Mentha pulegium (Lamiaceae) aka, pennyroyal

[Peter Warner]

I'm interested in any information anyone might have about Mentha 
pulegium (pennyroyal), in particular about its ecology and methods of 
management.  Native to Europe, the plant is well-established in northern 
coastal areas of California, growing in seasonally (winter/early spring) 
sites of moderate to severe disturbance, such as roadside ditches and 
heavily grazed pastures.  Domestic livestock seem to avoid eating it 
(for good reason!), so grazing as a control technique seems out of the 
question.  Does anyone have any documentation about its invasiveness, or 
an propensity to displace native plant species, in any habitat?  Thank 
you for information -- please, no herbalism-oriented responses!