Be wary of your redwoods with recycled water

In his State of the City address earlier this month, San Ramon Mayor Bill Clarkson observed that the city had been working hard to expand use of recycled water for irrigating landscaping.
The Dublin San Ramon Services District, which serves the Dougherty Valley and the older, southern half of San Ramon, has been aggressively installing additional purple pipes to carry recycled water for landscaping.
The newer Dougherty Valley was dual piped with potable water systems and recycled systems when it was developed. Not so for the southern half of the city where building subdivisions started in the 1960s.
Clarkson observed many successes in his address, but one unexpected downside—the coastal redwood trees, which have been planted widely in valley landscaping since the 1970s and 1980s, do not do well with recycled water. The recycled water is higher in salts than the groundwater or water imported from the Delta.
The redwood trees, which thrive in cool, foggy coastal climates (think of where we find natural forests of them (Marin, Sonoma, San Mateo, Santa Cruz counties to name a few) and it is clear what environments benefit coastal redwoods. If it grows banana slugs, it likely is ideal for coastal redwoods.
The trees have grown well in the valley with plenty of irrigation—developers and homeowners have favored them because they grow rapidly. The catch now is they are not suited to the higher salts in recycled water.
For areas with lots of redwood in the landscape and plans for recycled water, owners should plan to replace the trees over time. Currently, the situation is quite stressful for the trees receiving recycled water in addition to the effects of the four-year drought. Stress also is common for redwoods irrigated with potable water because they have been receiving significantly less water than they are used to because of the drought restrictions.
Recycled water works well for most plants, although water regulators were concerned enough about the salts that they required the Zone 7 water agency to build a reverse osmosis treatment plant for its groundwater to limit the buildup of salts percolating from recycled water.