We normally see black medic in shade areas only (purple leaves & looks like oxalis which has light green leaves). Ester herbicide & a Shurfo backpack w/flat fan nozzle fries it. Are you sure it's not oxalis?...cuz oxalis is most common in full sun.
Regarding the PG, I do not think it provides proper coverage. A z-spray or a T3000 would be much better. When we see black medic, our T-Men set the cruise control & drive around while using the spray wand (eliminating the need for lugging around a spot sprayer). my 2 cents
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