I like this idea, but the challenge will be building effective ammunition (I suspect depleted uranium might not be the best option here) for frequent engagement, and minimal (read: no) collateral damage.
Thinking about it, shotgun shells should work. Flechette rounds would probably be a good choice.
Thinking about it though, i could get absolutely burried in details rrying to think of counters and counters-to-counters haha.
It would probably work and be the cheapest solution.
After reading nogridbag's solution, I'm starting to think golf driving range nets[1] that are used to keep golf balls from flying into the road. You would need to put a top net and probably still have to have an active defense for that one drone to blow the net, the second to run into the plant situation.