Remember the old Erma Bombeck book, The Grass is Always Greener over the Septic Tank? I’m sure that she meant it as a joke considering that comedy was her stock in trade but perhaps there some truth in this. Think about what your septic tank does and how it works. All of the household waste goes into the septic tank, both liquids and solids. The solids sink to the bottom, the lighter weight scum rises to the top. In between is a layer of water. As more water enters the tank the level rises and it drains through a series of baffles that allow only liquids to exit. This water goes into a series of perforated pipes that allow drainage into a gravel leach field. This is where most of the harmful bacteria is removed by soil bacteria before the water goes into the ground water system. If there is a good layer of soil over the leach field some of this water moves up into the soil providing an additional source of water for anything growing over the leach field. Plus it may enrich the soil.
Under ideal conditions what has been produced is that elusive moist but well drained area in which a number of plants thrive. So perhaps it would be more accurate to say that the grass grows greener over the leach field. Because it is a well-drained area problems that develop with roots in wet areas that retain water are eliminated. The soil might also contain additional nutrients and have a closer to neutral pH than the rest of the yard in low pH areas. These conditions will vary depending on what goes into the septic system with household cleaning products tending to raise the pH.
Is it safe to plant in this area? That depends on what is planted and certainly planting it is a better idea than leaving it bare although, as we all know, nature does not allow ground to remain bare for any length of time. Although evaporation takes care of most of the excess water plants will prevent erosion and keep the ground from becoming overly wet. Plants also encourage oxygen exchange in the soil. Oxygen is necessary for the health of the soil bacteria that cleans up the effluent.
So what can be planted in the leach field? It is definitely not a good place to put a vegetable garden. There is always a chance of contamination and it is advisable to wear gloves when working in soil that is part of a drain field. A healthy leach field should be safe but caution is advised. Vegetable gardens also require tilling and cultivation, which are not advisable over a drain field. The less the soil is disturbed the more likely the septic system is to work properly. Putting in a raised bed is also not recommended as the excess soil can interfere with the function of the field preventing oxygen exchange and evaporation.
A flower garden is a possibility, although it depends on how deep the soil is. There may be as little as 6 inches of soil over the leach field and there is rarely too much more. Planting perennials that require frequent dividing would require the replacing of soil that was removed when the plants were dug so soil volume can be maintained. Conversely, excess soil may not be added, nor can the area be heavily mulched. The inability to safely mulch the area might make it difficult to control weeds without the type of cultivation that is not appreciated.
Vigorous digging in this area is not recommended so the size of the root system of a plant that might have to be removed or divided should be taken into consideration. While grasses are a good choice, large grasses such as the various Miscanthus species have far too massive a root system for safety. Some of the fescues or switch grasses, for example, would be a better choice. The primary consideration is to choose plants that will not interfere with the actual function of the system by clogging up the drainage pipes with roots. Shallow rooted plants that need little attention are appropriate. While annual would qualify their constant need for care might cause the compacting of the soil, which is also detrimental. Annuals frequently need watering, and adding water to this area is not beneficial. Also, if water softeners are used the salinity of the soil might be higher than usual and this should be taken into consideration when choosing plants.
It is also a bad place to plant deeply rooted tree and shrubs. These will get into the pipes and cause problems. Trees such as, red and silver maples, beeches, birches, elms and poplars should be kept as far away from this area as possible. Weeping willows are the poster child for trees that should not be planted anywhere near a septic system. They, as well as pussy willows and Japanese willow shrubs, will seek out a water source with their fast growing roots and clog up the drainage pipes. It is probably best to keep the drainage field free of trees or shrubs. However, if this is the major planting area on a smaller lot then choose fibrous rooted shrubs and small trees. Dogwoods, Japanese maples and eastern redbuds are potential trees and azaleas, boxwoods and hollies are acceptable shrubs. It is best to plant them between the drainage pipes rather than over them if at all possible. And do remember that shading the drainage field or allowing a leaf buildup may interfere with evaporation.
While ground covers can be a viable solution care must be taken in what is planted. Choosing a dense ground cover such as ivy or pachysandra may interfere with evaporation of excess water. They will also catch leaves, which can further acerbate the problem and act as a mulch interfering with oxygen exchange. Depending on whether the area is in sun or shade, some suggestions are lungwort, thyme, low growing catmint or lily of the valley.
The ideal is to simply plant grass over the leach field. If landscaping considerations make this undesirable choosing low maintenance plants with shallow root systems are the next best thing. A mixture of wild flowers, bulb and small grasses would be ideal for a more natural landscaping. For a more formal look at least choose native plants. These will require little to no fertilizing and less maintenance than nonnative plants. Joe Pye weed, black-eyed Susans, lambs ear, liatris and New England aster are all plants that are low maintenance and will cause little trouble in a drainage field. While they may be plants that spread, they can simply be mowed off if they expand over their allotted area and their root systems are not likely to cause the kind of problems that those with septic systems dread. Many of these plants are beloved by bees and butterflies so they will be serving a double purpose.