I know ornamental grasses are generally okay to plant on drain fields, but does this include larger grasses such as pampas grass? How about larger Miscanthuses? Arundo donax? There are mature pampas grasses already planted on the site I’m planning for. I’m wondering if they should really “go,” and if so, is there something else that would give some height in this area without impacting the drain field.
Here is a link to the Miller Library’s Gardening Answers on planting ornamental grasses in drain fields.
Here is the relevant section:
Grass is the ideal cover for drain fields. Grasses can be ornamental, mowed in a traditional lawn, or left as an unmowed meadow. You can also try groundcovers and ferns.
The key to planting over the drain field is to select shallow-rooted, low-maintenance, low-water-use plants. When tank covers are buried, keep in mind that plantings over the tank–from inlet to outlet–will have to be removed every three or four years for inspection and pumping.
Planting your drain field will be much different from other experiences you may have had landscaping. First, it is unwise to work the soil, which means no rototilling. Parts of the system may be only six inches under the surface. Adding 2 to 3 inches of topsoil should be fine, but more could be a problem. Second, the plants need to be relatively low-maintenance and low-water use. You will be best off if you select plants for your drain field that, once established, will not require routine watering.
SOURCE: WSU Cooperative Extension – Clallam County
Thurston County, Washington, has some information about landscaping a drain field, including plant suggestion.
Additionally, the Pacific Northwest Gardener’s Book of Lists (1997, by R. & J. McNeilan) offers a number of groundcover lists for various situations, including groundcovers for dry sites, slopes, and sun and shade. The Miller Library has this book.
Here is an article from King County.
I would be concerned that the larger Miscanthus plants might develop massive root systems which go far too deep for the site’s needs. Also, Arundo donax is an invasive plant, so you should not use it in your landscape. The Washington Noxious Weed Control Board has further information on this plant.
You may wish to remove the Pampas grass (Cortaderia) as well, as it is potentially invasive, has a deep root system, and is a prolific reseeder.