Probably most kids in Southern California are familiar with this weed, from having the burrs stuck in their socks after playing outside at recess or lunch (I remember being fascinated by the burr's spiral construction!). There is rather a lot of it in our front yard this year for some reason, so I decided to look it up. It is Medicago polymorpha, also known as burr medic, toothed bur clover, and, for some reason, California burclover -- it is not native to California at all, but to the Mediterranean, presumably the reason why it grows so well here. In lawns it is usually short, most likely because it keeps getting mowed off, but when given its head it is long and sprawling, either along the ground or borne up by nearby plants.
(Quite a number of online sources remark on its being called "California burclover" while native to the Mediterranean, but none of them wants to even hazard a guess as to why it is associated with California!)
