Can plants be grown over your septic drain field? If so, which are the best plants, which are the worst? In this article, we cover landscaping and planting ideas for septic tank owners in British Columbia.
Landscaping and Planting Ideas for your Septic Drain Field & Septic Tank
In most rural and regional areas of British Columbia, water is an incredibly important resource and it’s hard to come by access to a public sewer. This means that, to be able to function, many of BC’s families and businesses require a septic system – both to save water and because there are no other waste disposal options when outside of city sewer.
If your family or business relies on a septic system to treat waste, you will be very much aware of the investment you have made in purchasing, installing and getting the system maintained. A well operated and maintained septic system can last for at least 25 to 30 years. So it is incredibly important to understand not only how your septic system functions, but also how landscaping and planting can impact the longevity of your septic field.
Although I cover the function of septic systems extensively in my other articles, let’s quickly re-hash the conventional septic system.
How does a septic drain field work?
It’s easy to forget that the waste we flush down the toilet or wash down the sink has to go somewhere. If your home or business doesn’t have access to a public sewer, the wastewater from your toilet, shower kitchen sink, dishwasher and washing machine flows to your septic system.
Understanding how a septic system works is crucial to making good decisions about how to best look after and plant over your drain field. So… how does a septic system work?
As we have covered in previous articles, the bacteria inside your septic system helps break down organic wastes. Over time, the waste inside your septic system separates, with liquids rising to the top and solid, inorganic waste (sand, synthetic fibres, bits of plastic) setting as sludge on the bottom. Grease and oils, which take a long time to break down, settle on the bottom as well.
Tank sludge must be pumped at scheduled intervals, normally every few years, to ensure the septic system functions properly and retains longevity. But wastewater can be treated and either pumped back into the property as completely clean, reusable water, as irrigation for crops, or it’s simply released onto a ‘soil absorption area’, this depends on the type of septic system chosen for the design.
Sewage system, the recycling
This ‘soil absorption area’ is also called a leach field or a septic drain field. Septic drain fields most often consist of a layer of perforated piping that is set in gravel tranches and buried around 12 to 18 inches below the ground.
It is here that, after a few initial stages, the final treatment and distribution occurs. The treated wastewater seeps from the pipes, into the gravel and slowly makes its way into the soil.
Many different organisms and microbes in the soil filter and cleanse the treated wastewater before any remaining pathogens have a chance to reach the groundwater. The effluent needs to drain at an adequate pace to ensure the organisms function optimally. Most of these organisms rely on oxygen to survive and soil that is oversaturated with water will not contain a sufficient amount of oxygen.
A note on Mound Systems
In some septic systems, namely Above Ground Septic Systems or Sand Mound Septic Systems, the drain field sits above ground in an engineered mound.
Developed in the 1940s at the North Dakota College of Agriculture and originally called the “NODAK disposal system” after its place of origin, the mound septic system was a breakthrough alternative to traditional underground septic systems and drain fields.
The system was developed for use in areas where certain environmental conditions (a high water table, shallow soil cover and/or when the soil drains too quickly or too slowly) don’t allow the use of a traditional septic system.
Mound systems function very much like traditional, below-ground systems, however, the mound itself must be stabilised to avoid erosion and other disturbances that can damage the drain field.
Septic Field Landscaping, is it necessary?
In short – yes. And there are several reasons for this.
Most compelling, perhaps, is the reason that planting on the septic drain field can help stabilise the area and reduce the possibility of the soil cover eroding. Erosion can leave the drain field susceptible to damage, which can be incredibly costly to fix because the drain field is usually the most expensive part of a septic system.
Plants can also remove excess nutrients and moisture from the soil. This can make the final treatment process more effective because it means less work for the organisms and microbes in the soil as well as less chance of over-saturation.
The capillary action from the roots of the vegetation will also draw some of the wastewater from the soil, this also helps in not only treating the water but it also helps get rid of some of the soil moisture.
