the line itself. Water and nutrient make a very conducive atmosphere for many organisms. They’re fed, sheltered and protected, which allows them to take root, grow and reproduce. This is biofilm and it moves when pieces break off and travel with the flow of water.
Some common problems that can be associated with irrigation line cleanliness are water-borne pathogens such as Pythium and Fusarium. They can easily become attached and thrive in biofilm. Pieces will eventually get swept up, move downstream and are then deposited everywhere water splashes. Clogged emitters are indicative of a severe problem. In order to eliminate this, the irrigation lines should be as much a part of the clean-up as the greenhouse surfaces themselves.
If you regularly shock and flush your irrigation lines, consider maintaining them by treating your irrigation water throughout the season. This is a very effective way to keep free-floating micro-organisms from attaching and growing. There are several options for treating irrigation water on a continuous basis. Take time when making your choice to consider your facility, budget and what your goal is.
Written by Sarah Brackman. Sarah is Technical Sales for BioSafe Systems. She can be reached at firstname.lastname@example.org.