I thought the caption said mandarines..
These WERE mandarines indeed.
The story in short is, that Buk had a biiig bunch of those from the trees in his garden, filling up to two huge trash-cans (AFTER Linda took to the 'Dewdrop-Inn' as much as she could use as long as they were good). The time was March 1980.
He didn't know what to do with them and Benno Käsmayr, who was up for a visit at the time, suggested to simply bury them. That's the process we see in the picture as well as in the following one:
The guy helping Bukowski in the pic is not Benno, but Rainer Wehlen, the translator of the German edition of 'Shakespeare Never Did This'.
(I'm sorry, I can't find those pics in color right now. They must be on a very old hard-drive or CD-R/DVD-R, since it's over 10 years ago, when Benno sent me the original photographs to scan them. The reason I easily found it in b/w is, that I've published this one in our yearbook of 2004 (p.58) along with the background-story written by Benno himself.)