Mold on Microgreens- How to Combat and Prevent Mold

Mold is a problem that many microgreens growers face and it can become a consistent problem if you aren't prepared for it. Having the knowledge to identify and troubleshoot mold can help save a lot of headaches, wasted trays, and time in the future.

To prevent mold on microgreens, we need to better understand what causes it. How can we prevent it? Preventing mold comes down to a few key issues that cause excess moisture in the growing space - a prime factor in mold growth.

Is it mold or root hairs?

"Is it mold or root hairs?" has to be one of the most common questions new growers have when growing their first tray of microgreens. To the untrained eye, they can look the same, but with some practice and reference photos, you will easily be able to identify their distinct differences.

Root hairs are thin, featherlike cilia that grow out from the roots to increase the surface area of the new seedling to help with nutrient uptake. They will not have an odor and will appear only around the sprout's root.

Root hairs look kind of like fluffy cotton, and they are most prominent at the beginning of the sprouting stage. These fuzzy bits at the base of your microgreens are the root hairs, and they're nothing to worry about. However, they're easy to confuse with mold until you've developed a more trained eye. 

If you're unsure whether you have root hairs or mold on your microgreens, the rinse test, discussed in more detail below, will help you determine whether you are dealing with mold or fuzzy root hairs.

How to Identify Mold on Microgreens

When you're identifying mold on your microgreens, there are some key factors to keep in mind. Mold, often appearing as spider-web-like strands, can overtake a tray of microgreens when given the right conditions. Mold and fungi thrive without proper drainage, air circulation, and lighting.

Here are some key things to look for: 

• Offensive odor
• Grows above the soil and directly on the greens
• Slimy to the touch
• Will not disappear with rinsing
• Can be black, purple, or blue spots on leaves

Trays can also fall flat due to mold, also referred to as damping-off.   

 Damping-off in Microgreens

Trays can also fail due to mold, known as damping-off.  Damping-off is a common issue faced by microgreens growers and is caused by pathogens that attack seedlings at their base, eventually killing them off. This usually results in the loss of the whole tray. Learn more in How to Prevent Seedling Damping Off from the University of Minnesota Extension.  

These pathogens thrive in very specific environments. These environmental issues are mostly due to airflow problems in microgreens. When plants are not receiving adequate airflow, soil-based fungi begin to take over and spread from plant to plant.

It attacks both the root and the stem of the microgreens. The mold begins to deplete the microgreens of nutrients, causing them to collapse at the base. The roots may change to a brown color and will become slimy.

As they fall over, airflow issues worsen, leading to wet microgreens and an anaerobic environment. This environment encourages the pathogens to continue to take over, and ultimately causes total tray loss.

Why do my microgreens smell bad?

When your microgreens smell bad, that's a key indicator of the presence of mold. Microgreens should smell earthy and plant-like, while varieties like onion or broccoli smell like their full-size counterparts.

However, if your microgreens smell rotten or "swampy," something is wrong. Those smells in a microgreen tray indicate the tray is too wet, which creates an ideal environment for mold to take over. If your microgreens smell rotten, you should troubleshoot the growing conditions for the future, but toss those microgreens - they have mold and are not good to eat.

Where does mold grow on microgreens?

Mold grows on microgreens on the seed hull or on the stem and leaves of the plant itself. While root hairs are often mistaken for mold by beginner microgreen growers, they appear exclusively on the roots, as the name indicates.

Mold on microgreens can also appear at the base of the stem, but it won't pass the rinse test (discussed below). The mold can also appear on the stems or on the leaves.

The Rinse Test for Microgreens

The rinse test for microgreens can help beginner microgreens growers determine whether their microgreens have mold or if it's just the root hairs. To perform the rinse test, spray your microgreens a few times with water. If there is no reduction in the number of cilia, that indicates mold.

If the fuzziness of the root hairs disappears, you know for sure that you are dealing with root hairs only and not mold. If the suspected mold has not changed at all when wet, then it's time to throw out the tray and look at your grow space for possible issues.

Top 10 Ways to Prevent Mold on Microgreens

1.  Plant in trays with drainage holes                                                   

2.  Use clean growing mediums that allow for drainage                     

3.  Control the humidity of your grow space

4.  Provide trays with proper air circulation with the use of fans

5.  Set up proper lighting conditions allowing for 6-10 hrs. daily

6.  Presoak and disinfect "dirty" seed varieties

7.  Avoid seeding too densely 

8.  Properly disinfect trays between plantings

9.  Disinfect flood trays regularly with H202 

10. Use the bottom watering method for microgreens

Causes For Mold in Microgreens

Mold and fungi multiply quickly if given the opportunity. Plan the optimal grow space for your microgreens to ensure long-term success.

• Poor drainage. Often, when you're dealing with mold in microgreens, it's the culprit. Microgreens need frequent watering to grow, but pooling moisture left on the bottom of containers is often a breeding ground for mold. This article discusses how to bottom-water trays without overdoing it. 
  
  We recommend microgreen growing trays with holes to prevent this issue. The trays with holes allow water to drain away from your microgreens and root systems into a flood tray. The flood tray can then be emptied, reducing excess humidity in the grow space. Read  How to Use 1020 Mesh Trays for Seed Starting and Microgreens to learn more. 
  
