Michael Charters from Calflora.net explains that the pronunciation of the generic name for this species has an emphasis on the first syllable, so it is said “SIL-i-bum”. However, that doesn’t prevent certain coworkers of mine from repeating the humorous version of the name when they encounter the plant or images of such (I expect today’s “word of the day” at work will be “sillybum”).
Humour aside for the moment, blessed milkthistle is a plant native to the Mediterranean area. Where agriculture has gone, though, it has followed, and it is now found in mild temperate areas around the globe. Seed production of this plant in infested areas can reach and exceed 500kg/ha (or 446 lbs/acre)–a massive amount, and an indication of why this plant can become a problem.
This write-up by Caitlin Bean provides an excellent overview of the species: Silybum marianum. From her overview, another reason why it is problematic:
…One reason control methods have been sought after is due to the toxic potential of the thistle. Silybum marianum has caused some of the worst cases of stock poisoning in northwest Tasmania. The poisonous principle is nitrate (Macadam 1966). Cattle and sheep eat the plant material which contains potassium nitrate and break it down by means of ruminal bacteria into the poisonous form (Knott 1971). “The nitrite ion…combines with haemoglobin to form methaeglobin …[which is] incapable of combining with oxygen. If large amounts of methaeglobin are present in the blood stream, affected animals will begin to show respiratory distress for lack of oxygen.” (Knott 1971) Poisoning threats are increased when the plants are wilting after being cut or partly turned under during plowing and in wet weather (or) when soil moisture is high. In dry conditions they are not considered dangerous (Parsons 1973)