A Word from the Water Wise

Last year I wrote about a gorgeous native plant garden for a local magazine. The piece was a general interest story on gardening with the California flora, but the homeowner and garden's designer, Eric Callow, impressed me with his enthusiasm for what I sometimes regard as the dismal science: irrigation.

Callow, a financial adviser and member of the Rancho Santa Ana Botanic Garden board, had transfered is his talent with numbers and spread sheets to the garden, calculating exactly how much water individual plants in his garden needed.

As someone whose watering regime is often douse plants when I remember to, Callow made my head spin. So I asked him to send along some tips. Here is the first of two parts:

When my wife Elisa and I were looking for a home eight years ago, we had the good fortune to find a mid-century modern house designed by Lloyd Wright Jr. The house came with twenty thousand square feet of yard begging to be re-landscaped and I already had a deep interest and some experience with California native plants. So, it was the perfect opportunity to design and install (with help) my own California native garden-including hundreds of feet of irrigation pipe and tubing.

The garden has matured wonderfully in many ways over the last five years and has been included on the Theodore Payne native garden home tour over that period. Certain areas in the yard have been real challenges, however, with apparently healthy plants just up and dying or other plants hanging on but not prospering. I decided it was time to consult an expert, so I called horticulturist Lili Singer.

Singer is special projects coordinator for the Theodore Payne Foundation, where, among other things, she produces the above referenced home garden tour. As she checked on my garden, noting a variety of problem plants and areas, a clear theme developed: most of the time, the problem lay in how the plant was being watered. It was not just a matter of too much or too little either; Lili also analyzed cases of inconsistent watering (too much, trying to make up for too little) and uneven watering (supplies not reaching some of the roots). As a result, I’m reviewing and revamping my garden irrigation system, circuit by circuit, again.

Now, not everyone wants to start digging up and replacing valves and sprinkler heads, let alone laying pipe, but two issues argue for at least understanding some of the big concepts.

For one thing, we’re all going to be pressured to use less water in the years ahead. In fact, the package of water bills recently passed in Sacramento stipulate that urban users will reduce their usage by 20% and local water districts are required to have a plan in place to do so sometime this year. The legislation raises some serious issues regarding the justice and necessity of singling out urban users, but there’s no question that we as gardeners need to do our part to save water.

Secondly, in my experience, the average hired gardener can’t be trusted to know what they are up to when it comes to analyzing or designing an irrigation system. Indeed, I’ve had to rework an extensive drip system because a professional landscape contractor’s employee didn’t understand the following precepts.

So, here are what I consider to the big concepts for understanding garden irrigation.

1. Hydro-zone: an area of plants with similar water requirements, watered by a single irrigation valve and circuit of spray heads, drip emitters, bubblers et cetera. The valve is like an electrically controlled faucet. When the controller/timer tells it to turn on, water is released into the circuit of pipe or tubing and distributed over the area being watered.

2. Efficiency: laying down water evenly over the hydro-zone and only in the hydro-zone.

3. Pressure: the force, measured in pounds per square inch (PSI), available at the irrigation valve.

4. Flow: the volume of water that will flow to the hydro-zone at a given pressure.

5. Precipitation: the amount of water delivered to an area (hydro-zone) measured in inches.

A hydro-zone can be simple, like a rectangle of turf, or complicated such as an irregular shape or even overlapped. It boils down to a grouping of plants that can be efficiently watered from a single valve. For the health of your plants-and your own sanity-you want to place plants together that need similar amounts of water in the same seasons.

Efficiency has to do with getting water to all the plants in the hydro-zone and not to anywhere else (like the driveway, etc.). Even application is important too since an under-served section of the zone will force wasteful over watering of the remainder. Professional irrigation designers assume that their best design for placement of the sprinkler heads will be only 90% efficient. I think a well designed low-pressure drip or micro-spray (think of a tiny spray head as opposed to a pop up sprinkler) system can do better. If you use more heads or emitters, each covering a smaller area, you can cover an area more precisely.

The pressure depends first of all on what your utility delivers. It’s usually too much, so you will likely already have one or more pressure regulators to reduce it to something manageable. For example, I have 185 PSI at the street, which is reduced to 75 PSI in the yard. That’s still way too much for the faucets inside the house, so there’s another pressure regulator to bring it down to 25 PSI. So, if your irrigation isn’t working well, the first thing to do is to check the pressure with an inexpensive gauge that screws onto a garden faucet. If it’s really low, maybe someone ran your garden plumbing from inside the house!

More often than not, however, the problem is not pressure but flow. Water flowing through a pipe creates friction and turbulence so that the more one tries to cram through it, the more pressure is lost by the time it reaches the other end. Even if the pressure can be increased, there’s only so much water you can get through pipe of a given diameter. Irrigation designers refer to charts to tell them how much of the pressure they’re starting with at the valve will be lost for every 100 feet of pipe the water is being pushed through.

Let’s say your irrigation circuit is designed to pump out ten gallons minute (not unusual), but the water is being pushed through 100 feet of half inch pipe before coming out the heads. The result will be really anemic because over 30 PSI will have been lost before the water gets to the sprinkler head. If there’s 200 feet of pipe, over 60 PSI will have been lost-perhaps more than you had to begin with! Of course, whoever designed the system should have used three quarter inch or even one inch pipe, but believe me, this sort of thing happens all the time.

When the weatherman says we had an inch of rain, it means that a baking pan left outside would have that depth of water in it: that’s precipitation. Precipitation is also how irrigation is measured, that is, in inches of water per month. For example, according to WUCOLS III (a water-use classification of landscape plants created for the California Department of Water Resources) an area of low-water plants--buckwheat, sage, ceonothus and the like—in Pasadena needs about two inches of precipitation in July. This may be more than the plants need to survive, strictly speaking, but is meant to be an estimate of what they need to be healthy and look good in a garden setting.

In any case, I am convinced that having an efficient irrigation system controlled to deliver planned amounts of precipitation is a least as important as your choice of plants in saving water.

Of course, most people will continue to estimate water needs by the old “seat of the pants” method, but I predict that as the conservation screw keeps getting turned, measuring precipitation will become the norm. How to do that is another conversation, complete with charts and graphs, which I’m writing up for the second installment of Irrigation Insights.

*Estimating Irrigation Water Needs of Landscape Plantings in California

The Landscape Coefficient Method and WUCOLS III (Water Use Classification of Landscape Species)University of California (a free publication available online)