Breaking the Mold

mold, mildew, greenhouse, Growing Spaces, mold prevention

Powdery Mildew on a Turnip Leaf

Mold and mildew are both types of fungi that grow best in warm, humid environments. Most fungi reproduce and spread via spores. Spores emerge from fruiting bodies of fungi and are most commonly dispersed via breeze or transported by water. Much like plant seeds, spores can survive environmental conditions that aren’t conducive to fungi growth, only to begin growing when the time is right. Once spores land on a growing medium, they germinate and penetrate the medium, beginning to grow and mature.

According to the United States Centers for Disease Control and Prevention (CDC), no one knows how many species of fungi exist, but estimates range from tens of thousands to over three hundred thousand. Generally speaking, molds and mildews become prolific in damp, warm conditions, like those often maintained in greenhouse environments. Knowing optimal growth conditions for common greenhouse fungi allows farmers greenhouse gardeners to prevent growth of harmful fungi by maintaining a greenhouse environment that doesn’t optimize growth conditions for fungi. So, the best solution to mold is prevention!

Optimum growth of common greenhouse fungi including grey mold, powdery mildew and black sooty mold occurs when localized relative humidity is above 85%, temperatures are warm, air circulation is minimal and standing water exists on plant foliage, soil or other greenhouse surfaces. Prolific mold and mildew growth in a greenhouse environment can damage plants, infect soils and other growing mediums and create an unpleasant and even unhealthy environment for people.

The effects of greenhouse molds and mildews on people is highly varied and depends primarily on the sensitivity of individuals to mold spores. According to the CDC, those more sensitive to molds and mildews may experience stuffiness, eye irritation, skin irritation an wheezing when in the presence of varied types of fungi. People with more serious allergies may experience more severe reactions.

When investigating a fungal issue in a greenhouse environment, it is important to look for both signs and symptoms of excessive fungal growth. Symptoms include leaf spots, blight, cankers, rot, and damping off among other plant reactions a gardener may notice as a result of fungal infection. Observation of signs means actually seeing the fungus or fungi that are causing damage or disease. Have you noticed any of these symptoms or signs in your greenhouse?  Of course, it is best to prevent excessive fungal growth, so there is no need to investigate a problem in the first place.

The CDC doesn’t recommend routine mold sampling from affected environments because it is expensive and the reactions of individuals to fungi aren’t always specific. Additionally, there are currently no established standards for judging what is an acceptable quantity of mold or mildew in an environment based on human health concerns. If you are interested in paying for environmental sampling of molds, be sure to establish with your consultant criteria for interpreting test results, asking what they will do or what kind of recommendations they will make based on the results. When sampling molds, always consider the environment from which they are taken.

Mold and mildew can be a troublesome in a greenhouse, but fungal problems are actually fairly easy to prevent. The best way to avoid problems with mold and mildew is to maintain conditions that aren’t conducive to the growth and reproduction of fungi. Keep general and localized relative humidity below 85%, avoid over-watering, and be sure to adequately thin plants for good air circulation.  Tightly packed plants not only compete for light water and nutrients, they create an ideal localized environment for the growth and spread of fungi.  To learn more about adequate plant spacing, check out our blog post about thinning vegetable crops.

If you do see signs of a fungal issue in your greenhouse, ask yourself:

Could I be over-watering my plants, or getting the leaves too wet?

Are my plants far enough apart to allow for air movement?

Does my greenhouse have adequate ventilation?

Is the humidity in my greenhouse really high?

Greenhouse ventilation and good air circulation can be achieved by opening greenhouse vents and adding fans.  If your greenhouse is located in a humid location, a dehumidifier or air conditioning unit may also help to facilitate air movement and decrease humidity. Maintaining healthy soil or clean growing medium in a greenhouse is also key to preventing the growth and spread of fungi. In the event that mold and mildew do take hold, infected soil and plant material should be removed from the greenhouse and any other surfaces affected by fungi should be thoroughly cleaned.