  
• Using the wrong growing medium. The same type of moisture collection can also occur if you are using a growing medium that does not encourage efficient drainage. We find that potting soils with a good peat moss-to-perlite ratio promote better air circulation and drainage. Pairing this soil medium with trays with holes may be all you need to solve your mold problems.
  
  
• Excess moisture. Moisture in the environment can also contribute to mold growth. The humidity level of your growing space should ideally be between 40% and 60%. If you live in an area with ambient humidity often above recommended levels, or your grow space tends to retain moisture, adding a dehumidifier can help regulate humidity.
  
  
• Poor air circulation. Mold also prefers stagnant air for breeding and multiplying. Ensuring proper air circulation is key to minimizing this problem, especially if your humidity levels tend to be on the higher end. We recommend setting up circulation fans in your grow space, like these ones designed for grow racks.
  
  
• Lack of proper lighting. Mold prefers warm, wet, and dark environments. Full-spectrum light, like our LED plant lights, provides adequate lighting for plants and disrupts the mold life cycle.

If you're having a lot of issues getting your grow space regulated to the right conditions, we highly recommend checking out a pre-built grow rack set up like this one. We've done all the calculations to ensure the right conditions are created for seedlings and microgreens, from fans and lights to the water system and shelf spacing.

Disinfecting or Sanitizing Microgreens Seeds

Many seed varieties require a pre-soak before planting. It is advisable to add H2O2 to the water during soaking. Adding hydrogen peroxide to microgreens seed soaks can help prevent seed-borne diseases that could result in tray loss.

Another option is to soak your seeds in store-bought white distilled vinegar for 15 minutes, as recommended by the Kansas State University extension.

Some seeds are notorious for being "dirty" seeds. Although you should sanitize all your microgreen seeds, the "dirty" seeds are the ones you should sanitize properly every time. A few varieties that really benefit from sanitization are sunflowers, cilantro, and peas. 

Bootstrap tip: Avoid soaking mucilaginous seeds like basil and chia. 

Seed Density for Microgreens

If seeds are planted too densely, they cannot breathe or drain. The roots will mat up as they grow too closely together, which prevents efficient drainage. As the tray grows, it often becomes too thick to allow effective air circulation. This creates the perfect storm of mold growth. To prevent root matting and overly dense growth, plan the appropriate seed density before sowing. 

Seeds vary in weight and size depending on the type. Different types of seeds require different densities. You can't use a single density for all seed types. Knowing the appropriate planting density and accurately weighing out the seeds are the best ways to start a uniform, healthy microgreen tray.

We have compiled a guide, The Ultimate Microgreen Cheat Sheet, with many of the most common varieties and their recommended seeding rates in grams.

Preventing Mold on Microgreens in Your Grow Space

When it comes to prevention, keeping a clean growing space and equipment is always best practice. We recommend cleaning trays, grow racks, and harvesting equipment with a solution of soap, hydrogen peroxide, or vinegar.

Cleaning your equipment regularly, both between plantings and during plantings, not only helps prevent mold but also staves off other diseases that could cause germination problems and microgreen losses.

Scrub trays with a mild, basic soap, then mist with a diluted solution of food-grade hydrogen peroxide. Optionally, mix one part of food-grade 35% hydrogen peroxide with 11 parts of water. Put this in an amber bottle and spray on all trays.

You can also use a diluted vinegar soak to help disinfect your trays. However, do not mix vinegar and hydrogen peroxide in the same container. They each make great disinfectants when used individually or sequentially, but are dangerous when combined.

Let the trays air-dry completely before stacking to prevent mold contamination during storage.

Bottom-watering Microgreen Trays

Watering microgreens from the bottom ensures more even coverage for the tray. Otherwise, water can oversaturate areas of the tray, causing mold and rot. This also prevents damage to the delicate microgreens that can occur when top-watered.

To bottom-water your microgreens, nest your trays with holes into the same depth flood tray. Add 1/2" of water to the tray, wait a couple of hours, and then dump the water out. Do not leave standing water in the bottom tray. 

Our automated grow rack uses a flood-and-drain system to maintain optimal moisture levels, but it can also be operated manually.

Consuming moldy microgreens

Don't be tempted to try to salvage your microgreens once they've started molding. Even if a tray is only partially covered with mold, spores could still be present throughout the tray. Mold on microgreens is just as bad as mold on any other type of food. 

Some growers will use 33% food-grade hydrogen peroxide (diluted to 3%) or grapeseed oil to control light mold. In our opinion, it isn't worth the risk.

Bottom line when it comes to mold and microgreens

If your microgreens are contaminated with mold, discard them, sanitize your trays, and address any issues in your growing area. After that, you can grow them again. If you don't correct the issues before starting new trays, you'll find mold is a persistent problem, and you'll incur significant tray losses. Mold starts as invisible spores, so by the time you discover mold on your microgreens, your whole tray is most likely contaminated.

Focus on prevention to improve success rates. Once mold growth begins, inspect your growing space and make necessary changes.