Oyster Mushrooms. In Colorado we have two main Pleurotus species and likely others. Pleurotus pulmonarius is generally found in the Spring and again in the Fall on Cottonwood logs and stumps. It can also be found on other dead wood and trees. Pleurotus populinus can be found exclusively on Aspen typically starting in July at higher elevations. More rarely you can find oyster mushrooms on conifers such as Douglas Fir.