Glysophate (Round-up) will work well but as stated you must use it in high concentration.

A product called "Cross Bow" ( a 2-4-5T generation, I think) is a brush killer where glysophate is a grass killer) and can be applied at much lower concentrations. Watch out for "Tordon" as even the "new model" has some leaching problems. Not that is not by far the best brush killer out there next to Remedy and Valpar.

Several of these require applicator certification to purchase.