With increasing grow ops in residential houses many home buyers, home inspectors and indoor air quality consultants wonder if there were some types of mould species that can be used as ‘fingerprints’ or indicators for homes formerly used for marijuana grow ops. Currently we are not aware of species of moulds that can strictly be associated with marijuana grow ops. However, the dominant moulds in a building environment are generally indicative of the moisture levels prevailing over a long period of time.

The environments tends to select and maintain certain micro-organisms as the “home microflora”. This “home microflora” is usually determined by the types of nutrients available, water activity, pH and temperature. In building environments, moisture is the growth limiting factor for moulds. Moulds differ in their water requirements. A small group of moulds requires high levels of available water (water activities of more than 0.9). This group, referred to as hydrophilic or tertiary colonizers can be used as indicators for water/moisture damaged buildings. Such moulds include species of Chaetomium, Fusarium, Memnoniella, Stachybotrys, Trichoderma, Ulocladium and a few others. The largest group of moulds requires continuous damp conditions (or water activities between 0.80 and 0.90). This group is referred to as secondary colonizers. Examples of secondary colonizers include common indoor contaminants such as Cladosporium spp and some species of Aspergillus and Penicillium. Another small group of noulds, the xerophilic or xerotolerant, is able to grow at water activities below 0.80. This group, also referred to as primary colonizers, include Aspergillus restrictus, Aspergillus versicolor, Eurotium spp., Penicillium aurantiogriseum, P. brevicompactum, P. chrysogenum and Wallemia sebi. Xerophilic moulds are indicators of relatively dry environments.

Moulds commonly associated with marijuana in storage include species of Aspergillus, Penicillium, Fusarium, Mucor, Rhizopus, Alternaria, and Curvularia. These moulds cannot be used as indicator moulds for marijuana grow ops since they are also found on many other substrates.

Secondary colonizers are likely to be the most common in houses used for grow ops but they are not unique to the grow ops. In conclusion no moulds are currently known to be strictly associated with marijuana grow operations.