|The leaves of this palm look like an accordion; why?|
The most common reason for this kind of disfigurement on palms is a lack of boron. Essential for the growth of palm trees, boron is fairly common in the soils of Southern California. Even though it is practically ubiquitous there are certain cultural factors that can prevent if from being available to the plant. If the soil is too acidic boron will easily leach out of the soil; one way to do this is to fertilize the lawn surrounding the palm with Ammonium Sulfate, a type of nitrogen source that can acidify the soil its applied to; another way to inadvertently raise the pH would be to keep the area to moist for a period of time -constant moisture and lots of organic material in the soil can lead to an anaerobic and acid condition. But ironically another condition that can make boron unavailable is soil that is heavy clay and too dry. With the preceding condition the boron becomes bonded to the clay particles and becomes unavailable to the palm.
The first remedy is to find out what physiological or cultural condition is causing the boron deficiency and fix it. Check the drainage around the plant, after a rain or a heavy watering look for water that puddles and doesn't disappear for a while. Check the pH of the soil too with a kit (you can buy these from your local garden center or on the web). Finally, there's the '20 Mule Team Cure'; 2-4 ounces of household borax added to 5 gallons of water for each medium sized palm tree. Make sure to keep the solution agitated because borax doesn't dissolve very easily in water (remember using borax to wash you hands back in the day?). Err on the side of less rather than more because borax in higher concentrations is an herbicide.
But its not just boron that can cause accordion leaf; anything that adversely affects the main growing point of the palm, the apical meristem, can also cause it as well. This means diseases like pink scale, a fungus that shows up on palms planted in poorly drained areas with poor air circulation, and herbicides can cause the problem too. So make sure to rule out all these causes before you start applying borax to your trees.
Descriptions of boron deficiency:
This document discusses the '20 Mule Team' cure: