Molds on growing media

Molds grow on almost any organic source if the humidity is sufficiently high. These molds are generally created by filamentous fungi that are not pathogenic to plants or humans. They are omnipresent in the air, and each species can have different preferences for certain types of organic food sources, from wood to plant debris.  Many of these organisms are used in the food industry to produce wine and cheeses and in the pharmaceutical industry for antibiotic production.

Peat moss is no different than other organic matter, such as bark, compost, coir, mulch, etc..  If the humidity is adequate, molds and other microorganisms may grow on peat moss.  This is true whether it is inside the packaging or used in a potting mix to grow a crop that is maintained too wet in the greenhouse. In unused growing media, molds may develop on the surface of the product just under the plastic bag.  In compressed peat-based growing media, many environmental factors trigger mold growth, such as nitrogen fertilizer and high storage temperatures.

Desirable points about molds
These saprophytic molds are found naturally in peat bogs, but their populations are very low.  The high acidity of peat bogs is not favorable for their development and this results in slow decomposition of the peat moss. These organisms are not pathogens to plants or humans, but are saprophytic, in that they feed on dead plant material.  Several have been reported to be beneficial for plant growth, like the Trichoderma sp. species frequently found in peat moss.  A few other species are found like Penicillium sp. or Aspergillus sp, and are common saprophytic fungi that grow on organic matter like leaves, hay or even bread.  They decompose the organic matter, release minerals and use the carbon provided by these materials as a food source.  Their presence in the forest soil for example is important in decomposing organic matter and recycling the minerals for plant growth. In peat moss based growing media, their presence can be a benefit for plants as well. 

Less desirable points about molds
Unfortunately, the presence of molds can result in excessive growth and unsightly colonies may appear on the surface of the growing medium and in a few cases may create foul odors.  When peat moss is amended with lime and nutrients, especially nitrogen, this changes the chemistry, and the micro-organisms begin to propagate in the peat mix.  In compressed packaged growing media, other organisms like yeast may also build up in populations due to the absence of oxygen.  This occurs however when storage temperatures are high in the summer and when nitrogen is applied as part of a basic nutrient charge in the media. In peat alone, this normally does not occur, because the peat moss is dried down and no limestone or nutrients are added which considerably limits their development.

What to do if molds are present
The fact that peat-based growing media occasionally will have molds and odors because of the microbial populations should not be considered a problem, because it will not negatively impact the plants.  When product arrives with these molds, it’s recommended to expose the growing media to fresh air and loosen the compressed media.  This will inhibit the microbial development by drying it out, the oxygen will shut down microorganisms that create the odors and the odors will have time to dissipate.

Immediately after transplanting, begin a fertilization program because the fertilizer charge has more than likely been consumed by the microbial populations in the unused growing media.  Plant growth should be normal.  The presence of saprophytic molds should be considered a benefit for plants because these organisms will contribute to good microbial diversity in the growing media and may limit the occurrence of plant pathogens.