Published: July 27, 2017

Benjamin Franklin once famously observed: "An ounce of prevention is worth a pound of cure". In actuality, Franklin was talking about fire prevention in the city of Philadelphia. However, he just as easily could have been talking about pest control in greenhouse and high tunnel crops. Prevention always is the easiest cure for pests; in certain cases it's the only cure.

Greenhouse and high tunnel growers who find themselves continually battling certain diseases and insect pests might not be doing an adequate job of greenhouse sanitation. Prevention is one of the basic IPM principles for both disease and insect management, and proper sanitation is the first step in prevention. It is advisable to sanitize a greenhouse as early as possible following the growing season opposed to waiting just before planting the following spring.

Cleaning is the first step in sanitizing and involves physically removing weeds and all living plant material from the greenhouse. The importance of this step cannot be overemphasized since living plant material is an ideal refuge for pests such as aphids, thrips and mites. Plant debris, spilled potting media, 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-IT™) 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 bare soil or gravel as a floor might consider the installation of weed barriers. In addition to preventing weed growth, weed barriers make algae management easier. Weed barrier should not be used below a gravel greenhouse floor since it tends to trap soil and moisture, creating an ideal environment for weeds, diseases, insects and algae.

After thoroughly cleaning a greenhouse, it should be disinfected. To a certain degree, a number of pathogens (e.g. Pythium or Rhizoctonia) can be managed effectively 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:

• Isopropyl alcohol (70%) 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.
• Sodium hypochlorite (household bleach) probably is the most widely used disinfectant in greenhouse management. Additionally, it is the least expensive. 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 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 since it is extremely harmful to the eye.
• Quaternary ammonium salt (e.g. Physan 20®, Green Shield® and Klean Grow™) 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.

For those who produce vegetables in greenhouses or high tunnels using soil culture, sanitation is equally important. As mentioned above, remove all of the debris from the previous crop as possible. Be mindful of weeds around the perimeter of the greenhouse or high tunnel, since weeds harbor insect pests that can serve as vectors for certain plant diseases. Deep cultivation should follow debris and weed removal to bury any plant residue missed deep in the soil.

Good sanitation practices provide growers with a longer pest free window, reducing the need for pesticide and reducing labor and expenses associated with pesticide application. The following graph illustrates how proper sanitation can reduce the need to apply pesticides:

Credit: Scott DiLoreto, Penn State Univ.

In summary, effective greenhouse and high tunnel management requires growers and managers to develop a "think clean" mindset. Proper 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 production facility leads to healthy plants, and healthy plants lead to happy growers and greater profits