Nothing conjures up the Wild West like the tall saguaro cactus (Carnegiea gigantea) amid the desert landscape. Their ruggedness evokes grit and, perhaps, a bit of nostalgia for the cowboy & the open range. An aged saguaro can be between one and two hundred years old and store hundreds of gallons of water. Indeed it takes fifteen years to get a foot tall, and 75 before it puts out its first arm. And the saguaro is the most iconic among the hundreds of species.

Cacti come in many sizes and shapes. Their hardiness recommends them as house plants. Their tough skin and need for little water, as well as their singular beauty, make them stand out in an indoor display. During the growing season give them plenty of light, a high-potassium (i.e. 10-30-10) fertilizer once a month and occasional water and they will be happy.

Cacti (cactuses and cactus are also the plural) are native to the Americas and owe their spread to the early European explorers who took specimens back on their return voyage. While their exotic appearance caused them to be quite the fad in the Europe’s glass houses of the mid-nineteenth century, some types of prickly pear are common to the Mediterranean. The ancient Greeks and Romans believed they had been brought from abroad as seeds by far-ranging birds.

"All cacti are succulents, but all succulents are not cacti." Cacti are a particular kind of succulent. Mention cacti, and people picture bulbous plants that have sharp spines, tough skins and no leaves, and that don’t need much water. Yet cacti also produce beautiful, brightly-colored flowers that make the desert blossom briefly in a dazzling display after rain.

What makes a cactus a cactus?

Spines are the most noticeable feature most of us would single out. They are sharp. They are often plentiful, arranged in star-like clumps in geometric arrays around the plant varied in size, shape, length and color. They evolved from leaves to reduce the surface area of the plant in the scorching sun. They offer protection from hungry animals and to the small birds that live among them. Spines serve as condensation points for nighttime moisture, funneling it to the stem. They also hold the air closer to the plant, allowing it to form a moister, cooler envelope around the stem so the air doesn’t dry the plant out. Some species also have numerous hairs which help with these functions.

But the spines are not the defining feature of the cactus. That distinction is reserved for the curious little pad-like structure around the base of the spines. This is called the aureole. It is a soft, round pad that may have evolved from primitive branches. The spines originate in the bottom of the pad while flowers spring from the top half of the aureole. Other succulents may have the tough skin and sharp spines, but they do not have aureoles.

With no leaves, how do cacti perform photosynthesis? We are familiar with the process of photosynthesis, whereby leaves allow plants to breathe. Photosynthesis in plants with leaves takes place during the daylight hours, taking in CO2 and water and releasing oxygen. But in the desert environment, any plant with leaves would expose too much of the plant to sunlight, risking sunburn and excessive drying out. Photosynthesis occurs differently in the desert environment. In a process with the fancy name of crassulacean acid metabolism (CAM), the cactus takes advantage of the cooler, nighttime temperatures to open its stomata (breathing pores) and exchange oxygen and carbon dioxide. It stores the carbon dioxide taken in during the night as malic acid. Then, during the harsh daylight hours the plant recovers this CO2 and continues with photosynthesis. There are no open stomata to allow water to escape and dry the plant out. At nightfall the process repeats.

Most cacti have a rough, creased appearance with deep accordion-like folds or bumpy protuberances that add to their ruggedness. During rainstorms the cacti’s root systems take up the water very efficiently. The wrinkles expand as the water is taken up and the cactus fills out. A fully grown saguaro cactus may take up hundreds of gallons during a single rainstorm.

The Opuntia species comprise a large number of the cacti offered for sale and you may have one in your home. This group is unique. Nestled among the spines in the aureole are glochids. These are tiny, harpoon-shaped spears with a sharp, fishhook-like tip. Virtually invisible, once they touch the skin they work their way in and are very hard to remove. Often the tip breaks off and remains in the skin. Need I say painful?

There is an eastern species of prickly pear cactus, Opuntia humifusa, that is native to Pennsylvania. It is found in eastern Pennsylvania and New Jersey. In fact, the species occurs over a wide range, from Ontario to Florida and Colorado to the Atlantic. It will survive our winters, although it shrinks up considerably. In spring it puts forth copious bright yellow flowers with a red center. This colorful center is found only in cacti growing east of the Appalachian Mountains. These are followed by plump rounded fruit. They will root very easily in the garden. You can either plant the end of a pad in the soil or just let a fallen pad lie on the soil and it will put out roots. A word to the wise, in addition to the spines, this is an Opuntia and there are numerous tiny glochids. Wear gloves when handling it or cleaning up the garden near it as these will be on the soil as well as on the pieces of plant matter. You have only to rub your bare hand through the soil once to learn this. These are excellent pot plants. I move mine to a sheltered area and cover them with straw in the winter.

Have fun sorting out your succulents from you cacti.