Have you tried a sodium chloride concentrate? That's salty water -- the saltier, the better.
Salt? On starthistle? Get serious, I want to kill the stuff but I don't want to destroy my land. Do you know how many plants are in a typical infestation? Do you know how small they can be when they breed? Do you know how hard they are to find? Do you know how many seeds a single plant can produce? Do you know how hard they are to remove mechanically? Do you know that they produce a neurotoxin that is worse than the herbicides used to kill it?
I've heard everything from fire to vinegar. The fact is that most pest infestations are complex and dynamic enough to require custom treatment plans. There is no substitute for being trained and equipped for multiple methods, and there is no substitute for intimate familiarity with the land.