A grassy lawn landscaped over a septic drain field is going to help in the prevention of soil erosion it will also improve oxygen exchange. In many cases, it is mandatory by code to plant grass or low root vegetation to help with the transpiration, erosion control as mentioned and insulative properties for cold climates.
Another reason is aesthetics. Some nice landscaping over the septic tank is going to hide the lids and access areas, planting on the septic field is going to give you a rich lawn and plant life. Planting on your septic drain field with the right grass and plants not only helps with the performance of the system, but it completely disguises any infrastructure in the ground from being visible.
All this said it is important to understand that planting certain types of vegetation on or near your drain field may pose a threat to the long-term functionality of your septic system. This is largely because the root systems of some plants can penetrate and ruin the piping or other components within the drain field.
It’s not a lot of fun to find septic effluent surfacing in your garden and to have to suffer the costs and inconveniences of repairs from roots infiltrating the underground piping network.
So, what should you consider when planting on a septic drain field?
Ideally, you want to select plants that will meet your home or business landscaping requirements, while also keeping the drain field as safe as possible from deep-rooted vegetative or weather threats. The easiest way to tell whether a plant is safe for a drain field is to find out the plant’s rooting habits and water needs.
Look for shallow-rooted herbaceous plants that either already grows in your region or are adapted to the region’s typical rainfall amounts. Grasses and perennial flowers that don’t require too much maintenance are perfect. If you want to reduce your workload completely, choose vegetation that is both shallow-rooted and drought-tolerant. These kinds of plants should require little if any irrigation.
Some of the plants that are safe for your drain field are:
- Holly Shrubs
- Boxwood Shrubs
- Azalea Shrubs
- Holly Hocks
- Wild Violets
- Spring Bulbs
Some trees that are septic safe, including fruit trees, include:
- Dogwood Trees
- Japanese Maple Trees
- Eastern Red Bud Trees
- Cherry Trees
One must be cautious when planting fruit trees near the drain field, especially when there is a surface breakout from the septic system. Possible pathogens such as Escherichia Coli and Enterobacter spp can transfer from the septic drain field to the trees.
North Dakota University recommends that a root barrier is installed, this creates a barrier from the roots infiltrating into the septic lines.
A general rule of thumb, although not in all cases, is to keep a distance relative to the height of the tree away from the septic system.
Some of the worst plants and trees to cover a septic drain field or septic tank are:
- Pussy Willow Shrubs
- Japanese Willow Shrubs
- Aspen Trees
- Lombardy Poplar Trees
- Birch Trees
- Elm Trees
- Maple Trees (other than Maple Trees)
- American Sweet Gum Trees
- Ash Trees
- Tulip Trees
- Walnut Trees
- Willow Trees
- Cypress Trees
- Pine Trees
Shrubs with vast root systems, such as Caryopteris (also known as Bluebeard or blue mist spirea), are also not recommended.
What you don’t want is to plant vegetation that has a deep root structure, water-loving roots that will grow aggressively deep and potentially clog or damage the pipes in the septic drain field. As mentioned, septic systems – particularly the drain field – can be very expensive to fix. A damaged system can also get very messy and have a devastating impact on the environment.
When you decide on the vegetation that you want to grow over your septic drain field, make sure your plant it with minimal soil tilling and in conditions that will allow the plants to establish cover as fast as possible. (For instance, don’t plant just before heavy rain). You want the plants to settle into place quickly, to lessen the chances of soil erosion.
While it might be necessary to water the newly introduced vegetation until the plants settle into the soil, long-term irrigation or fertilisation is not recommended. Irritating the drain field can saturate the soil to undesirable levels – preventing the effluent from evaporation and consequently increasing the risk of groundwater contamination.
Maintaining septic drain field vegetation
As much as you may want to plant over the drain field so that it blends seamlessly with your garden and lets you forget its existence, this is not the best idea.
Keep your drain field visible, or make people aware through other means, like a plant barrier or fence. Don’t hold large social gatherings, mowing the lawn is just fine, but certainly, limit the foot traffic.
