Dividing Pampas Grass

I have three huge pampas grass plants. How and when can I divide these?

Several different ornamental grasses are often referred to as pampas grass. The pampas grass of the southern and western United States (Cortaderia) is not reliably hardy in colder regions and if it does survive usually does not need dividing. Miscanthus and Erianthus ravennea (Saccharum) are often called pampas grass by northern gardeners. Both of these are warm weather grasses that prefer to be divided in spring or early summer.