Ask Mr. Smarty Plants

There are lots of members of the Family Lamiaceae (Mint Family) and they all share the characteristics of a square stem and opposite, simple leaves.  Many of them are aromatic.  You can see native California members of the Family Lamiaceae if you select "Lamiaceae" from the Family: list on our Native Plant Database page and then use the NARROW YOUR SEARCH on that list to choose "California" from the Select State or Province option.  You can then see the more than 50 species listed on the list.  There are, of course, introduced members of the mint family and your description reminds me of one of those, Lamium purpureum (Purple dead-nettle), a native of Eurasia.  Here are more photos and information from Virginia Tech Weed Identification Guide, Illinois Wildflowers and Connecticut Botanical Society.  The Southern Weed Science Society lists this plant on its Weeds of the United States and Canada and it is also included in T. D. Whitson's Weeds of the West. 

If neither this Lamium purpureum (Purple dead-nettle) nor one of the plants from the native California Lamiaceae is the plant in your yard, take photos of it and submit them to one of the plant identifcation forums for identification.   You can find links to several of these plant identification forums on our Plant Identification page.