English garden over septic field?

Q: I have a large, gently sloping front lawn, which is also the area of my gravity septic drain field. I would very much like to look out over an English garden rather than the lawn, but do not know what options are available that will continue to allow the drain field to operate properly. Also, are there any health concerns when working with plants near a drain field?

The weather here in Hansville is mild and on the edge of the rain shadow. We experience some offshore breezes but considerably less rain than Seattle does. The salt air does affect some plant species — especially roses! I would appreciate any suggestions you may have.

A: In considering a plan to make changes in your garden, it is helpful to understand what a drain field is — a series of buried perforated pipes. They are laid a minimum of six inches below the surface in gravel trenches that distribute the septic-tank effluent over a large area into the soil. Plants can help a septic drain field function by removing moisture and nutrients from the soil.

Grass is the plant that is usually recommended because it has a root system that will not clog the pipes.

Herbaceous perennials and shrubs may be used if they do not have invasive root systems, so a mixture of perennials and small shrubs can be used to give an "English garden" look, assuming you mean a cottage garden design, which is based on easily grown plants informally arranged.

To avoid shrubs with invasive roots, choose those with fibrous roots such as small-leafed hollies and boxwoods. This would give you a neutral green foil for your perennials. Small-leafed rhododendrons would tolerate the sun and have fibrous roots, too, although they would not be happy in the wetter areas because they need good drainage.

You will want to create a path system to provide a framework to the composition, and to define and give access to your beds. Stone paths would inhibit the evaporation necessary to the drain field.

Use the existing lawn for grass paths and terraces. You may want to edge the grass paths with brick or stone to provide a mowing edge and a touch of definition to the paths. Adding a bench and a few pots or urns will give a stylish touch, and you might build a small arbor in the center for focus.

You will find it helpful to know how far apart and how deep the pipes are so that you could plant larger shrubs between them. You may have a drawing of the field, but it might not be completely accurate. Careful probing with a metal rod will help you locate the pipes.

Preparing the ground for planting is a challenge over a septic field. The usual 4-6 inches of mulch tilled in 8-10 inches deep would disrupt the pipes and inhibit evaporation of moisture. I think the best thing to do is apply an inch of mulch and plant.

For special shrubs such as roses, you could amend 4-6 square feet around the plant, trying to stay between the rows of pipe.

Mediterranean plants such as rosemary and lavender would enjoy poor unamended soil but may not appreciate the excess moisture.

Planting small test areas with a few plants would make sense so you have an idea how the plants perform before making a large investment. There may be drier areas at the end of the drain field that is away from where the effluent enters the system, and this would be the place to put plants that like dry feet.

Selecting plants tolerant of salt air would make sense in drain-field gardens anywhere because the effluent may cause a buildup of salts in the soil.

The rugosa rose and all its splendid cultivars should do well in your seaside location. Common yarrow, Achillea millefolium, most of the artemisias including Artemisia ludoviciana 'Valerie Finnis'; cottage pinks, Dianthus plumarius, and Jupiter's beard, Centranthus ruber, are just a few of the perennials that should do well for you.

As for health concerns, use caution and common sense. Wearing waterproof gloves when weeding or digging with care in this area can protect your hands from any harmful organisms.

Phil Wood has a degree in landscape architecture and designs and builds gardens. Call 206-464-8533 or e-mail thegardendesigner@seattletimes.com with your questions. Sorry, no personal replies.