This perennial, spiny, prostrate cactus typically forms a broad mat, up to 20 inches wide. Pads -- each up to 2 inches long and 1 inch wide -- form chains, which sometimes branch. Spines on pads usually grow in clusters, and some can be longer than the width of the pad. The 2-inch-wide yellow flowers sometimes have reddish or greenish centers. The small fruits are dry and inedible.
This is one of many cacti called prickly pear from spine-covered fruits. Brittle refers to the ease with which the pads break away. Minnesota also harbors plains prickly pear (Opuntia macrorhiza) and ball cactus (Coryphantha vivipara).
Brittle prickly pear is not present in hot, dry deserts. It grows in dry sand and bedrock outcrop habitat throughout the western half of North America. It has scattered populations in Minnesota, Wisconsin, Iowa, Illinois, Michigan, and Ontario. It is relatively common in Minnesota's southwestern corner and is scattered along the Minnesota River valley. Isolated populations grow in the St. Croix River valley, Stearns County, and Lake of the Woods.
This species blooms in May, June, or July, depending on regional climate. Blooms last only a few days. In many populations, flowering is rare or nonexistent. Reproduction most often occurs when pads break off and take root to start new plants. Animals, water, and wind can carry pads. The minutely barbed spines can anchor in flesh, fur, or clothing, even shoes. This attachment ability and trait of easily releasing outermost pads are likely adaptations that allowed this species to expand its range across the Great Plains by hitching rides on buffalo.
Brittle prickly pear has the greatest freezing tolerance of any known cactus. Populations in Canada and the northern United States can be found where winter temperatures drop to minus 40 F. The plant's low, sprawling growth habit and relatively thick pads may help protect it against frost injury by keeping it close to the ground, where temperature changes are less extreme. As temperatures drop in the fall, pads become less moist and build up higher concentrations of abscisic acid, which might protect against frost damage. Brittle prickly pear doesn't tolerate competition from grasses but can thrive on fast-drying, nutrient-poor soils where grasses cannot persist, such as bedrock cracks.
On state endangered species lists in Illinois, Michigan, Iowa, and Wisconsin, it is protected in several other states and Canada. Brittle prickly pear is not considered threatened here, but individual populations can fail due to habitat loss or disturbance by human activity.
Dave Crawford, park naturalist, Wild River State Park