Start clean

Reoccurring problems with diseases such as Pythium root rot or insects such as fungus gnats and shore flies may indicate that your greenhouse and potting areas need a good cleaning. Clean and disinfect when greenhouses are empty to eliminate overwintering sites for pests and reduce their populations prior to the growing season. Greenhouse pests overwinter in weeds and protected areas in unheated greenhouses. Once cleaned and disinfected, if possible, allow the greenhouse a fallow period of four weeks to reduce pest populations.

1. Cleaning
Cleaning involves physically removing weeds, debris and dirt, and is the first step prior to disinfecting a greenhouse and equipment. Dirt and organic residues from plants and growing media reduce the effectiveness of disinfectants.

Begin at the top and work your way down. Sweep down walls and internal structures and clean the floor of soil, organic matter and weeds. Install physical weed mat barriers if floors are bare dirt or gravel, and repair existing mats. These prevent weeds and make it easier to manage algae. Avoid using stone on top of them, which will trap soil and moisture, creating an ideal environment for weeds, diseases, insects and algae.

2. Disinfectants
After cleaning, disinfect surfaces. Disinfectants are used to manage many pathogens such as Rhizoctonia or Pythium. Disinfectants are sanitizers and may eradicate certain (not all), pathogens with little residual activity. Most disinfectants are also labeled for algae, which is a breeding ground for fungus gnats and shore flies.

There are several types of disinfectants used in the greenhouse including those containing quaternary ammonium compounds, hydrogen dioxide plus peroxyacetic acid, hydrogen peroxide, peroxyacetic acid plus octanoic acid and sodium carbonate peroxydrate. While a 10 percent solution of household chlorine bleach is often used to disinfect pots or flats (not structures), it loses half its strength in just two hours and can be phytotoxic to plants.

Organic disinfectants include products with hydrogen peroxide plus peroxyacetic acid and hydrogen dioxide. Ethyl or isopropyl alcohol is also allowed.

3. Preventing disease contamination
Disinfect benches, preferably made of wire. Pots, flats and trays should be new or disinfected. Wood benches can be a source for root rot diseases and insect infestations. Algae growing on wood surfaces harbor fungus gnats and shore flies. Plant pathogens such as Pythium can grow within the wood and plants rooting into the wood can become infected.

Disinfect potting tables preferably made of a non-porous surface such as a laminate.

• Provide supports throughout the greenhouse and hang hose nozzles. Keep all containers and hose nozzles off the floor to prevent contamination with pathogens.
• Keep growing media covered and in a clean area. 
• Avoid carrying over plant material.
• Avoid accumulating dirty pots, old growing media or plant debris in the media mixing area. Keep trash bins covered.
• Use horticultural oil on vegetation/weeds outside, around the greenhouse perimeter to smother overwintering pests such as aphids.

4. Monitor overwintered pests
Before starting a new crop in an empty greenhouse, place yellow sticky cards throughout the greenhouse and on the floor after watering benches and the floor to detect overwintered pests such as thrips (emerging from pupae), fungus gnats and whiteflies. Close up the greenhouse and turn on the heat to break dormancy. Place a few sticky traps on the outside perimeter of the greenhouse, especially under vents to detect pests. Check weeds around the greenhouse perimeter for aphids. When weather permits, treat weeds with horticultural oil, around the greenhouse’s outside perimeter to smother overwintered pests such as aphids.

Smith is a frequent contributor to Greenhouse Management and an extension floriculture specialist for the UMass Extension Greenhouse Crops and Floriculture program.

Have a question? You can write Tina at tsmith@umext.umass.edu.