Chlorine works faster. I think chloramine, even though it is effective, is used more as an added safeguard with chlorine because it hangs around longer in the water which spends varied amounts of time traveling to your tap. Soaking the plants for a while may do what you want, but I haven't a clue as to the duration needed. If your plants can handle a bleach dip, you may be better off going that route. Bleach will kill just about anything in a relatively short time, including some plants, so read up a bit if you go with that method.