The Butter Bolete of California forms a distinctly dense fruitbody (when young). The cap color is quite variable, but it is easily told apart from B. regius by the saturated red colors giving way to cinnamon/tan or yellowish hues, sometimes paling out completely. Nomenclaturally, B. appendiculatus establishes Section Appendiculati, which it shares with B. regius and half a dozen other species (including B. subappendiculatus). And while our local B. regius follows pretty close the original concept, the same cannot be said about this taxon. The European descriptions of B. appendiculatus pinpoint the cap as solid chestnut-brown. In that respect our name appears rather provisional and it is hard to imagine that it will survive critical re-examination by contemporary means. In fact, B. pseudoregius (Hubert ex Estades) looks much closer superficially to what we have here.
The primary habitat locally is Live Oak and Tanoak and this matches well the accepted species ecology. Yet, there have been numerous reports of B. appendiculatus or a look alike taxon from Sierra Nevada (Spring) fruiting under conifers. Since Snell & Dick did report B. appendiculatus from Sierra Nevada in a coniferous habitat, it is safe to name our Sierran collections the same way too. At the same time that causes our California Butter Bolete to depart significantly from its originally described habitat and range, thus “seriously provoking” a critical study.