Home owners who have a septic leach field (septic drainfield) on their property are often a bit at a loss when it comes to planting over it. Most municipal authorities seem to automatically assume you’ll want to grow a lawn there and any suggestions they may give beyond that are often cryptic or even wrong. In actual fact, there is quite a bit you can do with a leach field when it comes to gardening.
Leach fields were designed to be covered with vegetation, so are not good places to put just plain gravel, a patio, or other inert products. Plants help the leaching system operate correctly by removing excessive moisture and minerals from the soil and reducing erosion. But ideally, you should only grow plants with relatively shallow roots over them, preferably herbaceous plants (perennials, biennials, annuals, grasses, etc.). Ground covers, for example, are particularly suitable because they usually require little maintenance (and you won’t want to be digging regularly over a leach field), but you can also put in flowerbeds or a wildflower garden.
When you do plant over a leach field, plant densely, because the soil should be well covered with vegetation. If you include plants with a naturally sparse habit (bearded irises, peonies, etc.), mulching would be a good idea.
Avoid digging too deeply when planting: you don’t need a hole any deeper than the root ball is high. And wear gloves when you plant or weed in the sector to protect yourself from direct contact with potentially harmful microorganisms in the soil.
Vegetables on a Leach Field
Your municipality will certainly insist you shouldn’t grow vegetables on a septic leach field, but in fact you can if you are very careful. After all, generations of Chinese have used “night soil” (human waste) to grow vegetables and are no worse off. We Westerners are much more squeamish about that sort of thing and there is a certain risk, but there are ways of minimizing it.
First, it’s best not to grow vegetable on a leach field if the soil is sandy, because potentially harmful microorganisms (bacteria, viruses, etc.) migrate more readily to the surface under those conditions. If the soil is essentially clay, though, microorganisms are generally confined to a few inches in the immediate area above drainage pipe and the risk is therefore much more limited. Even so, avoid placing your vegetable bed directly above the drainage lines themselves, placing it between the lines if possible.
It’s wisest to avoid root and leafy vegetables, as they are most at risk of contamination. Limit yourself instead to fruit-bearing vegetables (tomatoes, cucumbers, beans, peas, etc.). By raising these vegetables above the ground on stakes or trellises, the fruits won’t touch the ground and you essentially eliminate all risk of contamination. As a precaution, however, even then you should rinse your crops thoroughly before serving them.
You shouldn’t garden in raised beds over a leach field: the added height will make the system less effective in removing excess moisture. Plant rather directly into the soil as it is or add only a thin layer of fresh soil.
If you want to totally avoid any risk of contamination, of course, simply place vegetable bed to the side of the leach field rather than over it. There is no “exclusion zone”: even 1 foot (30 cm) from the field is fine.
Trees and Shrubs
Ideally, you should avoid cultivating trees and shrubs not only over a septic field, but even nearby. If you don’t have a choice, prefer small shrubs because their roots are usually not as long and are unlikely to infiltrate underground pipes and drainage areas.
Plant any tree or large shrub no closer than about 20 feet (6 m) from the leach field. Trees reputed for their invasive roots, like willows, poplars, elms, and silver maples, should be planted even further away: 50 feet (15 m). You can however plant a tree near a leach field as long as you take the precaution of installing a rhizome barrier (bamboo barrier) in the soil between the field and the tree.
How you maintain your leach field is up to you. Hopefully the information above can help you make the right decisions!