Several factors are known to influence spiral root occurrence, including media formulation. In the routine test in 2008, one formulation, media "X", produced 16% spiral root compared to 1 - 3% for the other nine media, including media "A" from the same manufacturer. The peat moss source for each of media "A" and "X" were the same, but 15% vermiculite was mixed in media "A" and 25% perlite in media "X". "X" had a lower nutrient charge added although most elements were not out of the range of those of the media tested, and there was a difference in the proportion of two sieved fraction sizes between media "X" and the other nine. A series of experiments were done to find the cause of this higher spiral root frequency. There was a very strong relationship between spiral root frequency and In(media "X" % + 1) when media "X" was diluted with media "A", even as low as 0.14%. Transfer of germinating seedlings between the media demonstrated that the spiral root induction started within the first three days and continued until at least the ninth day after sowing, and the effect is reversible. There was also a statistically significant, if not strong, propensity for an individual cell to produce spiraling roots. Increasing the charge of nitrogen, phosphorus and potassium at sowing did not significantly decrease the spiral root incidence. Autoclaving the media and floating the media on the filtrate of media "X" shaken overnight in an equal volume of water suggested that the cause was not a water soluble or biological factor. Regardless of how the sieved size fractions of media "X" were redistributed, spiral root frequency on media "X" was unchanged. There was no difference in the water retention curves of media "A" and "X".