After a significantly long dry spell, I decided to reflect on the joy of the first substantial rainfall of the season for southern California. On this day of January 9, 2018 in north San Diego County, we cained 2 3/4 inches of rain as of noon. With this in mind I was reminded of an Encephalartos in the ground with about an 11" diameter Caudex that a customer had offered to purchase. Although I hadn't really thought about selling that plant, after careful consideration, I agreed to the sale. My big concern in digging the plant was that I had just "deep soaked" the plant a day or two earlier. My initial thought was that the dig would get real messy, real quick. To my surprise, as I dug, I learned that the "deep soak" that I had completed, only penetrated the dry decomposed granite soil to a depth of a little more than four inches. All soil below that point was bone dry. I had no idea that my irrigation efforts with a garden hose had fallen so short.
Every soil is somewhat different. As growers it is important that we understand just how much water is needed to deep soak our plants sufficiently. With today’s first good rain in many months, I knew that much of my soil had not seen rain or artificial irrigation for quite some time. I decided to run a little test (although not very scientific). I had captured rain water that had fallen into an empty bucket that stood unobstructed in my nursery. After a significant amount of rain had fallen, I measured the depth of the water in the bucket. It came out to be a hair over three inches. Since the bucket was not a perfect cylinder, I felt it appropriate to reduce the actual water level to approximately 2 ¾” in depth to compensate for the slightly flared mouth of the bucket. This provided a more accurate measurement of the rain water that had actually fallen.
Next I decided to dig two holes in my natural dg soil, in order to measure the depth of the rain penetration into the soil. Since my property had not received any irrigation or rainfall in the areas I chose to excavate, the depth of water-soaked soil could be entirely attributed to our first rainfall of the season.
The first hole was dug into a very gentle slope of approximately 10-12 degrees. The rain water penetration in the soil at that location was approximately 6.5” deep.
The second hole was dug where the soil was flat (to the naked eye). The rain water penetration at that location had a depth of approximately 10 ¼” (significantly deeper).
Clearly the water penetration was greater when the water was given time to soak in. Conversely, less water was retained in the soil, and the soil had less time to absorb it, when the water fell on a downhill grade.
This simple exercise showed me that there is more to proper irrigation than meets the eye. It showed that it could be fairly easy to water an entire garden of plants and not really achieve the objective. You must know your soil, and then irrigate sufficiently to get water to the root zone. Many plants seem to be fairly forgiving, in that they continue to grow even without optimum irrigation practices. If much of your garden is on a slope, consider building a shallow, broad, level basin (See blog from February 2017, “Late Winter/Early Spring Cycad Garden Maintenance” for the description & tips on forming soil basins.) around your plant specimens to concentrate your irrigation where it is needed, and to give the water time to penetrate.
I guess the take-away from this exercise would be to know your soil, and understand the amount of water that is necessary to penetrate to the root zone of each plant.
Grow and prosper,