by Christina Mountanos
As I looked around my garden this past weekend, I felt myself sigh in both satisfaction and a little relief. No doubt summertime is here and the plants in my north-facing garden are enjoying the warmer weather and longer days as much as I am! The star jasmine that turn pitifully bare every winter have resurrected themselves once again, and my bougainvillea is back and bigger than ever, spilling its maroon blossoms wildly over my neighbor’s fence.
As a beginning gardener, this has been my most productive spring yet. In contrast to previous years, almost all of the projects I’ve been working on have yielded good results. I’ve been successfully coaxing two morning glories up a trellis, patiently shaping a small collection of rosemary topiaries, and the petunias I received for my birthday in April are still alive and thriving in the intensifying sun. Surprisingly even the poppy seeds I scattered at the end of May have sprouted and grown!
Maybe it’s safe to say that I’ve finally gotten a handle on how things grow, and it’s likely that working in water conservation has helped. But, as far as I’ve come, I do often find myself still grappling with some of the most basic of gardening concepts. One that I struggled with recently? Roots! What’s so complicated about roots, you ask? Well, let me start by saying that if you’re well-versed in water-wise gardening, you’re surely familiar with the principle of watering deeply, but infrequently to encourage more drought-tolerant plants. If you haven’t heard this before, watering in this manner can create plants with roots that grow more deeply. Not only can plants with deeper, more extensive root systems find water and nutrients in more places, but having roots further away from the surface of the soil means they also stay moist longer and plants don’t dry out as quickly.
It was this concept that got me thinking. How deep is “deeply,” exactly? Of course, watering my potted plants has always been easy; I simply water until it comes out the bottom (fool-proof!). But, as I graduated from the simplicities of planter gardening, I began wondering about the tall shrubs that run around the perimeter of my yard. How deep are their roots? And what about trees? Is it both possible and necessary to water their entire roots zones?
Well, what I have found is that plants and trees have portions of their root systems that can, in fact, grow very deep in the soil. You may remember Charlene mentioning in a previous post that some California annuals have roots that reach 20 feet! These deeper roots can serve as structural supports and to find water and nutrients in extreme conditions. Oak trees growing naturally on our watershed, and throughout Marin, typically have taproots that grow deeply for this reason.
However, I was surprised to find out that the vast majority of a plant’s root system is concentrated much closer to the surface than I originally thought. Roughly 80% of a tree’s roots, for example, are concentrated in the top 12 to 36 inches of soil. Quite amazing when you think of how tall trees can get! Roots are confined to this depth, for the most part, because this is where the most oxygen, minerals, and nutrients are readily available. These elements become less and less prevalent as depth increases, and thus roots do, too. Not surprisingly then, watering beyond a depth of 36 inches essentially wastes water and effort.
A great take-away tip that I found from the California Master Gardener Handbook, and one that I now use for hand watering, is the 1-2-3 rule. Water to a depth of one foot for small plants (like annuals and groundcovers), two feet for medium sized plants, and three feet for large shrubs and trees. How long it will take to reach this depth will vary depending on your soil type and the flow of your hose, so some initial experimentation is necessary. A day or so after watering, use a soil probe or a shovel to dig down and to see how far the water has traveled, then adjust accordingly. Consider using this same procedure to check that your irrigation runtimes are sufficient as well.
With this small token of wisdom comes another sigh of relief. So far, gardening has surely been a process for me. It’s been a piecemeal operation with successes, frustrations, a lot of listening to those wiser than me, and most importantly, enjoyment.