Ask Mr. Smarty Plants

Panicum virgatum (switchgrass) - perennial, 3 to 6 ft. tall, medium water use, sun or part shade, native to Virginia.

From Condition Comments on our webpage for this plant:

"Switchgrass is a clump-forming, warm-season grass. It has finely textured, reddish-purple seedheads. It tolerates seasonal poor drainage, making it a good choice for a dry creek bed or rain garden. Before new foliage emerges fresh from the base in spring, many gardeners prefer to cut back old vegetation to six inches above ground. Flower panicles are open, airy and delicate making it a very attractive accent plant in a garden or meadow."

'Cloud Nine' and 'North Wind' are likely trade names give to named selections of the plant, and should have the same characteristics as the original.  Patience is probably the way to stimulate thicker clumps. Since it is a clump-forming grass, you know it's going to do that sooner or later. You mentioned that the plants were very root bound, and we assume you are cultivating them in pots preparatory to replanting in the soil? If that is the case, we would suggest breaking up the clumps when you plant them in their permanent spot. These smaller clumps could then be planted fairly close together and would pretty quickly form a thicker body. If they are already in the ground, we would recommend leaving them alone, trimming them in late Winter, and let them do their thing. 

This website from Floridata, Panicum virgatum, gives considerably more information about this plant. Give attention particularly to the cautions about avoiding over-fertilizing, especially with high-nitrogen fertilizers.

Panicum virgatum

Panicum virgatum

Panicum virgatum

Panicum virgatum