How to Get Rid of Mold in a Greenhouse

how to git rid of mold in a greenhouse

Between light and temperature changes and unwanted pests, a well-built greenhouse is the best way to protect your plants from the many forces of nature that are ready to cause harm. Unfortunately, there are still some maintenance problems you might have to face: enter mold.

To remove mold from a greenhouse, use hydrogen peroxide or vinegar solutions, since they're safer for plants. The choice of chemical depends on the location and type of mold. However, long-term mold problems must be treated at the source: fixing the ventilation and humidity of the greenhouse.

how to get rid of mold in a greenhouse

Since mold is probably the last thing you were hoping to grow in your greenhouse, let’s dive in to the ways to get rid of it without hurting anything else and how to prevent it altogether.

Mold Background

Molds are fungi that grow best in warm, humid environments. Most kinds are microscopic, but they tend to be found in large numbers and reproduce quickly by releasing spores into the air. After landing on a surface, they’ll start to grow new colonies which will then release new spores, and the cycle continues.

Exposure to mold, which can occur by breathing in the spores it releases, usually won’t hurt people. Still, it’s not a risk you want to run, especially with those who have compromised immune systems.

People who are sensitive to fungi will likely experience the same symptoms as allergic reactions, including nasal congestion, runny nose, and a scratchy throat. In a worse case, the symptoms can include fever and fatigue, and, in the worst case, a fungal infection. Don’t get too worried, though; these instances are rare and any effects you see will probably disappear once the mold is eradicated.

Of course, in a greenhouse, mold will also affect the plants. Fungi aren’t like plants and can’t perform photosynthesis, so they’ll have to find an outside source to get nutrients

Some types of mold get their food through wood and dead leaves. The more invasive kinds use living organisms to feed off of; and the small breathing holes in plants called stomata are practically equal to a wide-open front door. The microscopic spores slip in easily, and the parasitism begins.

There may be up to a few hundred thousand types of mold, but only a couple have a good chance at ending up in your greenhouse.

  • Gray mold is the most common type, and it can take root in almost any plant.
  • Mildew, sooty mold, and white mold are the other likely culprits.

Despite the different kinds of mold and their different behaviors, you’ll find that the prevention and removal are mostly the same all around.

Identifying the Problem

In homes and office buildings, the buildup of mold can go unnoticed for long periods of time, since they’re often hidden from sight and don’t affect the people inside very much. In a greenhouse, though, it’ll be harder to miss--especially when it starts to affect the plants.

If mold finds its way into your greenhouse, taking care of it shouldn’t be too much of a hassle, but, since mold is the biggest problem you will have to deal with, it’s important to keep an eye out.

You can do this by checking the leaves of your plants regularly (even the undersides), which is where you'll see the most obvious signs of mold.

Gray mold

Gray mold, scientifically known as Botrytis cinerea, can be detected by gray leaf spots, flower spots and blight, and stem and crown rot. Damping-off may also be a sign of gray mold, which is when seeds are weakened or even killed before germinating. In spite of its name, the spots may be colored dark brown or even black. 

Powdery mildew

Powdery mildew is fairly common in greenhouses. As unappealing as it looks, with the dusty, white leaves and flowers it causes, it does very little harm.

White mold

White mold, known as Sclerotinia sclerotiorum, creates fuzzy white, off-white, or tan patches. Another symptom of white mold in a plant is sclerotia, which are black and oblong. If a plant is infected, these small formations will be seen on the stems.

black sooty mold

Black sooty mold

Black sooty mold is an additional kind of fungus that might sneak into your greenhouse, but what causes it is different from the others. Normally, the growth of mold is caused by certain environmental conditions, like high humidity or specific temperatures that allow the individual molds to thrive.

This isn’t the case for black sooty mold, which is caused instead by sap-feeding insects.

Sooty mold is the name given to a few species of fungus that share common theme among them: the presence of a dark, threadlike material on the surface of plants.

This mold doesn’t directly harm the plants, but can block access to sunlight, stunting photosynthesis and growth.

Others

Mold can also be found in soil, and there are some kinds that tend to be seen in specific plants, like leaf mold of tomato.

In a greenhouse, mold or mildew might be found along the walls or ceiling, too, and the impact may not always appear in the plants. This makes it more difficult to notice, and even more important to check for. Remember to have a look around periodically, especially in the places that are more damp than others.

