How to prevent and eliminate mold when growing your microgreens

Summary: Preventing mold group is about airflow, humidity, temperature, sterilization, amount of watering, and seed density.

Microgreen growers of any level of experience will experience mold growth. As a beginner grower, you will, at some point, loose trays to dampening off, mold, and fungus problems. However, there are several methods that can help regulate the process and prevent this from occurring on a regular basis.

Mold spores continually float in the air around us. This is a normal occurrence. There is also a difference between the unwanted mold and normal root hair growth. Click here to read our blog post about the difference between mold and root hair. Although there is no way to eliminate mold spores in a grow room or greenhouse, mold only causes problems when it replicates and is allowed to thrive. Thus, the best technique is to limit and control the key environmental factors that cause mold to flourish and colonize. Here are several vital tips to help prevent mold and remedy such growth to avoid crop loss.

Preventing Mold Growth

To safeguard a bountiful microgreen harvest, utilize the following methods to prevent the key microgreen predator – mold – from damaging your crops.

  1. Air Flow: Good airflow is critical to ensuring the production of a flourishing harvest. To achieve the best results and prevent mold spores from colonizing, make sure you have ventilation and/or circulation fans in multiple areas of the grow room or greenhouse. This airflow will prevent mold spores from landing and replicating on your microgreens because the air will be circulating at a pace faster than the spores are naturally equipped to handle, preventing the mold from settling and growing. To encourage airflow without fans, utilize shallow microgreen trays instead of the traditional deep ones. Using shallow trays will allow the base of the microgreens to grow closer to the top of the tray, which will encourage airflow across the base of the plants where microgreen mold typically arises.
  2. Humidity: Mold thrives in humid environments. Evaluating the conditions of your grow area will significantly limit mold growth. If you are growing your microgreens in an environment that is not sterile, has a history of mold growth, is naturally hot or humid, and lacks fresh or circulated air, your microgreen harvest will be significantly less, or none at all. These are not ideal environments in which microgreens thrive. Select an environment that is more sterile, is less hot and humid, and is more sterile. Then, run a dehumidifier 24/7 in your selected grow area. Microgreens thrive when the humidity level is between 50-60%. Ensure you are properly caring for your dehumidifier by emptying the contents several times a week to keep the air at a constant, lower humidity level while you are growing your harvest. In addition, be sure to carefully water your microgreen trays. Should you spill water on a surface and neglect to clean it, that water will evaporate, condense, and humidify your greenhouse. A good method of preventing spills, even unintentional ones, is to utilize a drip irrigation system. During the process of drip irrigation, small amounts of water is applied uniformly across a specific area. The water is then delivered directly to the crop root zone, which ultimately will eliminate runoff, evaporation/humidity, and drift.
  3. Temperature: Mold grown can occur in either warm or cold conditions. The ideal temperature at which to sustain healthy microgreens and eliminate excess mold is 20-25 degrees Celsius or 70 to 75 degrees Fahrenheit. If the temperature drops below or above this range, the environment will encourage moisture, resulting in mold and fungus growth. Importantly, seeds generate heat as they grow. Even if the air temperature is only 70 degrees, the temperature of the plants can reach into the 80s, leading to mold growth. Along with ensuring proper air temperature, also be sure to check your microgreen and soil temperature. To prevent a noticeable rise in plant temperature, reduce the germination process quickly after it begins. As soon as you notice that the seeds are rooted into the growing medium, remove them from the germination phase by placing the plants under lights, out of high heat, and into good airflow.
  4. Sterilization: A clean workspace is vital to microgreen health. Be sure to keep your growing tools and trays sterile by power washing and sterilizing your growing surfaces between harvests, and thorough cleaning your trays and flats by submerging them in a solution of water and bleach or water and food grade hydrogen peroxide for several minutes between harvests. Sterilizing in this manner, and on a regular basis, will kill off any remaining fungus or mold that may be invisible to the naked eye and remains in your trays and on your growing surfaces.
  5. Watering: The process of watering your microgreens is a critical skill that will take time to develop and perfect. Overwatering will lead to excess moisture, thus encouraging mold growth and dampening off. This is one of the largest and most common problems that new growers face when it comes to mold. As discussed in #2, “Humidity,” above, investing in a drip irrigation system will maximize the watering process by allowing for proper moisture without the threat of spillage.
  6. Seed Density: Like watering, the ideal seed density will take time and practice. Too high of a seed density will trap moisture beneath the canopy of your microgreens and will prevent airflow, leading to mold growth. And, if seeds do not germinate properly, this will also cause mold as the purpose of mold is to break down plant matter. Experimenting with seed density will allow you to discover what works for you based on your grow set-up. Be sure to invest in high-quality seeds with quality germination rates.

Remedies to Eliminate Mold Growth and Avoid Crop Loss

Even if you are extremely careful and diligent, chances are most microgreen growers will still encounter mold from time to time.

  1. Properly Identifying Mold Growth: Recognizing mold is half the battle. Mold looks like rot and can be spotted by looking for any plant parts that appear to be breaking down, or “mushy.” Be on the alert for white fuzz on the husks of the microgreens that looks like a spider web covering the seed hulls. Mold will grow closer to the top of the crops, and not so much on the bottom. However, be aware that any white fuzz on the base of the root is NOT mold – these are usually the plant roots. Notably, sunflower, pea, and radish microgreens are the types most susceptible to crop loss due to mold overgrowth.
  2. When To Be On The Lookout: The most prominent growth phase in which mold occurs is directly after germination. This is because the microgreens have been in an environment that has high humidity, high temperature, and low airflow. This is the ideal climate in which mold will thrive. As discussed above, when you see this happen, simply take the trays out of germination by placing the trays under lights, and make sure water right away. Doing this will damage the mold colonies, spread the spores, and, if you have fans/good airflow in your grow room, this will further spread the spores to prevent them from re-creating those colonies.
  3. Quick & Safe Remedies for Mold Growth: There are three tried and true methods to safely eliminate mold from microgreens.
    • The first method is to remove the clump of soil where mold is growing. Then, after adding soil to fill in the hole, add a little more to go above that area so that the mold cannot continue to spread above the growing medium. This method will help to save the tray of microgreens and allow you to keep a decent harvest.
    • Spray a sanitizing agent, like food-grade hydrogen peroxide and water solution or white vinegar and water solution, lightly over the problem area. Do not water right away, even if the microgreens appear to be dry, as you want the solution to be able to sit on the microgreens and properly disinfect. Never use bleach on microgreens. Not only is it too harsh for the plants, but whoever eats the microgreens will ingest trace amounts of bleach as well. A commonly used food grade hydrogen peroxide solution is Zerotol 2.0, which can be viewed here: This solution is certified for organic growing and is a concentrated solution that is meant to be diluted with water before use.
    • Simply harvest above the mold by using a sharp instrument like scissors to cut the microgreens above the problem area.

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