Sanitizing Greenhouses

Published: October 1, 2013

Greenhouse growers who year-after-year find themselves battling root rots such as Pythium and insect pests such as fungus gnats might not be doing an adequate job of greenhouse sanitation. Prevention is by far the best cure for both disease and insect problems, and proper sanitation is the first step in prevention. Greenhouse sanitation is a two-step process involving both cleaning and disinfecting. This is especially important in the propagation area to prevent the transfer of infested plant material to an otherwise clean greenhouse.

Cleaning involves physically removing weeds and all living plant material from the greenhouse. This step cannot be overemphasized since living plant material is an ideal refuge for problematic pests such as aphids and mites. Plant debris, spilled potting medium, etc. also should be eliminated prior to disinfecting a greenhouse. Shop vacuums are useful in removing debris from concrete and covered floors. Additionally, there are cleaning agents on the market (e.g. STRIP-ITTM) that can make the job of algae, grime and fertilizer residue removal easier. Using a power washer with soap and water is another possibility. Care must be taken, however, since soap can leave residues that can inactivate certain disinfectants (i.e. the quaternary salts) used later in the sanitation process.

Owners of greenhouses with floors of bare soil or gravel might consider the installation of weed barriers to make cleaning floors easier. In addition to prevention weed growth, weed barriers make algae management easier to accomplish. Weed barrier should not be used below a gravel greenhouse floor, it tends to trap soil and moisture, creating an ideal environment for weeds, diseases, insects and algae.

Once the greenhouse is clean it is time to disinfect it. To a certain degree, a number of pathogens (e.g. Pythium or Rhizoctonia) can be managed by the use of disinfectants. Dust particles or other organic residue in the greenhouse might contain bacteria or fungus inoculum which disinfectants can eliminate. In addition to plant pathogens, some disinfectants are also labeled for managing algae. The latter tends to promote populations of fungus gnats and shore flies to develop.

There are four disinfectants commonly used in greenhouses. When possible/practical, rotation between these four is recommended. The four disinfectants include:

• Alcohol (70% isopropyl) is an effective disinfectant that kills microbes on contact. Since it is volatile, its effect is not long-lived. Alcohol is best suited for disinfecting propagation equipment such as knives or shears by dipping or wiping.
• Bleach (sodium hypochlorite) probably is the most widely used disinfectant in greenhouse management. Additionally, it is the least expensive product to use. Bleach should be used within two hours of mixing since its active ingredient (chlorine) is quite volatile. Typical household bleach contains 5.25%chlorine. A mixture of one part of bleach to nine parts of water produces a solution with 0.5% chlorine which is quite satisfactory for killing microbes associated with greenhouses and related equipment. When disinfecting pots or flats with bleach, first wash out all excess growing medium since organic matter tends to neutralize the chlorine in the bleach solution. Bleach should be used in a well-ventilated area for personal safety. Also, it should be noted that bleach can be phytotoxic to some plants, (e.g. poinsettia).
• Hydrogen dioxide is available under a number of brand names such as ZeroTol®, OxiDate® and SaniDate®. Hydrogen dioxide is a potent oxidizing agent that kills a wide range of microbes and their inoculum on contact. It is very effective in sanitizing benches, pots, tools, etc. as long as the solution used is still active. Kits are available which allow growers to test the solution to determine it has lost its potency. At such time more hydrogen dioxide must be added. Note: Special care should be taken when using hydrogen dioxide; it is extremely harmful to the eye.
• Quaternary ammonium chloride salt (e.g. Physan 20®, Green Shield® and Klean GrowTM) is an effective disinfectant that, unlike bleach and hydrogen dioxide, does not lose potency as it is being used. As with bleach, it is important to remove organic matter from the surfaces of items (pots, flats, etc.) being disinfected. Physan 20® also is labeled for use on seeds, cut flowers and plants. Carefully read and follow label instructions.

In summary, effective greenhouse sanitation requires growers and greenhouse managers to develop a “think clean” mindset. Proper greenhouse sanitation does requires time and attention to detail. However, the benefits of following good sanitation practices will be evident when used effectively and consistently. A clean greenhouse leads to healthy plants, and healthy plants lead to happy growers and greater profits.