Ask Mr. Smarty Plants

There are two palms that are referred to as Washingtonia palms; one is Washingtonia robusta, which is non-native to North America, and one is Washingtonia filifera (California fan palm), native to California, Nevada and Arizona.  We are hoping you have the Washington filifera, because it is native to North America. At the Lady Bird Johnson Wildflower Center we support the propagation and protection of plants native to North America and their use as landscape plants in areas where they are native. 

The Washingtonia filifera is also preferable because it is shorter, has a thicker trunk and is better for planting in dry urban landscapes. However, the answer to your question would be the same in both situations, as they are very closely related. The research we did indicated that it was better to trim off those lower dead leaves, or skirts, to cut down on fire danger, and to remove a hiding or nesting place for rodents and insects. This probably needs to be done professionally, even though your trees have not yet reached the 50-80 feet to which they can grow. Trimming can be repeated every few years. 

You are correct that it is a little surprising that they are growing on your property. The best we can tell from the USDA Hardiness Zone Map, your area alternates from Zone 7b to Zone 9b. The optimal zone is Zone 9 to 11. However, plants don't necessarily grow where they can grow best, they grow where they can get away with it. Your palms are obviously getting away with it. Here are some pictures of the Washingtonia filifera (California fan palm), both trimmed and untrimmed. Perhaps this will help you make the decision. 

Washingtonia filifera