Fact Sheets

Greenhouse Management / Engineering

Cleaning and Disinfecting the Greenhouse

First Steps to a Clean Greenhouse

If you have had re-occurring problems with diseases such as Pythium root rot or insects such as fungus gnats, perhaps your greenhouse and potting areas need a good cleaning. Over the course of growing a crop, infectious microbes accumulate and algae flourish on moist surfaces harboring fungus gnats and shore flies.

Attention to greenhouse sanitation and disinfecting are steps that growers can take now to prepare their greenhouses for the spring growing season. Some growers wait until the week before opening a greenhouse before cleaning debris from the previous growing season. It is better to clean as early as possible to eliminate over-wintering sites for pests to reduce their populations prior to the spring growing season. Pests are much easier to prevent than to cure.

Begin by thoroughly cleaning the floor of soil, organic matter and weeds. Install physical weed mat barriers if floors are bare dirt or gravel and repair existing ones. Weed barriers not only prevent weeds, but also make it easier to manage algae. Avoid using stone on top of the weed mat that will trap soil and moisture, creating an ideal environment for weeds, diseases, insects and algae.

Benches, preferably made of wire, should be disinfected and pots, flats and trays should be new or disinfected. Bench tops and work tables should be made of a non-porous surface such as a laminate that can be easily disinfected. Avoid using bare wood for these tasks. Hose ends should always be kept off the floor and growing media kept in a clean area and covered. Avoid holding plant material and accumulating contaminated pots, media or debris in the media mixing area. Next, disinfect the growing and plant handling areas, and irrigation system.

Benefits to Disinfecting the Greenhouse

Many pathogens can be managed to some degree, by the use of disinfectants. For example, dust particles from fallen growing medium or pots can contain bacteria or fungi such as Rhizoctonia or Pythium . Disinfectants will help control these pathogens. In addition to plant pathogens, some disinfectants are also labeled for managing algae which is a breeding ground for fungus gnats and shore flies.

Although disinfecting should be done routinely, timing does not always permit this extra effort. Take the opportunity to thoroughly clean greenhouses between crop cycles when greenhouses are totally empty.

Managing Algae

Algae are a diverse grouping of plants that occur in a wide range of environments. Algae growth on walks, water pipes, equipment, greenhouse coverings, on or under benches and in pots is an ongoing problem for growers. Algae form an impermeable layer on the media surface that prevents wetting of the media and can clog irrigation and misting lines, and emitters. It is a food source for insect pests like shore flies, and causes slippery walkways that can be a liability risk for workers and customers. Recent studies have shown that algae are brought into the greenhouse through water supplies and from peat in the growing media. Once in a warm, moist environment with fertilizer, the algae flourish.

Proper water management and fertilizing can help to slow algae growth. Avoid over-watering slow-growing plants and especially crops early in the production cycle. Allow the surface of the media to dry out between watering.

Avoid excessive fertilizer runoff and puddling water on floors, benches, and greenhouse surfaces. The greenhouse floor should be level and drain properly to prevent the pooling of water prior to installing a physical weed mat barrier.

Irrigation Systems

Non-recirculating irrigation systems can be cleaned using a new product, chlorine dioxide (SelectrocideT). It is labeled for disinfecting irrigation lines and greenhouse surfaces. To thoroughly clean irrigation systems, the company recommends using two consecutive overnight treatments, and then flushing the system with clear water, making sure to discard the dislodged debris. SelectrocideT can also be used as a continuous ultra-low dose treatment.

Copper ionization is also being used to control algae and pathogens in irrigation water. Growers in Europe have used the technology for several years and growers here are now beginning to use it. The Aqua-Hort® system consists of a control box, an electric treatment pipe and four sets of electrodes for the release of copper. The system automatically controls the output of copper ions according to the flow rate and electrical conductivity of the water. Copper ions are released within normal fertilizer standards during irrigation.

