Answer: In most instances using plastic, tarpaper or even weed barrier fabric under the path gravel will not stop the weeds from encroaching either over the surface or by seed, and those that do root will run along between the barrier and the gravel quite happily.
In my experience, there are several approaches you can try. One is to rake the paths periodically (about twice a week) to disturb any germinating weeds. Another is to change the pH sufficiently to make it an inhospitable environment for most plants. This is not a good idea if it adjoins closely planted areas or there is concentrated runoff, however. Finally, you might have luck using corn gluten (or a commercial product) to act as a pre-emergent inhibitor.
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