Oak Trees and Black Scum
Name: Frank C.
Date: Thursday, September 12, 2002
I have a large oak tree with limbs extending over my
wooden patio. For the past week there are multiple small 1/4 inch
puddles of a black oily appearing substance on the wooden deck. These
are more prevalent after a rain. Is this some type of sap leak? I see
othing particular in the tree itself.
Could be sap from the tree. Since the tree is large it would be difficult
to find the source-point but probably nothing to worry about except for the
puddles on your deck.
Anthony R. Brach, Ph.D.
It is possible that the black substance is air pollution from vehicles and
other burning sources. Dirt would collect over time on the leaves of trees
between rains, and get washed down to your deck during rains or when
sufficient dew collects on leaves to drip down.
Without more detail (is the wood on the patio new? has it happened in
other years/seasons?) it is a little difficult to diagnose. Maybe set out a
plate on your deck to see if it collects this oily grime, at least that way
you can determine if it's coming from above or below (e.g. leaching from
treated wood in your patio)
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Update: June 2012