Is It Safe to Plant a Vegetable Garden on Top of a Properly Working Leach Field?

A leach field consists of a series of buried trenches or perforated pipes that release the waste from the septic system to filter through the soil and decompose. Small residential lots with septic systems may have little free space that isn't part of the leach field. However, there are many good reasons not to use this waste processing field as the site for your new vegetable garden.

Even in a properly functioning septic system, there is likely some amount of bacteria and effluent spreading out of the leach field. The Clemson Cooperative Extension warns that the roots of the vegetables can reach contaminated areas. If waste is also reaching the top layers of soil, dirt splashing on leaves and fruit can cause direct bacterial exposure. Root crops like potatoes and carrots are also unsafe because they can pick up the viruses and bacteria in the effluent.

It is possible to damage your septic leach field by working the soil to create a vegetable garden over it. Tilling and turning the soil will compact the dirt just below the tines, trapping the water and interfering with proper drainage. Walking over the surface to weed and harvest vegetables further compacts the soil. Adding extra water for thirsty tomatoes or sprinkling in fertilizer will also upset the balance of the septic system, reports the Clemson Cooperative Extension.

The annuals primarily grown in vegetable gardens don't have the extensive root system of trees and shrubs. The University of California Cooperative Extension recommends leaving a 4-foot gap between any soil used for growing vegetables and the edges of your leach field. This ensures that the plant roots can't reach into soil that might contain bacteria from the waste. You should avoid installing growing areas for vegetables that are downhill from the leach field if it happens to be raised or located on the side of a hill.

Bringing in your own soil and building a deep raised bed may sound like a tempting way to use the space of your leach field without bacterial concerns. However, adding inches or feet of soil over the surface will inhibit moisture evaporation from the leach field, according to the University of Nevada Cooperative Extension. The water that flushes into the field needs to evaporate relatively quickly to keep the area from becoming a muddy mess that breeds bacteria and contaminates the groundwater.