The Colors of Cacti

Stem Color

• Epiphytic cacti with flattened leaf-like stems such as orchid cacti (Epiphyllum spp.) and night-blooming cereus (Hylocereus undatus) have bright green stems. These tropical species grow in shaded areas and are hardy to U.S. Department of Agriculture plant hardiness zones 10 through 12. The columnar Mexican fence post (Pachycereus marginatus) has deep green stems and is hardy to USDA zones 9 to 10. Some cacti have other plant pigments in addition to chlorophyll, such as rose plaid cactus (Gymnocalycium mihanovichii var. friedrichii), which can be variously shaded with purple or brown. This South American native grows in USDA zones 8 through 10.

Waxy Stems

• Cacti that appear to have blue, gray or white stems have the chlorophyll-bearing layer obscured by wax. Waxy layers help to keep the plant from drying out and also shade the skin. You can reveal the stem's basic color by rubbing away the wax layer in a small area. Blue-stemmed blue myrtle (Myrtillocactus geometrizans) is hardy in USDA zones 9b to 10a. The bishop's cap cactus (Astrophytum myriostigma) has the skin closely covered with dots of white wax so that it appears white to gray. Native to Mexico, it grows in USDA zones 9 through 11. Gray-stemmed pitayo de mayo (Stenocereus griseus), native to Mexico and Venezuela, has a waxy covering with a somewhat banded appearance. This columnar cactus is hardy in USDA zones 9b through 11.

Spine Colors

• Cactus spines are modified reduced leaves and range from gray, black, brown, tan, orange, red, yellow and white. Spines can be wide and flat, long and slender, straight or hooked, thread-like or hair-like. Some cacti have such a dense covering of spines that you can't see the skin. This serves to shade the cactus and to insulate it against extremes of temperature. The old man cactus (Cephalocereus senilis) is covered with long, white hair-like spines and is hardy in USDA zones 9 to 10. Golden barrel cactus (Echinocactus grusonii) is less densely covered with clusters of yellow spines along the ribs, but the spines give the predominant color to the plant. Valued for landscaping in USDA zones 9 to 10, golden barrel is also a popular container plant.

Seasonal Color Changes

• When subjected to cool temperatures and short day lengths, many cacti develop brighter colors in areas touched by direct sunlight. As plants shut down growth for the winter, chlorophyll content decreases and brighter plant pigments, normally hidden by the chlorophyll, become evident. Colors commonly seen are yellows, oranges, purple and red. The colors disappear when the plants begin active growth again. In the spring when cacti are actively growing, newly developing spines are more brightly colored before they dry up and harden with age. Young unexpanded pads of prickly pears are more strongly colored than mature stems, as shown by the bright purple new pads of "Santa Rita Tubac" prickly pear (Opuntia santa-rita "Tubac"). Winter pads also display a stronger purple color. This prickly pear is hardy in USDA zones 8 through 11.