When a pampas grass plant is 4 to 6 years old, the center of the clump begins dying. This is signaled by new growth appearing only around the younger, healthier perimeter of the plant. Cortaderia selloana can be propagated in August or September, but the best time is in the spring after the last predicted frost but before the appearance of new growth. This will give the plant an entire growing season to recover from the process and establish its root system well before winter dormancy. Wear heavy gloves and a long-sleeved shirt when working in such close proximity to pampas grass. The stiff leaves have spiked edges that can inflict nasty cuts.
Cut the pampas plant back to 1 to 2 feet tall so that you can clearly see its base.
Examine the stems at soil level. The inner section of the clump will be dead or dying and will be surrounded by healthy younger shoots.
Cut into the clump vertically with a sharp spade, separating a healthy chunk of younger growth from the center of the plant. Dig the division out of the ground.
Plant divisions at exactly the same depth that they occupied while growing from the mother plant. Water thoroughly and keep the soil evenly moist throughout the growing season.