Gardens Dig Greywater

With record setting rain and snow fall the past 5 months, 75% of California is out of the drought. Still it will take years to fully recover and recharge the groundwater deficit. As Jeremy Miller outlines in the New Yorker “California’s drought may be over, but its water troubles aren’t.” Now is not the time to go back to the old ways. Instead, keep looking for opportunities to eliminate water waste and conserve our extremely valuable drinking water. One opportunity is to reduce the amount of our drinking water you share with your garden. To that end, I’d like you to consider if greywater has a place in your landscape. Granted, greywater is not for everyone but I think it is for a lot more people than are currently using it. So I will attempt to demystify greywater and send you in the right direction to find out if it is right for your garden.

Greywater is gently used water from your clothes washer, shower, and lavatory sink. Not to be confused with blackwater from toilets, kitchen sinks and dishwashers. California Plumbing Code Chapter 15 lays out requirements for greywater system design and installation. What follows are some of the key requirements and tips for designing a successful greywater system.


The 3 Types of Greywater Systems

  • Laundry to Landscape – These are the simplest systems where water from your clothes washer is pumped through piping (using the washer’s pump) into mulch basins strategically placed around your yard. If designed per code, these systems do not require a permit.
  • Branched Drain – These systems can utilize shower, lavatory, clothes washer water or a combination. They drain purely by gravity through a network of pipes with branches that split the flow to send it in different directions. This system also drains into mulch basins. Branched drain systems require a permit.
  • Pumped Systems - These are the most complicated systems and involve pumps and filters to get the water up hill and/or to filter it to discharge it through a high flow rate drip system. Pumped systems require a permit.


Mandatory Rules for Greywater Systems:

  • Do NOT store greywater. Greywater must discharge and infiltrate into the soil within 24hours. Greywater has some organic matter in it and will go fetid if left in a storage tank.
  • Do NOT let greywater see the light of day. Greywater must be discharged at least 2 inches below the ground surface.
  • Do NOT discharge greywater close to a building or let it runoff your property.
  • Do NOT include water from toilets, kitchen sinks or dishwashers in your greywater system.
  • Do NOT discharge toxic chemicals in your greywater system.
  • You MUST include a 3-way diverter valve that allows you to direct the greywater into the sewer line. There will be times when you do not want to discharge your water onto the landscape – rainy season, laundry loads with harmful chemicals or biological contaminants, etc. The valve allows you to redirect the flow at the source into the sanitary sewer.
  • You MUST create a Homeowner Manual describing the system and how to maintain it. 


Best Practices for Success

  • Choose plants that are a good match for greywater and can handle some extra water and variation in supply. Not all plants like greywater. Most trees and other thirsty plants do well. You can use greywater for edible plants as long as the edible part is above ground and does not come in direct contact with greywater. Fruit trees and berries are a good match with greywater. Drought tolerant plants and xeriscaping are NOT a good fit for greywater.
  • You MUST use detergents and cleaning products that will not harm your plants. Avoid detergents with salts and boron. Do not use bleach and other harsh chemicals for cleaning.
  • Match the amount of greywater available to the amount of greywater needed by the landscape. You do not want to overwater or underwater.
  • Greywater systems require maintenance or they will fail. For systems that use mulch basins to receive the discharged water, the mulch needs to be dug out and replaced at least once a year. The mulch is an essential part of the system and provides a place for the microbes that break down the “chunky” part of the greywater and keep things from going funky. Over time the mulch itself is broken down and needs to be replenished.
  • Greywater systems with pumps and filters require more maintenance and are more difficult to maintain. Be sure you understand and are willing to do this maintenance or hire someone to do it.


Greywater System Design (simplified):

  • Determine which fixtures are accessible and could be incorporated into a system.
  • Estimate weekly grey water production. The code provides estimates: Laundry-15 gal/per day/ per occupant, Shower-25 gal/per day/per occupant). However, you can make a more accurate estimate based on the flow rate of the fixtures you are using and how many times or for how long they will be used each week.
  • Estimate irrigation needs of plants. As a general rule trees require about 10 gallons per week. WUCOLS (Water Use Classifications of Landscape Species) is an excellent resource for determining the water needs of various landscape plants for different California climate zones.
  • Discharge greywater into mulch basins. The mulch basin acts as a holding tank until the water can infiltrate. The mulch basins are sized to accommodate 24 hours of greywater discharge. The distribution piping will terminate inside a covered chamber (such as an irrigation valve box) and free fall into the mulch basin.
  • Determine the irrigation/leaching area needed to infiltrate the greywater within 24 hours. Different soil types have different infiltration rates – sand is fast and clay is slow. You can do an infiltration test on site or use the guidance provided in the code for different soil types. For example a square foot of sandy soil can absorb 4 gallons of water in a 24hr period, where as, the same amount of clay soil can only absorb 0.8 gallons.


There are more details to building a successful greywate system. However, I hope this gives you an idea of whether or not it is something that might work for your garden. For more in-depth information checkout the following websites: