It depends on the weed. Road salt kills many plants on the side of the highways but some survive and they are brown and ugly.

Also, rock salt will slowly leach into the ground so it will stop being effective and might kill desirable plants downstream.

You are best to use salted water directly on the weed rather than rock salt.

I found a good blog with other alternatives and attached below.
Source(s):
http://www.thisgardenisillegal.com/2006/05/7-deadly-homemade-weed-killers.h...

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Yes rock salt kills weeds and any other plant it comes in contact with including tree roots so be very careful if you use it.

BTW If you have a home with an older sewer system and have trouble with roots clogging it up every now and again rock salt is a great remedy for this, dilute a cup or two of rock salt in warm water pour down your drain and try and let it stand for a day or longer and your root problem will be gone.
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You can, but it does leach into the surrounding soil and eventually make it infertile, simply because there can be such high concentrations of it. And unless you want to take an enormous amount of trouble and expense removing the salt, it's not a good idea on a wide-spread basis.

Salting fields is a warfare tactic going back as far as written history: Rome used it against Carthage in the Punic Wars; it's mention in the Bible; Assyrians did it; it was used by Spain in the Middle Ages. Even the Red Army used it in the last century. Rome specifically did it so that the land around Carthage could never again support a city.
Source(s):
http://home.uchicago.edu/~vofunk/EnvironmentIstanbul.doc.

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