Identifying mold can sometimes be tricky. It could be many different colors, from brown and green to orange and white. The smell, which has been compared to dirty socks or rotting wood, could be a giveaway. 

Since it’s not possible to name every kind of mold, for the purpose of identification, if you find a patch of some substance in your greenhouse (whether on the walls, plants, or equipment) that makes you want to vomit but you also kind of want to touch it, that’s mold.

Removing Existing Mold

The treatment for the immediate removal of mold in a greenhouse depends on two things:

  • what kind of mold it is, and
  • where it’s at.

Removing gray mold & white mold

For gray mold and white mold, the plants will not be able to recover after becoming infected. Carefully remove the infected plants from your greenhouse. In doing so, the vital part is to prevent the spores or sclerotia from spreading when you move them. Burn and/or bury the plants after taking them out.

Removing powdery mildew

Powdery mildew on plants is much less harmful. Since it doesn’t hurt the plants directly, leaving it alone is an option; though it would probably be best to pick off the leaves that are altered, since it could block sunlight or spread to the other plants.

Removing black sooty mold

Removing black sooty mold is another story. Unlike the other types of mold, black sooty mold is caused by sap-feeding insects like mealybugs, aphids, and leafhoppers. The mold is the product of these bugs leaving deposits of honeydew. So, in order to rid your plants of the mold, you first have to rid yourself of the insects.

Also, ants will protect the sap-sucking bugs, because the bugs provide the ants with honeydew to eat. It’s a whole web of insect cooperation going on just to make this mold appear on your plants. First, place ant baits around the plants with the sooty mold, then apply insecticidal soap or oils to take care of the sap-sucking bugs.

Afterwards, the mold will start to die. If you’re feeling impatient to get it all off--and who could blame you--use a cloth and water to remove it. For stronger plants, you can use soap as well.

General mold removal in a greenhouse

And, finally, if you’ve simply found a patch of mold on the walls of your greenhouse, your best route is to use either hydrogen peroxide or a vinegar solution.

Pour at least 3% concentration hydrogen peroxide into a spray bottle, and apply it generously to the affected area. After about 15 minutes, wipe the surface down thoroughly, and clean the surface as you would normally.

Vinegar is another option. Add vinegar to a spray bottle and spray the area. Wipe it down after about an hour, and then use water to clean it.

To be on the safe side, you could follow either way (the hydrogen peroxide or the vinegar) and then use baking soda, which will absorb the moisture that molds thrive on. Dissolve a little bit of baking soda in a spray bottle with water and spray the area that had mold on it. Lastly, scrub and dry the area.

Hydrogen peroxide, vinegar, and baking soda are all safe for plants, making them the desirable options. In fact, after removing infected plants or plant parts, it’s in your best interest to follow one of these methods in the area around where the plant had been, even if you can’t see mold growth.

However, if you’re not extremely careful (and maybe even if you are), the mold could later show up somewhere else because some spores drifted off before you cleaned it. So, this will likely be a process that you’ll have to repeat.

using bleach to clean mold

Using bleach & fungicides to get rid of mold

You also have the option of a more aggressive treatment using bleach. ONLY use bleach if you’re sure that it will be far enough from the plants that they won’t come in contact with it. Bleach would kill a plant even quicker than the mold, not to mention the harm it would cause to the beneficial insects.

Fungicides are another harsh treatment option that can be utilized to remove mold from plants. However, not all plants will be able to fully recover, even after the mold is eradicated. All of this is to say that hydrogen peroxide and/or vinegar seem to give the best results with the lowest possible risk.

Preventing Mold

When it comes to keeping mold out of your greenhouse, the best approach is prevention. It doesn’t matter how meticulously you clean existing mold if you don’t change the conditions that allowed it to grow in the first place.

There are a handful of key prevention methods. Whether or not you’ve yet to have a problem with mold, these steps are important to follow. If you have had a problem with mold before, the long-term fix will be accomplished in making changes to one or more of these areas.

Water & humidity

By far, the most crucial part of keeping your greenhouse mold-free is avoiding water accumulation, both on plants and the building’s surfaces, and preventing the humidity from getting too high.

Most plants require a humidity level around 50 to 60 percent, and most molds survive best near 85 percent. Keep the humidity level as low as possible while still preserving the needs of the plants. One way to control this is by installing a dehumidifier.

