The exposed ash fall layers that comprise the most spectacular geological features of the Painted Hills Unit in the John Day Fossil Beds National Monument are nearly devoid of plant life. The reasons as to why start with the clay-rich soils. The initial part of any rainfall (and there is little – less than 41cm (16in) per year) is immediately absorbed by the clay component of the soil — think clay-based kitty litter. Once the top layer of clay is saturated with water, it becomes impermeable to any succeeding rainfall, so the rest of the precipitation quickly drains away; in the process, the draining water flushes away any organic matter that might have built up on the clay (through wind, perhaps), eliminating another factor in the establishment of plant life. Little available water and little organic matter equals a very inhospitable environment.
Where the underlying ash fall layers are not exposed, a number of plants can be found including sagebrush, a myriad of grasses, juniper and shadscale.
The Nature and Science of John Day Fossil Beds National Monument part of National Park Service web site goes into more detail – fascinating reading.