Sunday, September 7, 2003 - Page updated at 12:00 AM

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The Garden Designer / Phil Wood

Woodsy privacy border near septic drain field is a tall order

Q: We desperately need privacy and would like to plant a tall, woodsy evergreen border. Our problem is how to block off the neighbors along two of our property lines that are adjacent to our septic drain field. The pipes are 13 feet from the side fence running parallel to it and only 6 feet from the back fence. Can you suggest trees or tall shrubs that can be planted in the space between the pipes and the perimeter fence that will give us privacy? A previous column mentioned choosing fibrous over invasive root systems in septic-area planting. As a neophyte gardener, I've not found any books or field guides that mention how to distinguish between them.

A: You certainly have a limited space to accomplish an important task. I don't think you can grow woods in a 6-foot-wide space. In such a narrow space, consider bamboo. You would need to provide controls to keep it from encroaching into the nutrient- and moisture-rich septic field. One way to do this is to maintain a shallow trench and cut off any roots that cross into it. Another way is to place a root barrier between the bamboo and the drain field. Bamboo nurseries sell 18-inch-deep plastic barriers for this purpose. The nursery can suggest appropriate types of bamboo for your situation.

You also could grow small conifers or broadleaf evergreens with a root barrier in place, but certainly not forest giants such as Douglas fir, cedars or cypresses, because they would not thrive with a limited root range and might topple over when they reached any height.

A common hedging plant for narrow spaces is pyramidal arborvitae (Thuja occidentalis 'Fastigiata,' or its greener cultivar 'Emerald Green'). From my observation, it forms a dense root mass that would run into the septic field unless contained, but could provide a decent screen with a confined root run.

Perhaps your neighbors feel the same need for privacy and could be persuaded to plant a screen on their side, too, although if they planted invasive rooted trees, you'd have the same problem.

In the wider 13-foot space you could get by with larger plants, still with noninvasive roots. I'm not sure there is an easy way to tell a fibrous rooted plant from an invasive rooted one, except by the experiences of those who have grown them. Trees not to plant, because they are known to have invasive roots, are beeches, birches, poplars, red maples, silver maples and willows. More suitable trees would be crabapples, dogwoods, small pines and sourwood (Oxydendrum arboreum). One source I found mentioned hemlock as a possibility, too.

Also look at plants that do not need much summer water, such as upright junipers. Hollywood juniper (Juniperus chinensis 'Torulosa') reaches 15 feet in height and 10 feet in width. Wichita blue juniper (Juniperus scopulorum 'Wichita Blue') reaches 12 feet high by 6 feet wide. These will need good drainage, so check your planting area for free-draining soil.

Fibrous rooted shrubs include hollies and English boxwood (Buxus sempervirens). Boxwood is slow-growing but will eventually reach 15 feet or so.

One trick to control the roots of your plants is to prepare the soil well on the side away from the septic field so they will grow in that direction. Leave a 5-foot section of nonimproved soil between the plants and improved soil so the roots won't be tempted toward the drain field.

On another note: A reader called to comment on my discussion of palm trees several columns back, and I stand corrected. Windmill palm (Trachycarpus fortunei) is the hardiest commonly available palm for the Northwest. Mediterranean fan palm (Chamaerops humilis) is less hardy and would not survive a hard winter, so it is best used in containers. I very much appreciate comments from readers that keep accurate information flowing.

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

Copyright © 2003 The Seattle Times Company


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