Gardening over Lateral Lines

Avoid hydrophilic, or water-loving, plants. A leach field is a series of drain lines that allow the effluent from the septic tank to drain over a large area of ground. Eventually the lines can fill with roots and become clogged, resulting in an inconvenient and expensive job of replacing them. Especially notorious for their ability to clog lines are willows, maples, elms, birches, and beeches.

Properly situated lines are located 12 to 15 inches below the ground surface, resting on beds of gravel 18 to 24 inches underground. This suggests that shallow rooted plants might be suited to growing over a leach field. Some possibilities include fibrous-rooted shrubs, such as boxwood or holly, or small trees, such as dogwood. Medium to large trees should be at least 10 feet away from the nearest line. Herbaceous, shallow-rooted plants, such as flowering perennials and annuals, would be unlikely to cause damage to the lines, though you shouldn't be too enthusiastic in tilling the soil when setting them out.

When drainage trenches are cut at a wooded site, homeowners are often tempted to leave existing trees standing in the spaces between the lines. Besides the possibility of roots growing into the lines, this is not a good situation because the trees, having lost a great deal of their anchoring roots when the trenches were cut, may blow over in a storm. When planting shrubs in a leach field, place them between the lines when possible. At 6 feet apart, there should be room for one or two small shrubs without crowding them right over the top of the line where they could eventually cause trouble.

What plants do well over a leach field? This is determined by the condition of the field and the composition of the tank effluent. A properly functioning field is well drained, without soggy spots. Most of the effluent should percolate into the ground, but some will evaporate from the surface. If a field tends to be water logged, plant species that tolerate moist conditions. For a system operating at a less-than-optimal level with effluent rising to the surface occasionally, a planting of leafy shrubs or grasses may help the action of the field. Any moisture pulled from the soil via evapotranspiration by the plants will decrease the drainage requirements of the field.

The effluent will vary in composition based on what is running down the drain. Many household chemicals are highly alkaline and will raise the pH of the effluent, while the activity of the bacteria in the tank tend to lower the pH. The pH of the effluent will affect the pH of the soil, so have soil samples from the drain field tested at least once a year so you can adjust the soil pH as necessary for the plants growing in that area.

Salt levels are likely to be high no matter what the pH is, so plan to use salt-tolerant plants. Some plants that are both salt and moisture tolerant include hollyhock, bee balm, violet, arborvitae, redosier dogwood, inkberry holly, and blueberry. If your field is well-drained, you might try including yarrow, columbine, chrysanthemum, delphinium, daylily, peony, clematis, wisteria, and rose. Use of fertilizer may be cut down for plants growing over a leach field because many of the salts are forms of N-P-K nutrients.

Sometimes the only place to put a vegetable garden is over a leach field, raising the question of bacterial and viral contamination from the effluent. Soils vary a great deal in their ability to filter viruses and bacteria from the effluent. Clay soils work best, eliminating bacteria within a few inches of the drain lines, but sandy soils may allow bacterial movement for several feet.

A properly operating system will not contaminate the soil with disease causing organisms, BUT IT IS VERY DIFFICULT TO DETERMINE IF A FIELD IS OPERATING JUST AS IT SHOULD AND SOME PRECAUTIONS SHOULD BE TAKEN. Do not plant root crops over drain lines. Leafy vegetables could be contaminated by rain splashing soil onto the plant, so either mulch them to eliminate splashing or don't grow them. Fruiting crops should be safe; train any vining ones, such as cucumbers or tomatoes, onto a support so the fruit is off the ground. Dwarf tree fruits would be ideal for a leach field because their roots are not vigorous and are unlikely to cause problems in the drain lines and because the fruit is borne well off the ground. Thoroughly wash any produce from the garden before eating it. Do not construct raised beds over the field; they might inhibit evaporation of moisture.

Finally, decide what is more important to you - the leach lines or the plants. One gardener decided that the cost of a new system was inconsequential when compared to the enjoyment and shade she would get from a grove of silver maples growing near her leach field in the years before the lines clogged. That choice is up to each individual gardener.