Ask Mr. Smarty Plants

If we may be permitted an editorial comment before we answer your question, Pampas Grass, in our opinion, is a bad choice for landscaping anywhere. It is native to Argentina, and could become the monster that ate Tupelo. See this article Campaign Launched Against Pampas Grass to hear the points against it.

On to the Louisiana Swamp Iris; we had no plant by that common name in our Native Plant Database, but believe it might be Iris fulva (copper iris), which is, indeed, a wetland flower native to both Louisiana and Mississisppi. If there is a low area at the base of the hill and NOT in the street, we would suggest a small rain garden. 

Simply put, a rain garden is a depression in the garden that slows down the flow of water, allowing it to infiltrate the soil instead of allowing it to rush downhill carrying topsoil and pollutants with it.  The depression is planted with plants that are adapted to conditions alternating between very wet and very dry. You will find a very comprehensive publication about rain gardens, along with instructions and plant lists published by the Clean Water Campaign in Atlanta by following this link. Clean Water Atlanta also has a publication and a suggested plant list.

There are several plants, including the copper iris, that can withstand the sometimes wet, sometimes dry area at the bottom of a drainage area. If the water goes straight from the hillside into the street, we don't think an iris would be very successful, because its roots are rhizomes that sit high in the ground and can be washed away. What you need are fibrous-rooted native grasses (NOT pampas grass) that will hold the soil and help prevent erosion, as well as use the moisture to support themselves. Strictly speaking your Rain Garden would not be to clean stormwater pollutants, but to hold spring water in your soil. We are going to our Native Plant Database and find grasses native to the Lee County area, USDA Hardiness Zone 8a. Follow each plant link to our page on that plant for information on projected size, propagation, light requirements and so forth. Because you are dealing with a slope, you will probably want to put in plugs of the grasses you choose, because seed would likely wash away with the first good rain. 

Grasses for Tupelo MS:

Bothriochloa laguroides ssp. torreyana (silver beardgrass)

Carex blanda (eastern woodland sedge)

Chasmanthium latifolium (Inland sea oats)

Elymus virginicus (Virginia wildrye)

Schizachyrium scoparium (little bluestem)

Sorghastrum nutans (Indiangrass)

Tridens flavus (purpletop tridens)

From our Native Plant Image Gallery:

Iris fulva

Bothriochloa laguroides ssp. torreyana

Carex blanda

Chasmanthium latifolium

Elymus virginicus

Schizachyrium scoparium

Sorghastrum nutans

Tridens flavus