How much room do you have for them to grow?

A couple of feet would be enough, but the plant can get by with 10 to 12 inches if needed, but may not grow to full size.

Kt

The concern is for the water-proofing system on the roof. In layers from the surface: 2 feet of soil; Visqueen; 4" styrofoam; 4" bentonite clay; reinforced concrete.

Roots can wreak havoc with the system if they penetrate into it. (ask me how I know! We developed a leak and the culprit was a very lengthy elm root growing in from beyond the edge of the roof)

Thanks for the help!

Rufa is a clumper, so the rhizomes will be no problem and the roots grow more fibrous instead of growing like a tap root.

A few roots can grow 2 feet deep, but it should be no problem for you.

If you are still concerned, you could put a barrier down before planting, which would keep the roots from growing past it.

Good Luck

Kt