Marin Master Gardeners
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Marin Master Gardeners

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Know how to water plants to grow healthy roots

June 11, 2016
Barbara Robertson

Recently a kind and generous neighbor placed a few pots of brilliant red geraniums and sparkling white alyssum on my patio and said, “You need to deep water them.”

I looked at one of the shallow clay pots and thought, “OK. No problem.”

Everyone knows that plants need to be deep watered. But, how deep is deep? This time it was obvious, but until I attended a seminar by Lois Stevens who chairs the UC Marin Master Gardener help desk in Novato, I didn’t know how little I knew about deep watering a garden.

I had heard other UC Marin Master Gardeners praising Stevens’ “Deep Watering for Healthy Roots” seminar, and they were right. It is fantastic. Here are a few of my light bulb moments from the seminar I attended and a later interview.

According to some experts, California has been under drought conditions for the past 15 years. Climate change models indicate that in the future, rain will come, as we’ve recently seen, in larger storms in the shoulder seasons, which produces unwelcome runoff. With reservoirs full or nearly so now, it is the perfect time to plant natives that need two years to settle in before becoming drought-resistant. But, whether or not you have native plants, Stevens points out that we can maintain a healthy garden without necessarily increasing our water usage by watering plants and trees deeply.

“Deep,” in turns out, doesn’t mean sending water to the bottom of a tree’s taproot. In fact, fewer than 2 percent of all trees grow a taproot. Instead, roots spread wide, sometimes as wide as the tree is tall and not very deep. The same holds true for most other plants. Tree roots typically extend 18 inches to 3 feet down, shrubs 1 to 2 feet, and turf, ground cover and bedding plant roots grow in the top 6 inches to a foot of soil.

This is the light bulb moment. Seventy percent of a plant’s roots are usually in the upper half of the plant’s maximum rooting depth, and 70 percent of the water used by a plant comes from that root zone. At night, the deeper roots lift water up toward the surface for the feeder roots to access during the day. So, if the plant gets most of its moisture from the top half of the root-zone, shouldn’t you irrigate an area around the plant frequently?

“No,” Stevens says. “If you water shallowly and frequently, the roots don’t need to expend energy seeking water because it’s right there. It leaves them with no ability to deal with a drought.”

So, maybe you should water deeply and frequently? Again, no. More is not better.

“The roots will go deep, but they won’t develop a strong dense network,” she says.

With frequent deep watering there is the possibility of root rot not to mention wasted water.

“Water extracted by deeper roots can keep a plant alive, but these roots do not maintain optimum growth,” she says.

The answer is to water deeply and less frequently.

Stevens showed a slide illustrating the dramatic differences. Shallow, frequent watering produces a cluster of roots near the soil line and a few spindly roots below. Deep frequent watering produces a nice pattern of roots, but not many. Deep infrequent watering produces the desirable vast, dense network of water-seeking roots.

The amount of water that is required for optimum depth depends on your soil. The best way to know is to stick a shovel into the ground here and there or, as Stevens says, wiggle your fingers to feel the soil a foot or so deep using your own moisture meter. It should feel like a damp sponge. Remember to water wide — beyond the drip line to anchor that tree. Your garden will thank you.

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