Greenhouse Benches

If possible, use benches made of wire that can be easily disinfected. Wood benches can be a source for root rot diseases and insect infestations. Algae tend to grow on the surface of the wood creating an ideal environment for fungus gnats and shore flies, and plant pathogens can grow within the wood. Plants rooting through containers into the wood will develop root rot if conditions are favorable for pathogen activity. Disinfect benches between crop cycles with one of the labeled products listed below. Keep in mind that the following disinfectants are not protectants. They may eradicate certain pathogens, but will have little residual activity.

Disinfectants for Greenhouses

There are several different types of disinfectants that are currently used in the greenhouse for plant pathogen and algae control. They are quaternary ammonium compounds (Green-Shield®, Physan 20®, and Triathlon®), hydrogen dioxide (ZeroTol®, Oxidate®), chlorine dioxide (SelectrocideT) and chlorine bleach. Alcohol, although not used as a general disinfectant is mentioned here because it is used by growers to disinfect propagation tools. All these products have different properties. If possible, disinfectants should be used on a routine basis both as part of a pre-crop clean-up program and during the cropping cycle.

Organic growers have limited options for disinfectants. Oxidate® is the only material mentioned above that is currently listed by the Organic Material Review Institutes (OMRI), see www.omri.org . Ethyl or isopropyl alcohol is also allowed under the organic standards. Organic growers should always check with their certifying organization before using any material new to their farming practices.

Quaternary ammonium chloride salts (Green-Shield®, Physan 20® and Triathlon®). Q-salt products, commonly used by growers are quite stable and work well when used according to label instructions. Q-salts are labeled for fungal, bacterial and viral plant pathogens, and algae. They can be applied to floors, walls, benches, tools, pots and flats as disinfectants. Physan 20® is also labeled for use on seeds, cut flowers and plants. Carefully read and follow label instructions. Recommendations may vary according to the intended use of the product. For example, the Green-Shield® label recommends that objects to be sanitized should be soaked for 10 minutes, and walkways for an hour or more. Instructions recommend that surfaces be air-dried after treatment except for cutting tools. The label recommends soaking cutting tools for 10 minutes before use, then using the wet tool on plants. One way to do this is by having two cutting tools, one pair to use while the other is soaking.

Q-salts are not protectants. They may eradicate certain pathogens, but will have little residual activity. Contact with any type of organic matter will inactivate them. Therefore, pre-clean objects to dislodge organic matter prior to application. Because it is difficult to tell when they become inactive, prepare fresh solutions frequently (twice a day if in constant use). The products tend to foam a bit when they are active. When foaming stops, it is a sign they are no longer effective. No rinsing with water is needed.

Hydrogen Dioxide (ZeroTol®, OxiDate®) Hydrogen dioxide kills bacteria, fungus, algae and their spores immediately on contact. It is labeled as a disinfectant for use on greenhouse surfaces, equipment, benches, pots, trays and tools, and for use on plants. Label recommendations state that all surfaces should be wetted thoroughly before treatment. Several precautions are noted. Hydrogen dioxide has strong oxidizing action and should not be mixed with any other pesticides or fertilizers. When applied directly to plants, phytotoxicity may occur for some crops, especially if applied above labeled rates or if plants are under stress. Hydrogen dioxide can be applied through an irrigation system. As a concentrate it is corrosive and causes eye and skin damage or irritation. Carefully read and follow label precautions. Note that OxiDate® is the only product of the products mentioned in this article listed on the Organic Material Review Institutes (OMRI) website, in the newly-listed products section.

Sodium Carbonate Peroxyhydrate (GreenClean Granular Algaecide®, TerraCyte®) Both algaecides are granular and activated with water. Upon activation, sodium carbonate peroxhydrate breaks down into sodium carbonate and hydrogen peroxide. GreenClean is labeled for managing algae in any non-food water or surfaces. TerraCyte in addition to being an algaecide is labeled to control moss, liverworts, slime, molds and their spores and is labeled for use on plants. Non-target plants suffer contact burn if undiluted granules are accidentally spilled on them.

Chlorine Dioxide (SelectrocideT) Chlorine dioxide is a new disinfectant in the horticulture industry for controlling algae, bacteria, viruses, fungi and other microbial pests on greenhouse surfaces and in greenhouse irrigation systems. Currently it is labeled to be used to clean out irrigation lines with a periodic treatment at a moderate dose or to keep lines from becoming re-contaminated by treating irrigation water flowing through the system with a continuous ultra-low dose. Research continues to be conducted on this product to expand its use in the industry.

