the sensing side is reasonably easy, but the automatic watering gets hairy. OP was proposing one pump+reservoir for each plant, which adds up the cost and makes

the sensing side is reasonably easy, but the automatic watering gets hairy. OP was proposing one pump+reservoir for each plant, which adds up the cost and makes powering the system more demanding. And you probably want some sensing in the reservoirs too so that they don't run dry. Alternative would be to do plumbing to create a water network, but that brings its own challenges especially in residential setting where plants are scattered all over the place.