For those who want a "green" solution to weeds that actually works I can confirm that white pickling vinegar (7% acetic acid) makes an amazing plant killer. Spray it on any leaf you want to kill on a hot sunny day and within 10 minutes it's starting to turn brown and shortly afterwards the leaves are completely dead (none took longer than overnight). It works by stripping off the waxy coating on leaves that prevents them from dehydrating too quickly. No need for expensive or potentially dangerous chemicals, just make sure it's at least 7% acetic acid (stronger would be better but not as available but "Pickling" vinegar is usually at least 7%) and put it in a cheap plastic spray bottle and you have a plant killer. It will kill almost any leaf you spray it on depending on how waxy they are some will be more resistant (that's why 20% would be even better) but it works great on dandelions, thistles, grass etc etc. For dandelions just spray the leaves and the center of the flower. Dandelions and others with a long tap root will need a subsequent application because it won't kill the root, just all the leaves, but as long as the leaves are dead the root will die in pretty short order. I'm going to hack a Swiffer WetJet mop so I can refill it with vinegar and use it's built in battery powered sprayer to do large areas conveniently (I have a lot of gravel trails in the back of our property that I want to keep vegetation free), the spray bottle gets tiresome after a while. There are commercial formulations based on vinegar now and they contain surfactants to make the liquid spread more evenly on the leaf and not bead up, I'm going to experiment with that as well, maybe dish soap or citrus based organic cleaners. Kinda makes you wonder why there are so many complex alternatives like Roundup when plain old vinegar works so well.