Keep it clean

Greenhouse sanitation aims to prevent disease and insect outbreaks. Photo by Peg McMahon, Ohio State University

Why sanitize? Growers are often concerned about the costs of managing pests and pathogens. Proper sanitation is a key component in reducing those costs and improving plant quality.

Investing time and money in greenhouse sanitation is much less expensive than paying for the repeated pesticide applications and crop losses associated with unsanitary conditions. Like routine car maintenance, you may not notice the benefits immediately, but preventive measures save you money and time in the long run. Follow these practices to maintain a clean and profitable operation.

Think clean
Effective greenhouse sanitation requires growers and greenhouse managers to develop a “clean” mindset. Insects and pathogens are ubiquitous and may easily enter greenhouses.

It is critical to properly train anyone using the greenhouse to recognize pest and pathogen problems, and to understand what can be done to reduce their presence in the greenhouse.

It also pays to recognize the major sources of greenhouse pests, including floors and benches; weeds; tools, containers, and other equipment; trash; clothing; and new plants that have been introduced into the greenhouse.

Floors and benches
The greenhouse floor is a major source of pests and pathogens. Many pathogens can survive in soil and residue for extended periods. In the greenhouse, many pathogens can spread to container-grown plants through splashed water, they come in contact with plant roots or media or when watering nozzles are placed on the floor.

When possible, make sure greenhouse floors do not contain exposed soil or plant debris: concrete or gravel floors are preferable. At a minimum, cover floors with plastic sheeting or landscape cloth and change such coverings frequently. Some growers utilize the soil of the greenhouse floor to grow plants to maturity. In these cases, the grower should focus on preventing pests and pathogens from entering the greenhouse; removing weeds from the greenhouse floor and area immediately outside of the greenhouse; and properly sanitizing tools, boots, and other equipment used in the greenhouse. These areas all are discussed in the following sections.

Routinely sweep and wash greenhouse floors to remove soil and debris. Conduct transplanting and seeding in a location separate from the greenhouse. Discard any plants or growing substrate that falls on the floor. There is no “three-second-rule” in the greenhouse. Hang up watering hoses so they do not contact residue, soil, or potting media on the floor or benches.

Using raised benches is an easy way to reduce contamination from the greenhouse floor. Although they are more expensive, metal benches are easier to clean than wooden benches. Wood surfaces are difficult to sanitize and can harbor pathogens and problematic organisms such as algae.

Clean benches regularly to remove substrate or soil residue and potential contaminants. Water alone is not sufficient. There are many chemicals that can be used to clean benches and other greenhouse areas. Make sure all benches have been washed of all dirt and debris prior to using sanitation chemicals because organic matter may inactivate the compounds.

Chlorine bleach is effective for bench top sanitation (a 10 percent commercial solution allowed to sit for five minutes — most commercial bleach contains 6 percent active ingredient, so the sanitizing solution is equivalent to 0.6 percent active ingredient). More concentrated solutions are more effective at killing pathogens, but require several rinses to adequately remove residual bleach. Use chlorine bleach with caution: it is highly volatile, phytotoxic, can irritate mucus membranes and lungs, and can corrode metal. Products that contain quaternary ammonium are less volatile and more stable than commercial bleach, and work well when used according to label directions. However, quaternary ammonium products do not penetrate wood well, so do not use them on wood benches for sanitation.

Products that contain hydrogen dioxide effectively kill bacteria, fungi, and algae on contact but do not penetrate wood well. Chlorine dioxide is another product that effectively sanitizes surfaces. Chlorine dioxide is a gas, so it can penetrate and access greenhouse surfaces better than liquid products. Always use the proper protective gear and follow product labels when applying any sanitation chemical.

Weeds
Another source of pathogens and pests are weeds growing inside and immediately outside the greenhouse. Hand weeding may be sufficient to remove some weedy plants, but many weeds grow rapidly from seed, so they are difficult to control by this method alone.

Chemical applications are often the easiest and most effective means to control weeds. Products that contain glyphosate, diquat, or paraquat can be used for weed control immediately outside the greenhouse, but be certain to close the greenhouse vents and doors when applying these products in the vicinity to prevent damaging the plants inside.

Tools, containers and other equipment
Regularly clean tools and equipment with soap and water, and sanitize them often. Ethanol (70 percent) is very effective for tool sanitation. Make sure to sanitize cutting tools after pruning or taking cuttings of plants.

Many important plant pathogens such as viruses or bacteria are easily spread between plants. Sanitizing greenhouse tools effectively reduces the spread of these diseases in the greenhouse.

If possible, discard pots and flats after use. However, if it is necessary to recycle these items, take care to wash them thoroughly with soap and water, and to disinfest them with a product labeled for that purpose. If you have had issues in the past with diseases such as Fusarium, Pythium or Rhizoctonia, you should use new pots. Never store growing substrate or soil in the greenhouse.

Trash
Trash bins can harbor pathogens and pests, so always keep them out of the greenhouse. It is particularly important to remove discarded soil and plant material from the greenhouse for this reason. At a minimum, seal trash bins with a lid and empty them daily. Thoroughly clean bins after emptying them.

Clothing
Take care that everyone entering the greenhouse removes soil and debris from their shoes and boots. This can be another source of pathogens and insect pests. Place brushes and hoses outside the greenhouse entrance for this purpose.

You can also purchase special disinfectant mats (foot baths) that sanitize shoes and boots upon entering the greenhouse. If you work in several greenhouses during the day, work in the problem (dirty) greenhouse last to avoid infesting other houses.

New plants
New plants are excellent avenues for pests and diseases to enter your greenhouse. Turn away shipments of plants that appear to have insects or are diseased.

Train all greenhouse personnel to recognize disease and injury symptoms, and to discard any suspect plants. Send suspicious plants or tissues to a diagnostic lab (such as the Purdue University Plant and Pest Diagnostic Laboratory) for testing. Insect pests can often be managed chemically. However, you can help curtail outbreaks by preventing insects from entering the greenhouse. For example, you can use insect screens on vents to keep insects out. Sticky cards are useful tools that help assess the presence of insects in the greenhouse.

Routine cleaning
It is best to thoroughly clean and sanitize your greenhouse between crops. You should remove all plants, soil, and items from the greenhouse prior to sanitation.

Power washers can remove dirt and algae, and sanitizing agents can clean walls, benches, tables, floors and other surfaces.

Once clean, close all vents and doors and allow the greenhouse to sit for a day or two. Several hours of hot greenhouse temperatures (around 110°F) can effectively reduce pest populations and may adversely impact some pathogens. However, excessively hot greenhouse temperatures can ruin certain types of shade curtains and plastic coverings, voiding their warranties. Extreme temperatures may also damage lighting fixtures or structural components of the greenhouse.

These guidelines require time and attention to detail. However, the benefits of following these practices will be evident when used effectively and consistently.

A clean greenhouse leads to healthy plants, and healthy plants lead to happy growers.

Nathan M. Kleczewski and Daniel S. Egel are plant pathologists at Purdue University; nkleczew@purdue.edu; egel@purdue.edu.