If you asked most Americans to draw a cactus, even the most artistically challenged would probably produce a tall, cylindrical plant, its upraised “arms” beckoning you into the desert. The saguaro (Carnegiea gigantea) is undoubtedly the most recognizable cactus to most of us, but it’s only one among more than 1,700 cactus species. To complicate matters more, there are many other succulent plant species that are confused with cacti. What’s the difference?
Nearly all cacti are succulents, but not all succulents are cacti. (“Time for a Venn diagram here, class!”) Succulent plants are defined as those that have thickened, fleshy parts—typically leaves or stems—that allow them to store water. Aloes, agaves, sedums, stonecrops, certain euphorbias, etc. are succulent plants, but are not cacti.
Cactaceae is a large plant family, encompassing some 127 genera and all the true cacti. Like most plant families, all cacti have similar flowers—and DNA—that distinguish them from other groups. You might see a plant like the Canary Island spurge (Euphorbia canariensis) that does a pretty good imitation of a cactus, but is botanically unrelated, instead being a cousin of the poinsettia. Many other succulent plants—aloes, for example—apart from some degree of spiny-ness, don’t resemble cacti at all.
Cacti have evolved various ways to deal with dry climates. Extensive shallow root systems allow them to quickly absorb water from brief rains. Spines, actually modified leaves, not only provide protection from herbivores, but also limit air movement, maintaining higher humidity around the plant. And incredible as it may sound, the spines also provide some degree of shade. Thick, fleshy cactus stems have a much lower surface-to-volume ratio than leaves on other plants, again reducing water loss.
Photosynthesis in cacti takes place in the stems, rather than in large leaves, which can quickly lose water. (Some cacti do indeed have leaves that are very small, or quickly drop off.) In yet another water-conserving trick, cacti have evolved an alternative type of metabolism. Stomata, small openings on the stems, can close during the heat of the day, limiting water loss to the atmosphere.
Cacti come in a variety of shapes and sizes, with two types predominating: the cactoids, which may be columnar, globular or barrel-shaped; and the opuntioids, which have stems divided into distinct flattened “joints” or “pads.” Among the first type is the iconic saguaro. Although widely depicted as a symbol of the vast American desert, the saguaro is native only to southern Arizona, a similarly-sized area in northwest Mexico, and a tiny strip of California. The saguaro is not quite the world’s tallest cactus, but at over forty feet, it comes close, and nothing can match it for character. It may take about 75 years to develop its signature arms, but then it easily takes on a human appearance, lending itself to depictions adorned with cowboy hats, lassos, etc. Living up to two hundred years, they can grow as many as 49 arms, thereby losing the human appearance.
Like any resident of the desert, saguaros are challenged by their environment. As seedlings they need some shade from a “nurse” plant, and may only grow a quarter-inch tall in their first two years. In their search for water, their roots may extend 98 feet from the trunk, and in a significant rainstorm, they can suck up 200 gallons of water. The pleats in the saguaro’s trunk allow it to visibly expand as it absorbs water, and a fully hydrated plant can weigh two tons.
Saguaros are known as a keystone species in the Sonoran desert ecosystem. Their waxy white flowers can be pollinated by honey bees, bats and a variety of birds. Gila woodpeckers excavate a nest in the trunk, which other species may use in subsequent years. The ribs, or woody skeleton, of a dead saguaro can be used for fences and light-duty construction purposes. Native American people made them into poles for picking the edible fruits off the arms.
Almost as well-known as the saguaro is the prickly pear cactus, actually comprised of dozens of Opuntia species that occur over most of the United States, including three species native to Virginia. Opuntia pads vary in size and shape, but generally are equipped with conspicuous sharp spines. Far more insidious are the barbed, hair-like glochids, small spines that occur at the base of the larger spines. These easily detach and can work their way through clothing and gloves into one’s skin. Stating the obvious, handle prickly pears with extreme care, preferably with tongs.
Given the warning immediately above, why would you even attempt to handle a prickly pear? If you’re brave, the edible pads, known as nopales, can be disarmed and are widely used in Native American and Southwestern cuisines. The fruits, called tuna in Spanish, can also be eaten. Not having attempted consumption of either the pads or the fruits, I refer you to numerous online videos that detail their preparation and cannot be held liable for any consequences.
Cacti are easy to grow, especially as houseplants. Give them a very well-drained soil and bright light—and do not overwater! Keep them away from small children and curious pets. Many prickly pears are more cold-tolerant than one might presume, so they can be grown outside. The key is keeping them dry, especially in winter. Growing them in containers is the easiest method, but building a berm of gravely soil can also work.
And prickly pears are one of the easiest plants to propagate. Take a pad and chop it in half with a shovel. Let the chopped pad dry out for a few days or weeks to form a callus, then stick the callused end into the ground. A new prickly pear will grow quickly.