A brief article in the Royal Horticultural Society’s Magazine, The Garden, May 2012, p.23, examines the belief that caper spurge (Euphorbia lathyris) deters moles (Talpa europaea). It is claimed that moles ‘dislike the smell the [spurge’s] root exudates’, but if this is so, why doesn’t the sap of other spurges (Euphorbia spp.) act similarly?
It is suggested that caper spurge often grows in soil that does not support large populations of the worms and other invertebrates which moles eat, or in dry soil which moles do not frequent (presumably because such soil is unlikely to support a large number of worms). Thus people noted that moles were scarce in places where caper spurge grows and incorrectly assumed that this was because the spurge deterred them.

Image: J.G. Sturm, Deutschlands Flora in Abbildungen, 1796.