Ask Mr. Smarty Plants

When we are asked a question like this, we always try to determine which species of the plant we're dealing with, as there might be different conditions. So, we went to our Native Plant Database and searched for yuccas native to California. Of the 28 plants listed, there were three native to California, Yucca baccata (banana yucca), Yucca brevifolia (Joshua tree), and Yucca schidigera (Mojave yucca). This doesn't necessarily mean you have one of those plants, it just gives us a starting place for finding the answer to your question. Unfortunately, we found no specific instructions for trimming any of these yuccas, so we will provide you with a little generic advice from personal experience.

If you have a yucca that has developed a trunk, and that is where the dead foliage is appearing, those dead leaves eventually will peel off the trunk and drop to the ground. You're right, they are pretty unsightly, so you can hurry the process along by cutting the blade as close to the trunk as possible with a sharp knife. Please prepare to protect yourself, first. You will need heavy long sleeves, long gloves, preferably leather, goggles to keep from getting a pointed tip in your eye, and a knife or pruner with as long a handle as possible. The yucca will defend itself, and it is well-equipped to do so. Even the dead foliage is still tough and fibrous, but it can be gotten off. Bag up the detached material in a heavy paper trash bag and clean up the surrounding area to prevent insects or  diseases from harboring there. This is not material for the compost pile; the sharp edges and tips will persist for a long time.

Yucca baccata

Yucca schidigera

Yucca brevifolia