Water Basics

Families enjoy warm sunshine at a park in February.

It’s all about water – living in Southern California, that is. The sun shines brightly almost every day of the year. Temperatures are, for the most part, moderate, and due to low humidity, even the hottest days are followed by delightfully cool nights. Weather – meaning precipitation – rarely interferes with our fun, as those who watch the Rose Parade can attest. And in this glorious climate, it is said we can grow just about anything. This is nearly true, so long as we do not forget the addendum: Just add water.

Whether you intend to garden with supplemental irrigation or relying mostly on naturally-occurring water, success will elude you unless you understand water in your garden. As anyone who lives here knows, water, in our mediterranean climate, comes primarily during winter months in the form of rain. (See Is So. Cal a Desert? for description of mediterranean climate.) Annual fluctuations in rainfall are large, and successive years of drought are not uncommon.

Water, though, can be delivered to the garden in other forms. Fog, lifting off the ocean and blowing inland, bathes some gardens in moisture through much of the year. Some of these foggy gardens dry out by afternoon; others barely see a ray of sunshine through the mist. Higher elevation gardens, in foothill or mountains areas, may receive moisture in the form of snow or runoff from nearby snow banks. Inland gardens may receive occasional summer thunderstorms due to monsoonal conditions.

Mist and fog swirl around Angeles Crest Highway

Water also flows underground. Although the water table has fallen due to years of pumping, reduced infiltration, and the channelization of streams, water still travels below the surface. In our local nature park, a healthy red willow, growing vigorously on a mound, suggests the presence of subterranean water throughout the year. Water, rich in nitrogen, leaks from older sewer pipes in many gardens. The roots of trees often find this source, exacerbating plumbing problems for the unlucky homeowner. Low spots sometimes act as sinks where subsurface water collects.

Most gardens, especially here in the United States, have a ready source of irrigation. These systems deliver water both as intended and inadvertently through leaks and broken emitters. Runoff from a neighbor’s irrigation may be yet another source of water. Although you may not have control over the amount of water delivered from neighboring gardens, failure to account for it can result in plant mortality.

Runoff from impermeable surfaces like roofs, paths, driveways and roads can be directed into gardens. Gardens that intentionally make use of water from these sources are referred to as “rain gardens.”  Rainwater harvesting conserves water, reduces urban runoff, and increases rainwater infiltration, making it an important sustainable garden practice. Again, when planning a garden, consider this and all other sources of water.

Curb cut allows water diverted from street to flow into parkway rainwater garden.

Once you have a good sense of how much water you have, where it is coming from, and how it is delivered to different parts of your yard, you need to think about how it interacts with other physical characteristics of the garden. For example, gardens with lean, well-drained soil do not retain water as compared with gardens containing loamy or clay soil. Low lying areas with heavy soil may stay wet for months without any supplemental irrigation. Mulch also influences water retention in soil, as do wind and sun exposure. And finally, it should be remembered that gardens change over time. Heavy, compacted clay soil can develop improved structure and drainage as new plants take hold, extending their roots through the soil, while worms and other insects create pores and incorporate organics.

Into this amazing and complicated system we attempt to predict which plants will do best. Gardens sometimes offer wonderful surprises because they are complicated. The red willow in the nature park is an example. When I first saw this plant standing high and dry on the mound I thought someone had selected exactly the wrong plant for that location. Later I learned it just showed up on its own, and was exactly the right plant for that location.

Big arrow points to red willow (Salix lasiandra) growing on mound in nature park. It just showed up, uninvited and unannounced but very welcome. Since there is no irrigation in the park, there must be water flowing nearby.

Still, as gardeners, we try to make educated choices. Over years of growing plants horticulturists and nursery women and men have gathered information on growing needs. Good gardeners then try to select plants that will be most compatible with their garden conditions. But even here the picture is not as exact and precise as we may want. Plants come labeled with vague information, such as: provide average, regular, or ample water. Regular moisture is defined in the Sunset Western Garden Book (2001, p. 161) as “soil shouldn’t be too dry or too wet.” Plants that require little supplemental water may be designated as “California Friendly,” water wise, drought tolerant, low water use, or water conserving.

These vague terms usually ignore whether the water is needed throughout the year, or only in certain seasons. Native plants that grow in our dry, mediterranean climate are adapted to the dry, hot summer, but require winter rains to survive the annual drought. Many will succumb to disease if they receive too much summer water. In fact, over watering is probably the biggest cause of failure in native plant gardens. Still, some coastal plants being grown in inland gardens may need supplemental water in both summer and winter, while others that go dormant in summer may rot out in hot, moist soil.

Plant label reads “Most soils; drought tol.” Interesting since Juncus is usually considered a more moderate water consumer. My experience with this plant does suggest that though it will accept year around moisture, it can survive and even thrive on less.

So how can we make sense of all of this? First, it must be noted that water requirements are given for established plants. New transplants often require additional water until they have bulked up and developed roots that extend into the surrounding soil. This can take one to three years depending on the plant type. (Plant care during establishment is discussed in more detail in Keeping them alive after they’re in and Keeping them alive powerpoint presentation.) So whenever you see water guidelines remember that it means after the plant has become established.

The reason that water guidelines are so ambiguous is that gardens are complex systems, and plants are complex, living organisms. However, water guidelines do have value. They should be used to group plants with similar water needs. This practice, called hydrozoning, makes the garden easier to manage and much more likely to succeed.

Beyond that, you will need to experiment to determine what watering schedule works best for your plants. This takes observation and experience. Whenever I travel, leaving my husband in charge of the yard, he requests a list of plants with clear and precise instructions on when to water each. The best I can do is to tell him to water them when needed. He does not find this very helpful, and he lets me know. He is not on intimate terms with each plant, as I am, and so these vague instructions are useless. I try to provide better information, but a serious gardener should indeed follow the most obvious guideline, water as needed.

I planted this flannel bush – too cheap for me to pass up –  in the one place where there is room and sun (I don’t have any full sun). It is next to lawn which doesn’t get irrigated during t he summer. Flannel bush does very poorly with summer water but surprisingly has been doing great in this location since March 2007. The loamy soil, part sun, and nearby lawn are not conditions one would expect flannel bush to like, but here it is. (Now watch it croak to spite me.)

Other Wild Suburbia articles on plants and water:
Summer water
Water while you play (drip irrigation for vegetable garden)
First steps to a sustainable habitat garden