one plant completely filled my round 5 foot wide 3 foot deep pond, top to bottom- the root mass weighed a ton and came out in a solid bowl shape. we had to hack it into chunks to remove it. it was in there 3 years. no room left for fish!
if you like them, i suggest you contain them in a pot that you can remove and replace every year. min

Mine escaped too- and the rhizomes can travel fifteen feet under ground before popping up where there is water. I spend much of my summer pulling it out.

If it will have to go three feet deep, though, it may not get outside of the bed. I have not found any rhizomes more than two feet deep. But did you mean to say three inches? If you meant the concrete is three inches thick, then the reeds will certainly escape. You could try Bamboo Guard, but it's expensive and it's a lot of work to dig down 28". Ask me how I know. Ugh.

May I suggest a few alternatives? If the bed is big enough, Cape Rush is very beautiful, and there is a smallish variety. Louisiana and Spuria irises are upright and spiky. Horsetails also fall over easily and look untidy. They get rootbound easily and don't grow as well after a short period of time. I still love them, though.
Renee

Much safer to only plant in a container, or do use a bamboo rhizome barrier to the full 30" typical depth, and be sure to seal/glue the overlapping joint. It is definitely a real pain to weed out once it has escaped.

To leave the front of the bed the plant would have to travel down 3' and forward 8', all under concrete, before reaching lawn.

It can do that, yes.

Thank you everyone so much for the replies, very helpful!

Min, curious why it would need to be replaced yearly? I'm sure you can guess I'm not a fantastic gardener so please excuse the question if it has an obvious answer =)

Renee, I did mean 3 feet, except that it's only raised that high in front. It's situated on a slope, so on the back side the reeds would have easy access to escape. I've used bamboo guard before (on bamboo) but am worried it could still escape. The cape rush might be a good alternative, though I admit to still preferring the look of the horsetail =/

megan - because it grows so fast and gets so heavy so fast.
i think you would do very well to take renee's advice and plant one of her alternatives. min