Pennyroyal Plants, Cats and Essential Oils

I'm a master gardener and writer in Ann Arbor, Michigan. A few years ago, I created a garden for cats at a no-kill cat shelter. I researched a ton of plants and created the garden. (I've blogged about it here: gardenfaerie.blogspot.com/2009/02/garden-for-cats.html.)
During my research I was surprised at the amount of misinformation regarding toxicity of plants to cats, specifically in regard to baby's breath which everyone insisted was poisonous to cats, but it's not [it does have low doses of saponins in its roots, but (a) cats don't tend to eat plant roots and (b) the toxins are passed safely out a cat's system.
I'm writing a book about gardening for cats, based on this experience. A plant I don't have personal experience with, that someone recommended is pennyroyal (Hedeoma pulegioides or Mentha pulegium). I'd like to try this in my own garden and include it in my book, except of course I got all kinds of toxic hits, as well as some that dispel the notion. I liked your response to a Q&A to it, and my main question is whether you have or know of anyone who has grown the plant in proximity to cats.
I'm not asking about the essential oils, but the plants itself. Would they be toxic if cats rubbed on them or ingested the leaves?
I also can't find a great source about how essential oils are extracted, or, specifically, how essential oils compare to plant material in terms of amount of specific properties. (For example, 1 teaspoon of essential equals the properties of what volume of leaves?)

Thanks so much for your email. Whilst I agree that much misinformation exists regarding plants and animals, I cannot comment on your statement regarding Baby's Breath (Gypsophila paniculata) and cats. I agree that cats are unlikely to eat the root of this plant but most constituents found in abundance in one part of the plant usually show up in another part of the plant, albeit at a lesser amount. In addition, there are all kinds of saponins, with all kinds of effects. Hard to respond to your statement without much more research.
Next, Mentha pulegium (English Pennyroyal) and Hedeoma pulegioides (American False Pennyroyal) are not the same plant but are distantly related cousins. Both are members of the Mint (Lamiaceae) family and can contain up to 85% of the volatile oil pulegone, which in turn can account for 2% of the plant's constituents. Pennyroyal essential oil is usually made from Mentha pulegium. And, yes, it's the volatile oil content that is the basis of all your "toxic hits" due to it's association with severe liver damage (hepatotoxicity) and death. However, as you say, most of the references are regarding the essential oil, not the plant itself. As mentioned, the fresh plant may contain a percentage (2%) of volatile oil, so whilst the pulegone content is less concentrated in the plant as opposed to the essential oil, it still exists and could potentially affect an animal should it be ingested. Cats particularly are susceptible to volatile oils; thus there may be a higher probability of liver issues due to consumption (eating or licking) of Pennyroyal. If it was my cat, I would avoid having this plant in my garden. Rather be safe than sorry. And no, I haven't met anyone who has grown this plant in proximity to cats so cannot comment on this point.
As for your last question, essential oils are made through a distillation process. Different plants contain varying amounts of volatile oils so each essential oil requires a specific amount of fresh plant to produce say, one millilitre of essential oil. Fresh plants contain the highest amount of volatile oils as this constituent degrades over time once the herb has been dried.