Many visitors at Rocky Mountain National Park are surprised to find the park is home to two species of cacti- the mountain ball cactus (Pediocactus simpsonii var. minor) and the plains prickly pear (Opuntia polyacantha). In spite of occasional snow showers, early spring temperatures may be warm enough to encourage flowering, especially on rocky surfaces that hold and reflect warmth to the plants. Usually we expect mountain ball cacti to be blooming in May and early June in most locations. The plains prickly pear, in contrast, usually blooms in late June when temperatures are warmer. Fortunately, because the park has a wide range of altitudes, the blooming season for both cacti can extend across several weeks.
Most of us normally think of cacti in association with warm, dry environments, but we don't often think of them in colder, dry environments typical of the park. Our cacti endure freezing temperatures by shriveling in the fall and winter. This shriveling is similar to the much more familiar sap cycles in some trees in which sap "goes down" in the fall and "rises" in the spring. Not only do easterners depend on this sap cycle for harvesting maple syrup, the trees, and, in this case cacti, depend on it to prevent freezing and cell rupture in delicate leaf and stem tissues when temperatures fall below freezing.
Shriveling also tends to make cacti even less obvious and less palatable to herbivores (plant eating) animals like deer and elk looking for a winter meal. Each mouthful is dry and full of spines. However, animals do occasionally use mountain ball cacti as survival food.
So next time you visit Rocky Mountain National Park, look for the unobvious but fascinating cacti in montane areas of the park.