I want to build vegetable raised beds on a slope. At the top of the slope is my septic bed, so I don’t want the soil in the beds to be in contact with the soil on the ground under them. As I will still need good drainage in the beds, what material would you recommend I use to keep the soil in the beds isolated from what is underneath?
Thank you for your consideration,
Peter
We cannot recommend that you plant edibles in that location, nor can we recommend covering the soil with any material.
First, here is a response we provided another gardener with a similar inquiry:
“Generally speaking your septic system is a mini-wastewater treatment facility. It consists of the pipes that drain wastewater out of your home into an underground septic tank, which accumulates solid waste as it permits liquids to exit into the septic field. From the tank, a series of perforated distribution pipes set (usually) in gravel trenches slowly drain and evaporate the wastewater (this is the septic field or leaching bed). As the fluid effluent seeps into the ground, it is cleaned by the soil. These pipes need to be a certain depth below the surface in order to function properly, so it is important not to add much soil to the top of the drainage field. It is also critical to ensure that no plant roots damage the leaching bed. If you have swampy areas near the pipes, you may want to check with an expert to make sure the septic system is working properly. And if you have not done so already, I suggest that you get a copy of the plan that sets out the precise location/size of your septic system. You may have to ask your municipality or regional district for it. Mark out the area of your septic system so that you know its perimeter.
The best plants to grow have shallow roots – e.g., grasses and flowers. Do not rototill the top of the septic field when planting, do not dig deeply into the soil and avoid heavy mulching and watering the plants more than absolutely necessary – all of which could could interfere with the function of the field. Perennial flowers or ground covers and grasses are the best plants to select, as the less the septic field is disrupted by gardening the better. ”
Here is a simple fact sheet that reinforces our advice, from the US Environmental Protection Agency.
From the University of New Hampshire, more information on what / how to plant near septic systems. They include a list of appropriate perennials, but we would also recommend that you consider native perennials, some of which are described in Ontario Native Plants for Rock Gardens. Of course, your site is not a rock garden, but these are shallow rooted plants and many would be suitable.
Finally, have a look at our own resource page on native plants.
I think you will be able to create an attractive group of beds filled with plants that are safe to use in this location — and I hope you find another sunny spot far from the leach field for your veggies.