The LA Land Tree of the Week is Juglans californica var californica, also known as Southern California Black Walnut to you and me.
I've taken an interest in our native walnuts since going on the Payne Foundation garden tour and stopping to see a remnant walnut woodland in Encino where a road was once planned, but never put in. Walnut woodlands are far rarer than oak woodlands because they generally coincided with places people wanted to put homes. While there, we were encouraged to take several green walnuts with us, which I subsequently planted and which grew marvelously well without any special treatment. Several are now on the upper 40 slope at my parents' place as experiments and one is still in my back yard.
Aside: Dog latin plant names and plant nomenclature seem designed to trip one up. Here's one bit of trickiness that I think I've figured out. If you were to point to an arbitrary "Southern California Black Walnut" growing wild, then it would be "Juglans californica". However, if you were to purchase it from a nursery, you would get "Juglans californica var. californica" (frequently also "Juglans californica californica"), which is just a way of describing that the purchased variety was originally selected by the nursery trade from among a large number of wild Juglans californica plants, and may well be propagated from a cutting to ensure growth habits consistent with those known for the parent plant. (As a further aside, this makes it a clone of its siblings, which ought to be a consideration when thinking about biodiversity issues, though I would guess this is far from most gardeners' minds.)
I disagreed with some of the preferred habitat descriptions that were called out in the Tree of the Week, ("the SCBW prefers to grow on moist sites, northern slopes, and in streamside woodlands accompanied by oak or cottonwood") so I looked to some web resources at the Payne Foundation (my old favorite online plant reference guide) and Calflora (my new favorite online plant reference guide) . Consulting with both references, I found no preference for moist growing conditions. In fact Calfora states, "Wetlands: equally likely to occur in wetlands or non wetlands."
I was about to move on when I happened to look at some of the pictures of Juglans and noticed that the leaves were quite different from the plants that I have started so successfully in my back yard and on the upper 40. The leaves on my plants are pinnately lobed, but on Juglans they are invariably described and pictured as ovate to lanceolate (in other words singly pointed, not lobed). Is it possible that the juvenile growth habit of the leaves is different than the mature growth habit or did I just get a bunch of something other than Juglans but which resembled a walnut very much?
This is something I'll have to track down.
EDIT 30 Dec 2007: I asked my local nurseryman and the consensus is that the plant is Lavatera. I was told Lavatera maritima, but there's no Calflora listing under that species name. Comparing leaf shape and growth habit to the pictures I get Lavatera assurgentiflora (tree/bush/island/San Miguel island mallow), a California native flowering shrub to 15 feet tall. This is a nice plant with beautiful flowers that is happy to coexist with oak woodlands.