I used to take care of the fish tank at a vet clinic that had plastic plants. I'd throw them in a bucket, dump about 1/3 of a gallon of bleach in and fill a little over 1/2 full with warm water. Then I'd do some work on the tank and go back and scrub and rinse the plant. Then I'd soak them in fresh water and let them dry before I put it them back in. If I was short on time I'd add prime to the fresh water.