Garden & Greenhouse


Greenhouse Maintenance Tips

In order for a greenhouse hobbyist to get the most bang for his or her buck, they must implement some regular greenhouse maintenance. In many cases, the techniques and methods used to protect the plants from humidity- and temperature-related issues will also do wonders in extending the overall life of the greenhouse materials. Perhaps the most straightforward solution to both temperature/humidity-related issues and extending the life of the glazing and framing materials of a greenhouse is a well operating ventilation system. A good ventilation system will help boost a greenhouse’s productivity and longevity. Put another way, in order to maintain a productive greenhouse, the ventilation system should be high on a gardener’s priority list. A greenhouse’s temperature, humidity, and ambient CO2 levels are all regulated by the ventilation system. Greenhouse hobbyists have two choices when it comes to ventilation systems: passive or powered.

Passive Ventilation System

A passive ventilation system is a ventilation system with no mechanical components. In other words, no powered fans are used with a passive ventilation system. Instead, the greenhouse is ventilated via convection (hot air becoming less dense and rising) through the ridge vents. In addition to the ridge vents, a passive ventilation system needs intake vents to bring cool air into the greenhouse and displace the hot air as it rises. Intake vents are typically placed on the lower portion of the end walls of a greenhouse. In addition to convection, passive cooling techniques can also include the use of shade cloth to reduce heat created by solar radiation.

Powered Ventilation System

A powered ventilation system for a greenhouse is a ventilation system with a powered fan and intake vents. The size of the fan system will be determined by the greenhouse’s cubic feet. Powered ventilation systems are usually set up with a thermostatic controller that turns on the fan when the set-point temperature is reached. The intake vents are synced with the fan so they will open at the same time the fan is activated. In order to maintain the highest level of performance, the thermostat sensor should be placed at plant height. In addition to fans, powered ventilation systems may include other equipment, such as evaporative coolers, foggers, or wet wall systems.

Importance of Ventilation for the Prevention of Molds

A few examples of destructive fungi that are transmitted through the air are powdery mildew, black spot, and botrytis. When given the right conditions, these opportunist fungi quickly cause problems to otherwise healthy plants and have the capability of destroying an entire crop in a short period of time. Since the spores of these fungi are invisible to the naked eye, they are often overlooked by greenhouse horticulturists. If a greenhouse contracts one of these destructive fungi, it takes serious work to eradicate the issue entirely. As with most problems that can occur within a greenhouse, prevention is key to success.

By using an intake air filter, a grower can do away with many of the spores and pest insects that could potentially end up in a greenhouse. HEPA (high efficiency particulate air) filters offer some of the best protection against pest insects, diseases, and spores. Constructed from superior blends of synthetic fiber filter media, HEPA filters remove over 90% of mold spores and bacteria from the air. In addition to intake filters, stand-alone air purification systems can be installed within a greenhouse to further reduce the likelihood of molds and diseases.


A relatively easy, but very effective, preventative measure against diseases, pest insects, and molds is general cleanliness. Dead or rotting plant material, used soil, and stagnant water are all ideal breeding grounds for many molds and pathogens. In addition to keeping a tidy growing area, it is wise for a gardener to clean him- or herself before entering the greenhouse. Put another way, it is not a good idea for a horticulturist to enter his or her greenhouse right after visiting a plant nursery or doing yard work.

Sanitizing the Greenhouse

Deep cleaning is important greenhouse maintenance and should be done annually (at the very least). If a mold or disease issue arises, a gardener should thoroughly clean his or her greenhouse immediately. In addition to mold and disease treatment, a good sanitization will combat or prevent other issues, such as algae build up. A gardener should start by removing any plants that are currently in the greenhouse. These plants may require their own treatment before reentry. When it is empty, hose the entire greenhouse down with soapy water. In most cases, soapy water will be enough to prevent any algae from building up on the glazing or framing of the greenhouse. If algae have become an issue, a good pressure wash can easily rectify the problem. It is very important for owners of polycarbonate greenhouses to avoid using window cleaners of any kind. Window cleaners can actually damage the polycarbonate panels and make them less transparent.

