If you're a farmer or a rancher, you probably don't want ragwort growing on your land. As seen above, it looks like an oversized dandelion but unlike dandelions, ragwort is more than just a weed that's more than just a minor annoyance. If horses or cows eat ragwort, there's a good chance it will make them sick, causing cirrhosis of the liver. It's native to Europe and in those areas, ragwort is less of an issue because cows and horses avoid eating it; its bitter taste is enough of a turnoff to prevent the livestock from grazing where it grows. But in Australia and New Zealand, the cattle and horses aren't so smart, I guess, because they tend to eat the stuff and get sick. 

So, of course, farmers and ranchers try to get rid of ragwort, particularly if they're in Australia or New Zealand. Both nations have regulations that not only encourage the eradication of the weed but, in some cases, require it. 

And about a century ago, these efforts caused an odd epidemic: farmers pants began to explode.

Modern land management, farming, and ranching techniques have ways of controlling ragwort -- they're not perfect, but in general, the efforts are effective (although temporary). Turn back the clock to the 1930s, though, and ragwort control was a lot more trial and error, with "error" winning more often than not. One of the chemicals used back then was sodium chlorate, a compound now primarily used to bleach wood pulp so that our paper is a bright white color. But sodium chlorate is also really good at killing plants. It's not a great herbicide because it doesn't discriminate between weeds and crops, but if you really want to nuke some ragwort, it'll work. We don't use it that way today very often because sodium chlorate is also toxic to humans -- you don't need to ingest a lot of it in order to be in a heap of trouble.

Oh, yeah, and because of the exploding pants. New Scientist sets up the problem perfectly:

In 1931, the peace and quiet of the New Zealand countryside was shattered by a terrifying new phenomenon: suddenly and apparently at random, men’s trousers began to explode. Some pairs detonated on the washing line, others as they dried in front of the fire. More seriously, some were occupied when they started to smolder. At first, there were just a few isolated reports, but soon the nation was in the grip of an epidemic of exploding trousers.