Also, be careful not to over-water your plants. If there’s too much for the plants to drink up, the mold will kindly offer to do so.

To avoid standing/stagnant water on your plants, water them earlier in the day rather than later. Photosynthesis, which is the main way plants use water, will only occur when the sun is out. Mold, on the other hand, requires no light to flourish, so it’s best to limit the amount of water that sits in plants overnight.

Occasionally use a rag or sponge to wipe down the surfaces of the greenhouse where condensation builds up--and don’t forget to check for moisture on the floors. And, finally, if there are any leaks, make sure they are properly fixed, and that any water-damaged areas are repaired.

Ventilation & air purification

If you had mold in your greenhouse once, some microscopic spores are probably still present in the air. Once a spore settles, more will start to grow. In order to destroy those tiny nuisances, you should set up an air purifier in your greenhouse that specifically kills mold spores. The clean, dry inside should keep new cultures from forming out of the old ones.

In the same vein, you want to keep the air circulating to outside the greenhouse. Ventilation is already important in helping to regulate temperature, but it’s also needed to keep fresh, healthy air coming in and the dirty (and moldy) air going out.

So, open your vents and add fans, if needed. Keep the air moving.

Spacing 

Spores can spread through air, water, and by direct touch. For this reason, the prevention of mold can be aided by keeping plants spaced a few feet apart from each other and not letting the leaves of various plants become entangled. This is good for the plants, too, because it supports air circulation, sun exposure, and a quicker drying time after watering.

Likewise, remember to clip and thin plants when they start to look overgrown.

Cleanliness

There’s nothing that can’t be helped by some good, old-fashioned cleanliness. Clean the floors, walls, and surfaces of your greenhouse as the material it’s made out of allows. If you want to take the extra step, it can’t hurt to occasionally clean surfaces with hydrogen peroxide or vinegar.

Don’t let dirt or water build in any equipment you use, either. Mold will find its way into potting soil and then to the plants, if given the opportunity. Wipe down gardening tools after use and take cuttings or any trash out sooner rather than later.

Mold vs. Other Plant Diseases

Sometimes a plant may appear to be affected by mold, but is actually infected with bacteria or a virus. If you notice rotting, yellowing flowers or leaves, damping off, or strange-colored blotches without also seeing patches of mold, you’re likely dealing with a type of bacteria or virus.

While various kinds of mold often have huge ranges of possible hosts, many other plant diseases are specific to certain families, such as the clubroot afflicting plants in the cabbage family and aster yellows appearing on flowering plants

Preventing plant diseases is much like preventing mold:

  • Rotate crops,
  • keep out the bad bugs, and
  • make sure that there aren’t any serious nutrient deficiencies among your plants, which would make them susceptible to pathogens. They need plenty of calcium and nitrogen.

It’s worth noting that when a plant is infected by a fungus or bacteria, it’s usually because the plant has already been weakened or harmed by something else. Take care of your green babies with lots of love.

And, if you do notice strange symptoms in your plants, default to hydrogen peroxide. In addition to being antifungal, it’s also antibacterial.

Finally, to ensure the well-being of your plants, triple-check the health of new plants before introducing them to your greenhouse. The last thing you want is to let the newcomers ruin all the fun.

mold vs other plant diseases

Related questions 

How do you keep greenhouse humidity at the correct level?

Measure humidity levels with a hygrometer. To lower the humidity level, open/install vents and fans, wipe down condensation, and avoid overwatering. Install a dehumidifier, if necessary. To raise humidity levels, use a shade covering and spray water onto leaves instead of directly into soil.

How should plants be arranged within a greenhouse?

Keep plants at least one or two feet apart to allow proper air circulation and light exposure. It’s a good idea to maintain specialized areas for related plants. You can organize them by plant types, harvest time, edible/non-edible, similar requirements, etc.

Can plants be brought into a greenhouse from an outside garden?

Plants can be moved from an outdoor garden to a greenhouse. This is typically done to protect plants from harsh winter conditions. However, first ensure that the plants are healthy. Allow the plants to acclimate to the new conditions slowly, and be careful when digging up plants from the garden.

About the author

Greg Volente

Greg Volente holds a Naturalist Certificate from the Morton Arboretum, and worked for The Nature Conservancy leading environmental education programs and doing natural areas restoration. Besides gardening, he's an avid wildflower enthusiast, and loves botanizing, hiking, and backpacking.