Chlorine bleach. There are more stable products than bleach to use for disinfecting greenhouse surfaces. Chlorine bleach may be used for pots or flats, but is not approved for application to walls, benches or flooring. When used properly, chlorine is an effective disinfectant and has been used for many years by growers. A solution of chlorine bleach and water is short-lived and the half-life (time required for 50 percent reduction in strength) of a chlorine solution is only two hours. After two hours, only one-half as much chlorine is present as was present at first. After four hours, only one-fourth is there, and so on. To ensure the effectiveness of chlorine solutions, it should be prepared fresh just before each use. The concentration normally used is one part of household bleach (5.25 percent sodium hypochlorite) to nine parts of water, giving a final strength of 0.5 percent. Chlorine is corrosive. Repeated use of chlorine solutions may be harmful to plastics or metals. Objects to be sanitized with chlorine require 30 minutes of soaking and then should be rinsed with water. Some would say that rinsing is not necessary. Bleach should be used in a well-ventilated area. It should also be noted that bleach is phytotoxic to some plants, such as poinsettias.

Alcohol (70 percent) is a very effective sanitizer that acts almost immediately upon contact. It is not practical as a soaking material because of its flammability. However, it can be used as a dip or swipe treatment on knives or cutting tools. No rinsing with water is needed.

Disinfectants should be used on a routine basis both as part of a pre-crop clean up program and during the cropping cycle.

This information is supplied with the understanding that no discrimination is intended and no endorsement implied. Due to constantly changing regulations, we assume no liability for suggestions. If any information in this article is inconsistent with the label, follow the label.

References

Aqua-Hort® Copper Ionization of Irrigation Water. http://www.aqua-perl.dk/

Bennett, K.C, Sanderson, J.P., Weston, L.A. , Weiler, T.C. 2006 Cornell Guide for the Integrated Management of Greenhouse Floral Crops.

Billings , J. (Ed.) How can algae buildup on porous surfaces in greenhouses be removed? 2000. Greenhouse Business 6(8):41-42.

Lopes, P. and L. Stack (Ed.) 2005-2006. New England Greenhouse Floriculture Guide: A Management Guide for Insects, Diseases, Weeds and Growth Regulators. New England Floriculture Inc.

Konjoian, P. SelectrocideT chlorine dioxide as a New Product for the Control of Algae and Other Microbial Pests in Greenhouse Irrigation Systems. KFES Newsnotes, Spring 05-1

OMRI Brand Name Products List. 2005. Organic Materials Review Institute, Eugene , OR . http://www.omri.org/

Powell, C. 1993. Disinfectants or Sanitizers and Their Uses in Greenhouses. Tips on the Use of Pesticides on Floriculture Crops. Ohio Florists' Association, Columbus , Ohio pp 78,79.

Pundt, L. 2000. Algae Management in the Greenhouse. Yankee Grower . 2(2):5-6.

Rahilly, C. and Thomas P. 1993. Defeat Algae by Limiting Elements Vital to Survival. Greenhouse Product News pp 14,18, July issue.

Vandiver, V.V., Jr. 2002. Biology and Control of Algae. University of Florida Extension. Fact Sheet SS-AGR-34. http://edis.ifas.ufl.edu/AG142

Pesticide Labels:
http://www.biosafesystems.com/labels.html (ZeroTol®, Oxidate®, GreenClean®, Terracyte®)
http://www.wmmg.com/products/labelmsds.asp (Green-Shield®)
http://www.physan.com/PAGES/home.html (Physan 20®)
http://www.ohp.com/Products/triathlon.php (Triathlon®)
http://www.griffins.com/tech_service/pdfs/selectrocide_label.pdf (SelectrocideT)

Updated 2007

Prepared by:
Tina Smith, Extension Educator
Floriculture Program
Dept. Plant, Soils and Insect Sciences
University of Massachusetts, Amherst