I have a Washingtonia filifera that I planted in an especially well-drained location hoping that it will survive damp North Carolina winters (Colington, NC 8b/9a).  It has been in the ground 3 years and is doing well - except that the leaves yellow prematurely.  It has drip irrigation and gets Palm Gain and/or Carl Poole palm food every spring and early summer.  It is south-facing and gets almost all day full sun.  Soil is sand/silt from old sand dunes.  I've tried adding organic matter and even lime (although pH seems to be around 7). 

What should I try to get this thing to keep green leaves longer?

 

    

If your ph is 7 with the addition of lime, I think that may be the issue. They love ph in the 8 range. 

Once established I doubt it would need supplemental water in your climate.  They grow a root ball larger than the crown.  

I'll try some more lime.  Thanks!

From the native groves-

Most soils supporting
California palm are high in pH (average pH 9.2) [40].  Soils in its root
zone, however, are generally less alkaline than topsoils [33

 

https://www.fs.usda.gov/database/feis/plants/tree/wasfil/all.html#15

2 hours ago, jwitt said:

 

https://www.fs.usda.gov/database/feis/plants/tree/wasfil/all.html#15

Thanks for this great reference!!

In Southern California it is rare to see one of these yellow like that . Even in fields or on the side of the road where they volunteer, they look green . They seem to grow very well even without water supply or care they stay green. I would say a combination of climate and soil problems as others have mentioned. Harry

I have one growing closer to the house that is green (but it is dwarfed by the larger W robusta in this picture).  The yellowing one is by the street on a sandy hill where a pine tree used to grow.  It gets drip irrigation with county water that does have some chlorine in it,  It was yellowing before I gave it any drip, though.  It's possible that some herbicides were used in that area in the past.