Conceal on-site septic systems with decorative grass

Published on Wed, Jan 20, 2010 by Peg Keenleyside

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The talk of the town over the past couple of months has been the impact of the new Whatcom County septic system regulations, with more than a few folks having to upgrade their existing system or install a new one.

Cottage owners seem to have been particularly impacted by these new environmental protection measures.

On the often-smaller cottage lot, septic systems can leave you somewhat aesthetically challenged in the landscaping department. With this in mind, this month we offer a sampler of septic field and mound landscaping tips and some online resources to get you planning for this year’s adventures in gardening.

A drive around the county will tell you that most people don’t generally landscape their septic mounds (alas), however the Washington State Sea Grant Program and WA State University Extension Internet sites both note that planting on a septic area is actually encouraged because plants help in oxygen exchange and evaporation - the exchange of air and water being one of the key functions of your septic “eco-system.”

Hardscaping, however – the use of barks, rocks, gravel or paver stones set in sand on your mound or field; including in your reserve area – are definitely out.

Non-starters for a septic area garden also include trees and large shrubs since their invading roots can, over time, damage or break pipes. Avoid mat forming plants like the rhizome-spreading Bearded Iris and invasives like the charming but insidious English Ivy.

Of course, high on the no-fly list of gardening options for septic fields and mounds are vegetables and other plants intended for eating.

Shallow rooted grasses appear to be among the most recommended choices at both the Sea Grant and Washington State Extension sites, and while most people around the county seem to have opted for the ubiquitous lawn grass, there are a wide variety of native and drought tolerant ornamental grasses to consider for achieving height, foliage variety and color as you run up a wish list to take with you to the nursery or garden center this spring.

Shallow rooted drought tolerant ground cover plants, such as the native Kinnickinick (Arctstaphylos), are also among recommended choices for septic areas since watering your field or mound on a regular basis is not endorsed; for the reason that excessive water will inhibit system performance.

Grass choices for height interest include Fountain Grass (pennisetum alopercuroides), with its two-foot tall arching stems and bottlebrush “flowers.”

Blue Oat Grass (Helictotrichon sempervirens) with its stiff blue-gray evergreen blades is another recommendation.
For a good list of other recommended shallow-rooted grasses and plants for various degrees of sun exposure, download the WSU Extension article “Landscaping Your Drainfield” available at

Get some fabulous visual inspiration for designing with grasses and native plants with a visit to the Dutch master gardener Piet Oudolf’s site: piet-oudolf. Odoulf also has several great books available, Gardening with Grasses, being one I’ve enjoyed perusing over the winter.

Before-You-Plant tips

Whether you have an existing grass covered field or a bare mound of earth staring at you from the yard, the “call before you dig” maxim seems to be the recommended order of the day before you put your septic area planting plan into action.

In the case of existing grass covered fields or mounds, you may need to get acceptable digging and soil add-on depths from the Whatcom County on-site sewage system (OSS) Program office for your specific sub-area (360/676-6724).

The general published recommendations for Washington that we found allow for no more than a six inch depth for digging, and no more than two to three inches for topsoil add ons.

If you have a new system, be sure to take the time to check specific dig or soil add-on depths with your septic system designer/ contractor to make sure your plan is compliant with your system’s operating guidelines and – we emphasize – warranty conditions.

In your planting plan, consider incorporating some garden-friendly marking structures for your septic components that need inspection, whether these are located at grade or not; a sundial, a free-standing pyramid trellis or a birdfeeder mounted on a tall, colorfully painted stake rising up out of a drift of long stemmed grasses are all possibilities.