Cat peeing outside litter box

Ash

My old cat will not stop peeing in our house plants. We have two litter boxes and both of them are clean (we clean them every day) but he keeps in going in the plants instead. We threw out the plant he was peeing in cause it was dead (from his pee) now he’s digging into the plant in our bedroom, which has boards in it specifically to stop him from doing this. He’s been to the vet he does not have a uti or anything like that. Any advice?