Dear Master Gardener; I have a lot of black medic in my yard. I do not have grass but river rock that was put in by the previous owner. Unfortunately, no landscaping materiel was used underneath so the black medic is liking the compacted sandy soil. I remove as much as I can by hand weeding, but can not keep up with the whole yard. Could you suggest a way to keep this problem under control without using a lot of toxic chemicals? Thank you K Bottomley
That's quite the challenge with the Black Medic growing amidst the river rock. You should concentrate your hand pulling on Black Medic plants that are not yet flowering. You definitely want to try to prevent them from going to seed.
You could also try spot spraying with a household white vinegar (no more than 5% acetic acid) and dish soap solution - Mix one gallon household white vinegar with one tablespoon dish soap (dish soap is a surfactant that will help keep the vinegar on the plant longer). This is a contact solution so it may kill the above ground parts but may not prevent resprouting from the roots. Use a spray bottle or a pump sprayer. Try to spray young plants for best results. Remember it is non-selective so don't get it on any desirable plant.
You could also put down black plastic and try to "cook" the plants using the sun's heat. Maybe try covering a small part of the river rock (and Black Medic plants) as an experiment to see if it works. If so, try it in other spots.