Fall 2022

Sustainable Gardening Practices: Using Recycled Water

by Jim Farr

Water is precious and recycling preserves this resource.

In California, about 10 % of waste water in municipal and industrial usage is currently recycled and about 20% of that recycled water is used for landscape irrigation. Over time, this percentage is expected to greatly increase. The need for recycled water continues to grow, prompting more Californians to use recycled water in their yards than ever before. Using recycled water will help alleviate water shortfalls from the drought and help consumers save on high water bills.

The use of recycled water extends across most of the counties in the Bay Area and includes Alameda County. Some consumers have noticed certain plants in their yard beginning to turn brown and die after using recycled water. This is an issue that consumers should be aware of; understanding what is happening will give consumers strategies to avoid this problem in the future.

Recycled, Reused, Reclaimed, Gray Water Defined

According to the California Department of Water Resources, recycled water is “highly treated waste water from various sources such as domestic sewage, industrial waste water and storm water runoff.” This water has been through three levels of treatment including filtration and disinfection.

The terms “reused” and “recycled” are often used interchangeably depending on where you are geographically. Reclaimed water is not reused or recycled until it is put to some purpose. Gray water is untreated, non-disinfected household waste water that does not include toilet waste. It may be sourced from showers, baths, and washing machines.

Differences Between Recycled Water and Clean, Potable Water

Recycled water has been highly treated and is good for irrigation but not for drinking. While recycled water is very similar to clean potable water, there are some differences. One of these differences is that recycled water has a greater level of dissolved salts than potable drinking water. In a recycled water analysis, available online, the salt concentration is expressed as higher numbers for total dissolved solids, conductivity, and salinity. While it is outside of the scope of this blog to discuss each of these in detail, these higher numbers found in recycled water represent an overall higher concentration of salt in the water. 

Why is the Salt in Recycled Water a Problem for Plants?

Higher levels of salt in recycled water can damage salt-sensitive plants. 
As plants transpire (i.e. release moisture into the air through their leaves), roots pull water out of the soil, carrying some of the salts in the soil with it into the plant. If chloride, sodium and/or other ions reach damaging concentrations, leaf chlorosis and/or burn symptoms develop.

Three major effects of high salinity on plants and soils are:
1. Prevents water moving from the soil up into the plant (sometimes referred to as increasing osmotic pressure);
2. Salt ions, notably sodium, chloride, and boron, accumulate in plant cells to a harmful concentration;
3. Slow water infiltration and soil permeability due to excess sodium breaking down soil aggregates leading to more compacted soil.

Plants Vary in Their Tolerance for Salt

Roses are one of the many common plants that are salt sensitive. Plant species vary widely in their tolerance or sensitivity to salts and specific ions (see references at end of article). Plants with moderate to high salt tolerance, including many drought tolerant plants, are adapted to sites that contain elevated concentrations of salt that may be too high for salt-sensitive plants. Examples of these more extreme salty environments include arid areas where the salt concentration is high in the soil or along coastlines where there is exposure to salty air and ocean sprays.
However, some plants familiar with homeowners in their front and backyards are highly salt sensitive and the higher level of salt in recycled water can harm these plants. For example, some common plants with very low tolerances for salt are Camellias, Butterfly Bushes, and Roses. Trees that have a low tolerance to salt include Liquid Amber, Albizia, Red Maple, Ginko, Crepe Myrtles, and most types of Citrus.

Bay Area water municipal districts have highlighted the issues associated with the higher salt concentration of recycled water on their websites. However, it is something not commonly known to consumers. For example, the Dublin San Ramon Service District includes this message at the bottom of the web page on recycled water quality:

Most plants thrive with recycled water's higher nutrient content but some plants are sensitive to the higher salt content. The Tri-Valley Water-Wise website has a list of plants known to tolerate higher salt content, as does East Bay Municipal Utility District's award-winning book, Plants and Landscapes for Summer-Dry Climates of the San Francisco Bay Region.”

A variety of tables are available to determine which plants and trees have low salt tolerance, including Trees, Shrubs, Ground Covers and Vines, and Ornamental Grasses. Researchers at the University of California and at other research institutions continue to study this issue and update lists of plants that have low, medium, and high salt tolerance.

What To Do If Recycled Water is Affecting Your Plants

If you are using recycled water and notice that some of your plants are showing signs of browning and don’t look healthy, check the tables above to see if these plants and trees are on the list of low salt tolerance. If you do choose to replace plants that have died from higher salt levels in the water, make sure to replace them with any of the many plants that have high salt tolerance.

Also, if you notice trees and shrubs in your local park beginning to turn brown, let your city parks and recreation department know. Many parks in the Bay Area now are irrigated with recycled water. Parks and Recreation Departments are sometimes not fully aware of the impact of recycled water on the plants in their parks. This can result in the early death of trees in parks – in particular, maple trees and some evergreens have low salt tolerance.

Recycled Water Conserves Precious Water But Understand Its Effects on Plants

Lastly, watering with recycled water will help save precious potable water and contribute to dealing with water shortages which is a good thing. The intent of this article is to give you more knowledge when using this water, so you can determine which plants and trees might be harmed. If you are just beginning to put in new landscaping, this is the perfect opportunity to use the plant lists here to choose plants and trees that have high salt tolerance. If you are interested in measuring the salt concentration of your water, several devices are available typically referred to as hydrometers, refractometers, or conductivity meters. Test strips are also available for measuring salt concentration. These can be readily purchased online and can cost as little as $10 up to over $100

References & Tips:

• Tables of Salt Sensitive and Salt Tolerant Plants
  Tolerance of popular landscape species to salt spray and soil salinity are presented for trees (Table 2_02), shrubs (Table 3_02), groundcovers and vines (Table 4_02), and ornamental grasses (Table 5_02).
• Mulching
• University of California Urban Water Use Study
• Outstanding Plants for Alameda County
• EBMUD Water Smart Plants
• UC Davis Arboretum All-Stars
• Water Use Classification of Landscaping Species (WUCOLS) – Very comprehensive database with a great search tool
• Water Wise Tips for Container Gardens:
  • Self-Watering Containers
  • Growing Vegetables in Containers
  • Container Gardening 101

Jim Farr is a UC Master Gardener of Alameda County
Photo credits: Jim Farr, Hedwig V., and UCANR