After using soapy water and/or a pressure washer, it is a good idea to further sanitize the greenhouse with a sterilizing/sanitizing agent. A diluted bleach solution is a sanitizing agent commonly used by greenhouse horticulturists. However, the odor and fumes created by bleach can be overwhelming, especially when used in a confined area, like a greenhouse. Also, a diluted bleach solution works fine for sanitizing the greenhouse structure, but should not be used on the plants that will be coming back into the greenhouse. Even a diluted bleach solution would be harmful to the plants. Molds and pathogens often enter a greenhouse by hitching a ride on a plant or in the soil of a plant brought in from outside. A good solution is to use a greenhouse-specific cleaning agent that can be applied on both the greenhouse and ornamental plants. There are a few different cleaning agents available that can be used as an algaecide and a fungicide. These products can also be used directly on ornamental plants and their soil to kill any pathogenic fungi, bacteria, algae, or viruses.

Treatment for Molds

Molds, especially powdery mildew, are quite common in high humidity environments like a greenhouse. If powdery mildew is contracted, one treatment option for greenhouse horticulturists is sulfur burners. Sulfur burners are effective at controlling a number of molds and mildews. As the sulfur is vaporized, a “dusting” of sulfur residue is left on all of the leaves in a garden. This changes the pH level and creates an undesirable growing environment for the pathogens.

Another common mold to affect greenhouse crops is botrytis. Botrytis mainly affects tender tissues, such as flowers, fruits, and seedlings, but it can also enter the plant’s tissue through pruning scars or other distressed or wounded tissue. Lower, shaded sections of a plant are usually the first to show signs of a botrytis infection. The first symptom shown by a plant with a botrytis infection is a water-soaked, browned area. After the initial browning, a silvery-gray fuzzy mat will develop on or around the browned tissue. Sections of a plant that have botrytis need to be carefully removed immediately in order to avoid spreading it to other areas of the garden. If possible, bag the section of plant that is infected with botrytis before cutting it. This will reduce the possibility of spreading the spores as the area is disturbed. In fact, all sections that are infected with botrytis should be slowly and carefully removed to reduce spreading the fungus. Be sure to sterilize the pruning shears with rubbing alcohol after each cut and before they are used again in the garden. Once the infected sections of plant tissue have been removed, the rest of the garden can be treated with a biological fungicide.

Increasing the Longevity of the Greenhouse’s Glazing and Framing Materials

Greenhouse horticulturists spend a lot of time and money on their greenhouses. In order to get the most out of his or her investment, a greenhouse grower should do everything he or she can to extend the life of the greenhouse materials. By controlling excess humidity and temperatures with an effective ventilation system, a greenhouse grower automatically increases the life of the greenhouse materials. In fact, excess heat plays a big part in the degrading of poly-based greenhouse glazes.

Paint the Rafters White

A fairly easy way to help extend the life of a greenhouse’s glazing and framing materials is to paint the internal rafters and/or frame white or another reflective color. A reflective color can significantly reduce the temperature of the glazing material where it meets the rafters. Excess heat created by dark colored rafters will degrade the glazing material much more quickly than if the heat was reflected. A white or reflective interior will also make the entire greenhouse brighter.

Sand or Wrap the Rafters to Reduce Abrasion

Over time, the wind can negatively affect a greenhouse’s frame and glazing. The wind causes the glazing material to rub back and forth at its connection points. In other words, there can be a lot of abrasion where the rafters are connected to the glazing material. By wrapping the rafters with another poly material and/or sanding them to be completely smooth before installing the glaze, a greenhouse grower can significantly extend the life of the glazing material.

One significant factor influencing a greenhouse’s effectiveness and cleanliness is its ventilation system. The ventilation system of a greenhouse manages the humidity, temperature, CO2 replenishment, and pathogen prevention. When possible, devices, such as HEPA filters, should be employed to reduce the likelihood of air-borne pathogens. If pathogens do rear their ugly heads, prompt treatment and thorough sanitation are keys to eliminating problems and preventing a relapse. Proper humidity and temperature control, regularly deep cleaning the greenhouse, and a couple of relatively simple structural modifications will go a long way in preserving the integrity of both the glazing and framing materials of a greenhouse. All in all, a greenhouse hobbyist who implements a proper ventilation system, while taking into consideration the fact that excess heat, humidity, and wind can negatively affect greenhouse materials, will be able to protect and get the most value out of his or her investment.

Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse senior editor and can be contacted at

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