A Question of the Week


Planting over a septic leach field (drain field) is possible if it is done with care. If you have limited space on your property where you can garden, the leach field may be the only spot for landscaping. Growing shallow-rooted plants over the drainage area is recommended because they help remove excess moisture and nutrients from the soil and reduce erosion. Although turf grass is the typical choice, a variety of other herbaceous perennials, annuals and groundcovers can be safely and effectively planted. Vegetable gardening over a leach field is not recommended.

About Septic Systems

In rural areas where city sewer lines are not available, most homes have their own septic systems that consist of a septic tank and leach field. The septic tank breaks down organic matter and separates oil, grease and solids from household waste water. The remaining effluent is discharged from the septic tank to a series of underground perforated pipes that allow the liquid to slowly drain into the soil. In a properly functioning septic system, percolation through the soil removes harmful bacteria and nutrients before the water reaches the water table.

Planting Considerations

Planting over a leach field deserves special consideration because plant roots can clog the drain pipes and potentially damage the drain field – an expensive problem to fix. Many herbaceous perennials are pretty safe options because their roots will not grow deep enough to reach the pipes. Drought tolerant species are preferred because they do not require much irrigation and their roots will not attempt to reach into the constantly saturated soil around the drain pipes. Avoid water-loving plants that are apt to try to find additional moisture. Additionally, limiting the amount of water applied over the leach field is important, because saturating the soil may reduce effluent evaporation and increase the risk of groundwater contamination.

Placing trees or shrubs over or near the leach field is risky. Woody plants have deeper roots that may clog drain pipes in relatively short order. Water-loving species are especially chancy and should be avoided, such as willow, poplar, elm, red and silver maple, birch and beech. If you simply must plant a tree, position it towards the end of the drainage line where there is less water to draw roots in the direction of the leach field. Try to consider how far the roots will spread as the tree matures. Roots will typically extend at least as far from the trunk as the tree is tall.

Before planting, it is a good idea to check the soil pH. The detergents and cleaning products that go down the drain are usually alkaline and may elevate the soil pH over time. Depending on what you want to grow, you may need to amend the soil to adjust the pH. Moreover, household effluent generally contains high levels of salt, especially if you have a water softener. Plants that can tolerate salt exposure are more likely to thrive.

Vegetable gardening over a leach field is a bad idea. Though properly functioning septic systems won’t contaminate the soil with harmful pathogens, there is no easy way to guarantee that the crops grown over a leach field will be safe to eat. Furthermore, vegetable gardeners may not be keen on growing their food plants in soil that is frequently inundated with household chemicals. Unfortunately, placing raised beds over the drainage area isn’t a good solution either. The added soil depth of the beds may inhibit evaporation and limit the effectiveness of the septic system.

Suggested Perennials

Astilibe species
Epimedium species
Barren strawberry
Waldsteinia ternata
Penstemon digitalis
Rudbeckia hirta
Blanket flower
Gaillardia species
Blazing star
Liatris species
Butterfly milkweed
Asclepias tuberosa
Nepeta racemosa
Aquilegia species
Geranium species
Hemerocallis species
Dianthus species
Globe thistle
Echinops ritro
Solidago species
Hens and chicks
Sempervivum species
Hosta species
Knautia macedonica
Lamb’s ears
Stachys byzantina
Lupinus species
Moss phlox
Phlox subulata
Verbascum species
Papaver species
Purple coneflower
Echinacea purpurea
Russian sage
Perovskia atriplicifolia
Euphorbia species
Sedum species
Coreopsis species
Wild bergamot
Monarda fistulosa
Woodland sage
Salvia nemerosa
Achillea species

Got questions? The Ask UNH Extension Infoline offers practical help finding answers for your home, yard, and garden questions. Call toll free at 1-877-398-4769, Monday to Friday, 9 a.m. to 2 p.m., or e-mail us at answers@unh.edu.

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Got questions? The Ask UNH Extension Infoline offers practical help finding answers for your home, yard, and garden questions.
Call toll free at 1-877-398-4769, Monday to Friday, 9 a.m. to 2 p.m., or e-mail us at answers@unh.edu.