To remove rust from concrete without damaging nearby plants, use Singerman Laboratories Rust RemoverorIron OUT Outdoor, which are formulated to be safe for vegetation. For a natural, plant-safe approach, apply undiluted white vinegar or lemon juice directly to the stain, let it sit for 15-20 minutes, scrub with a stiff brush, and rinse thoroughly.

You need to adjust the spray on your sprinkler heads so they don't hit the concrete, or get a different type of head/spray pattern that will accomplish the job without hitting the concrete. The heads are typically pretty easy to adjust and switch out, if needed.