Mold on Dirt in Crawl Spaces, Basements, Floors
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• How to recognize and remove mold contamination from dirt or soil such as in crawl spaces or other dirt floors
• Yellow mold in crawl spaces may be destructive Meruliporia fungus
• White mold found on crawl space or basement dirt floor may be a basidiomycete.
• Photographs of mold growing on dirt crawl space floors

Many readers have asked us what to do about mold found growing on crawl space dirt or on a dirt basement floor. I've often seen orange and yellow molds and yeasts growing on wet dirt in these areas.

Our photo (left) shows a fungal fruiting body or mushroom growing out of a concrete block basement foundation wall. The yellow fungus growing on the mushroom may be a second opportunistic parasite.

Depending on genera/species a mold growing on dirt might be toxic, pathogenic, or allergenic, though in our experience (warning, I am not a mycologist) I haven't come across dirt-growing molds that were species that were airborne - at least not at the time of our inspection and under wet conditions.

So the chance that such a mold passes throughout a building as problem spores is lower than otherwise. Still, one would want to be careful not to get mold in a cut, or in your eye, even if it does not appear to be an airborne species that can be easily inhaled.

Mold might also be growing apparently on dirt but actually on something that spilled or was left atop the dirt - which could mean other fungal species than the soil fungi we just cited.

In any case if there is only a small amount of mold - a few square feet - it's reasonable for a homeowner to remove the visible mold and surface soil - just an inch or less. There may be fungal components deeper in the soil, but we're going to address that by drying out the area rather than digging up the whole house.

Our photo (left) of white-colored mold on dirt , was contributed by a client whose crawl space contained Meruliporia sp. fungus on the dirt (see below). We did not process a sample to identify this white fungus in the lab. It may be a different species from others in the crawl area, and its growth may have been related to items such as cardboard or wood left on the dirt floor, or due to a buried concrete block - notice the roughly rectangular pattern of the white mold growth at the center-right in the photo.

But before sampling what looks like "white mold" on dirt or on a masonry surface, take a look at Efflorescence & white or brown deposits - very often the white crystalline looking substance in these locations is not itself mold, though it is a moisture indicator, meaning mold may be nearby.

If the area of mold on dirt is large, more than 30 sq.ft., and if it's not a harmless cosmetic mold, professional cleanup would be appropriate, but only after a competent and independent (independent from the cleaning company) inspection diagnosed the extent and cause of mold and thus provided you with a mold cleanup plan.

If you see a large area of moldy soil that appears homogenous in character (color, pattern, growth surface, etc) then it may be economical to collect a sample to send to an independent lab for identification (mine or any independent mold testing lab - we give instructions on how to collect mold samples at Mold Test Kit Procedures.)

It's essential to also find and correct the cause of mold growth: Any mold cleanup anywhere in a building, whether its on crawlspace dirt or basement soils, must be followed by a diagnosis and correction of the moisture source that invited the mold growth, or the mold problem will simply recur.

The fungus in this photo mailed to us by a client was identified as Meruliporia incrassata - a mold that can attack wood structures causing significant damage. We discuss this fungus at Meruliporia Mold Photographs. Although this recently-built home, located at 5500' in the Sierra Nevada mountains had crawl area ventilation, snow melt draining towards the structure caused chronic wet conditions and was surely a significant contributor to this mold growth.

Furthermore, and possibly more important, soil mold in a basement or crawl space is an indicator of wet conditions, which means that more-problematic molds may be growing on wood or paper or other organic surfaces nearby - and may be more of a problem for building occupants. For example if insulation were installed in this crawl space ceiling it would be likely to have mold contamination, though more likely from Aspergillus sp. or Penicillium sp. - different fungal genera/species prefer to grow on different materials. See FIBERGLASS INSULATION MOLD.

An inspection of wood framing and subfloor over a damp moldy dirt-crawl space or basement should be conducted using light and careful examination of the surfaces - see our online article about how to use light when looking for mold.

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