Plants of Southwest U.S.: Low Precipitation, High Diversity
The sandy beds of this collection are home to plants that have evolved to survive long periods of dry weather. These adaptations have led to many species unique to the American Southwest, and we have acquired those that can deal with the climate of northern Illinois.
Contrary to assumptions about the desert, this region is very rich in species. Desert flora includes cacti, Agave, joint-fir, and others. Deserts, while they can be very sparsely vegetated, often have incredibly diverse flora and fauna. Some of this diversity has resulted from the evolution of traits to help species survive in the desert's challenging climate.
This collection was started in 1985. In order to accommodate this unique plant habitat on the Arboretum grounds, the collection site was chosen on the West Side, on a southwest facing slope just east of Lake Marmo. The site, covering just less than 2 acres, was excavated and filled a mixture of sand and gravels to facilitate drainage. The collection now contains 44 different kinds of plants and 113 specimens.
Yucca, a ubiquitous plant in desert habitat but a rarity here, is represented by three species in the collection: narrow-leaved yucca (Yucca angustissima), Bailey's yucca (Yucca baileyi), and soapweed (Yucca glauca). Yuccas are actually in the Lily Family, the same family as tulips! The cacti in the collection are the pincushion cactus (Escobaria vivipara), cholla (Opuntia imbricata), desert prickly pear (Opuntia phaeacantha), and plains prickly pear (Opuntia polyacantha). The oldest living tree in the collection is a black maple (Acer nigrum) planted in 1922.
Prickly-pear (Opuntia), the most cold-tolerant cacti, is the only cactus found in the eastern U.S., and can extend as far north as Canada. Chollas (also in the genus Opuntia) grow in cylindrical segments with large barbed spines. These barbed spines can hook into skin or clothing and break stems off of the main plant. This helps to spread the cholla, because broken stems can take root when they fall to the ground.