Answer: It sounds like you are doing all the right things, but these plants take a lot of effort to eradicate. Digging the tubers and pulling seedlings should help, also be sure not to allow any survivors to go to seed. (An extreme method would be to sift the soil to remove additional seeds.) A heavy mulch should help reduce the number of seedlings that emerge. Finally, if you use glyphosate, make sure that you wait the full prescribed amount of time for the chemical to translocate to the roots and do its job and that the plants are in active growth when you use it. In addition to carefully following the label instructions you might also contact the manufacturer for any special usage instructions and to be sure you are using the correct form of the product for eradicating four o'clocks.
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