Ultimately, fungal growth in any enclosed area where plants are grown is inevitable.  Prevention of fungal growth and proliferation is key to long term growing success in greenhouses. Armed with a good understanding of potential fungal issues, a good greenhouse set up and continued garden sanitation and maintenance, a greenhouse can be a comfortable and healthy environment for both plants and people.

Have good ideas or suggestions about how to prevent mold growth or how to treat existing greenhouse mold?  Share them below!





  1. In a gardening class I took last year, the instructor said to use cinnamon sprinkled liberally on the mold or mildew. Haven’t had to do this yet, but wonder if anyone else has heard of this or tried it.

    • Thank you for the tip! Maybe we’ll give that a try here at Growing Spaces if we run into a little mold. Let us know how it works if you use this technique!

  2. I get fuzzy gray mold on the soil around my plants when I use my organic fertilizer. It works well to spray it with a solution of about a tablespoon of dishwashing liquid diluted in a quart of water.

  3. How successful would aquaponics be in a growing dome. This article makes me think it would be a big problem do to the massive amount of water involved with the growing processes.

    • Mold issues can be a real bother in all greenhouses, not just the Growing Dome, because they are enclosed growing spaces. The Growing Dome is in fact fully customizable and aquapnoics systems can be used successfully in them. Aquaponics systems can be tricky to set up, given the intricacies of water chemistry and environment, although a number of our customers have successfully done so! If you are interested in more information on this topic, you may want to check out this video of Udgar Parsons and myself visiting a successful aquaponics operation in a 26′ Growing Dome in Santa Fe, NM. The controlled environment agriculture specialist in the video is Eric Highfield, a researcher and teacher at Santa Fe Community College.

  4. The Growing Spaces team has also heard that Thieves Essential Oil is an excellent organic mold and mildew fighter. Used in different ways, Thieves oil can also be used to ward away undesirable insects and other pests.

  5. I want to use my green house for storing my white plastic garden furniture over the winter Oct to April. As I have spent two days bleaching and scrubbing it; I do not want it to go green over the winter. So what is the best thing to do about ventilation. I have a temperature thing in the roof that opens the top window but I do not know if it will automaticly open and close under winter conditions. It is machanical with a piston thing that makes it work.

    I thank you in advance for any help you can give me. Steve

    • Hi Steve!

      We recommend using a garden shed or garage for storing your furniture. The heat and UV rays will degenerate the plastic of your furniture. Hope this helps!

  6. Fungus is a soils 2nd best friend without it plants will starve. Please be conscious of this and simply brew a microbial compost tea to keep the microbial population bacterial dominant as many crops need. Overwatering is just that and opps …you problems began before you did…check soil structure and ammend as needed. Keep up on your calcium levels apply gypsum if needed. Possible kelp added to earthworm casting tea.

  7. Neem Oil is a relatively safe organic pesticide and fungicide. It is from the neem tree in India and local people often chew on a piece of the tree bark to clean their teeth (which supports its nontoxic quality). We have found it to work great on fungal blight in an open garden, but any crop leaves that have already been infected will be decimated when Neem Oil is applied as treatment. Healthy leaves will thrive. Squashes are very sensitive to application, but other hardy crops can handle it well. It is best used as a preventative, not a cure. Pick up a concentrated bottle or prepared solution and use according to manufacturer instructions. It can be found at most agricultural product stores. I plan to use it in my 22′ dome, we have just started up a full blown surge of white powdery mold. It is a new dome and we are still getting used to humidity control. We do plan to start aquaponics, so after we filled the tank, the mold began a week later. We need to get more air circulation in the dome and reduce the density of our crops.

  8. Mix a tablespoonful of Baking Soda, with a teaspoon of liquid hand wash mixing together in a gallon of water- it does work and a at a minimal cost

    • Basically, change the PH value (acidity) enough on wherever the fungus grows.
      Baking soda is basic (“anti-acid”), and eg. Citric acid is a mild* acid (* depending on water/acid mixture).
      Switch it around a few times & the mold should die since it can’t compensate fast enough.
      Be careful to not mix it TOO strong, because it could damage the leaves.

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