According to most literature, Lithodora diffusa should be propagated by softwood, or semi-hardwood cuttings. By chance, I discovered a new method for multiplying stock.
Given Nova Scotia's wet, and sometimes quite cold winters, I make it a habit to shelter a few valued plants in our unheated polytunnel greenhouse, as insurance against loss. During the winter of 2012-13, my stock Lithodora was placed on the sand floor of the greenhouse, where it rooted in place. By the time I discovered this, I noticed that there were green shoots sprouting from the small exposed root area where the root left the drainage hole in the side of the pot, but before it disappeared into the sand.
After lifting and separating the new plantlet from the pot, I decided to run a test. The parent plant was tipped out of its pot so I could harvest a few more roots. These were potted up in a sandy substrate, with the cut surface positioned at, or slightly above soil level(exposed to light). All roots initiated several new shoots, and grew into robust plants more quickly and reliably than by the old method of using softwood cuttings and rooting powder.