What's your existing soil and drainage like in the spot where you want to install it? It's favorite setting is in fertile, damp but well-drained soil in as much sun as you can give it. "Fertile" does not necessarily mean full of compost. I wouldn't use mulch as an additive to the soil. If you need to increase drainage, go down as far as you can possibly dig (Pampas builds very deep root systems), and use inert material like sand and chicken grit as additives to good loamy soil. What you're after is healthy soil which has all the needed ingredients for the support of plant growth:

Fertile soil has the following characteristics:

It is rich in nutrients necessary for basic plant nourishment. This includes nitrogen, phosphorus and potassium.
It consists of adequate minerals such as boron, chlorine, cobalt, copper, iron, manganese, magnesium, molybdenum, sulphur and zinc. These minerals promote plant nutrition.
It contains soil organic matter that improves the structure of the soil. This enables the soil to retain more moisture.
The soil pH is in the range 6.0 to 6.8.
It has a good soil structure which results in well-drained soil.
It consists of a variety of micro-organisms that support plant growth.
It often contains large amounts of topsoil

Although it appears to be contradictory, "retaining moisture" and "well-drained" do not compete with each other. The moisture in healthy soil is retained at a molecular level in organic matter, while well-drained means it doesn't puddle or remain soaked.