Always ask your Septic System Installer if you are unsure of any possible situations that can negatively affect your system.
But don’t be alarmed if any of these trees are already growing on your property. This is fine, so long as they are at least 50 feet away from your septic system and drain field.
Trees that can be safely planted closer to the drain field include ornamental trees like dogwood, cherry and crabapple, hemlock of the red, scarlet and white oak varieties, and small pines such as Mugo pines. Keep these trees as far away from the septic tank and drain field as the mature height of the tree, or at least 20 feet.
Can I plant a vegetable garden over the Septic System?
The wastewater seeping through the drain field pipes can make the soil rich in moisture and nutrients. So it may seem like a brilliant idea to take advantage of this and plant a fruit or vegetable garden over the drain field.
Unfortunately, this is not recommended – particularly when it comes to creeping herbs and root vegetables. This is because the vegetation will come into direct contact with soil that is likely to be contaminated with disease-causing pathogens such as viruses and bacteria.
Root vegetables like carrots and potatoes are obviously the most susceptible to contamination. But leafy crops that grow close to the ground are also a risk as these can become contaminated by irrigation water that has splashed onto the foliage.
It is impossible to tell from appearance if leaves, fruits or vegetables have been contaminated.
General speaking, the taller the crop, the lower the risk of contamination. But there are many other factors to consider. For instance, if your household has a water softener, it is highly likely that your water softening system adds salt to your septic system each time it regenerates. This means the wastewater that ends up in your drain field will be salty and could, therefore, harm salt-sensitive vegetation like beans and cucumbers.
Also, depending on your septic system, the wastewater that ends up in your drain field might still contain residue from household chemicals like laundry detergent. It is important to consider the effect of this on any vegetables or fruits you are considering planting on your septic field.
And not to mention gardening activities like ploughing, deep digging, rototilling and fence post installation. All of these practices can potentially harm the piping under your drain field. As for raised garden beds, these can significantly interfere with the normal evaporation rate of the wastewater from the soil.
Herbaceous plants: guidelines and considerations, in summary
The go-to plants for your septic drain field will be herbaceous plants like annuals, perennials (including bulbs) and ornamental grasses. The advantage of ornamental grasses is their fibrous root system, which will hold soil in place and offer ground cover all year round.
We have already covered the advantages of shallow-rooted, herbaceous plants over other deep and woody-rooted choices. However, even when planting shallow-rooted plants on a drain field, there are some general guidelines to consider:
- Soil cover – Don’t feel tempted to add additional soil over the drain field unless the amount is minute or being used to restore an area that may have been eroded or pulled up by removing another plant.
- Tilling the soil – Avoid this if you can. Remember that the piping of your septic system drain field could be as close as 12 inches from the soil surface, sometimes less.
- Gloves – Be sure to wear some when working with the soil from your drain field. The water seeping from your septic system into the drain field may contain pathogens that could make you very ill if they came into contact with your skin, eyes or mouth.
- Groundcovers – If you are choosing a groundcover for your drain field, as a native grass or creeper, do not use species that are known to create a thick, dense canopy. Effluent in the drain field needs to be allowed to evaporate for your septic system to function optimally, and this cannot happen if the ground cover is too thick.
- Native species – Choose plants that are well adapted to your region. This will reduce the need for you to fertilise or water the drain field area.
- Foot traffic – To reduce the chances of damage, make sure to keep foot traffic on the septic drain field to a minimum. Select low maintenance species that won’t require much tending or mowing.
A selected listing of plants for use on septic drain fields
Here are some more ideas of plants that work well on drain fields in British Columbia. This list is not all-inclusive, so make sure to do your own further research to ensure the plants you choose will thrive in the conditions typical to your location.
Wildflower meadow mixes
Groundcovers for sun
Carpet heathers (Calluna)
Groundcovers for Shade
Sweet Woodruff (Galium)
Wild Ginder (Asarum)