The saguaro cactus (Carnegiea gigantea) is the iconic plant of the Sonoran Desert. It ranges from southern Arizona to western Sonora, Mexico, restricted by rainfall and temperature (It is too dry in the Mojave desert and too cold most other places). Most saguaros flower and produce fruit during April, May, and June, but individual plants may bloom at other times of the year. I’ve seen one along Kinney Road flower in January.
Saguaros commonly reach 40 feet tall, a few reach 60 feet to 80 feet. Growth rate depends on rainfall and soil conditions. In Tucson, which averages 12 inches of rain per year, saguaros take about 10 years to get 2 inches high and 30 years to get 2 feet high. Arms may form when the saguaro is 50 to 100 years old and 7 to 12 feet high. Saguaros growing on sandy alluvial fans tend to be bigger and have more arms than plants growing on steep, rocky slopes. The saguaro has a central root that may extend two or three feet deep. Most of the roots are within 4 inches of the surface, ready to suck up water from a brief rainstorm. As a rule of thumb, roots extend outward at least as far as the saguaro is tall. That makes saguaros hard to transplant because it is difficult to dig out all the roots.
The saguaro is covered by a thick, waxy skin which makes it waterproof and restricts water loss through evapotranspiration almost exclusively to the stomates (pores for gas exchange). Water loss is further restricted by the metabolism of cacti and succulents. That metabolism is called CAM, which stands for Crassulacean acid metabolism. To avoid water loss, most pores are closed during the heat of the day. The pores open during the cooler nights allowing the cactus to take in carbon dioxide it needs for photosynthesis. But photosynthesis requires sunlight, so carbon dioxide is stored as an organic acid for use the next day, one of the things that makes the water in cactus not potable. This CAM metabolism allows the cactus to survive on about one-tenth the water compared to leafy plants, but the price is very slow growth. For a more detailed explanation on how CAM works, see this article..
Most of the tissue in a saguaro is for water storage. A fully hydrated saguaro is about 90 percent water by weight and a large plant can weigh 80 pounds per foot of height. This water-bearing tissue helps protect the cactus from temperature extremes. Heat is absorbed through the surface during the day and is stored, resulting in a small temperature rise that does not reach a lethal level. At night, the heat is slowly radiated back into the air. This same thermal inertia usually keeps the tissues above freezing on cold winter nights, but there are limits. The cactus would be in grave danger if the temperature stays below freezing for several days.
To support the soft tissue, the saguaro and other columnar cacti contain a cylinder of 13 to 20 woody ribs near the center of the cactus stem, running the length of the main stem and branching into the arms. In the upper part of the stem, the ribs are separate; as the stem ages the ribs continue to grow and fuse into a latticed cylinder. The outside of these woody ribs contain the vascular, or water transporting, part of the plant.
When looking across an expanse of desert containing many saguaros, one notices that there is not a continuous range of saguaro heights. Rather, there seems to be just a few “cohorts” of saguaro of similar heights. This happens because the seeds need several consecutive days with moist soil to germinate and these conditions do not occur every year.
Flowers occur near the top of the main stem and on the tips of arms. The white flowers are about 3 inches in diameter and smell like ripe melon. Each flower opens at night and remains open until mid-afternoon the next day. Nectar produced at night attracts bats. A second batch of nectar is produced in the morning to attract birds and insects. Bats and white-winged doves are the main pollinators. Doves, Gila Woodpeckers, Cactus Wrens, and House finches eat the fruit and disperse the seeds.
After the flowers are pollinated and fruits mature, the fruit opens to expose its red interior leading some people to think there are red flowers atop the saguaro.
By the way, notice in the photos of saguaro fruit, the brown dried up flower. These contain a very sharp edge that can be used as a knife to open the fruit.
Many large saguaros contain holes excavated by Gila Woodpeckers and Gilded Flickers. The birds go in and down, removing the fleshy part of the cactus. The cactus produces scar tissue which quickly becomes very hard and impervious to bacterial infection. This material, often in the rough shape of a boot, survives after the cactus dies and rots away. Woodpeckers generally excavate a new hole each year, leaving the old hole for other cavity-nesting birds. Cactus spines deter animals from climbing the cactus. Apparently, however, they don’t deter snakes. I’ve seen photos of a gopher snake climbing a saguaro to prey on the dwellers in a hole.
Most saguaro seedlings die from drought, frost, and predation. Seedlings up to a foot tall are eaten by rodents and rabbits.
Some mature saguaros are killed by lightening strikes or blown over by high winds. However, the chief agent of mortality in the Arizona Upland is freezing. Mortality by freezing depends on the seasonal timing and duration. A healthy middle-aged saguaro can stand a few hours of temperatures as low as 10 degrees F in mid-winter. On the other hand, 12 hours of 20 degrees F in late fall causes damage and death.
According to Natural History of the Sonoran Desert, cited below, a frost-damaged saguaro may survive for another decade or longer, but eventually weakens until it can no longer resist infection. “Bacterial rot caused by Erwinia cacticida turns the flesh of weakened plants into an odoriferous black liquid.” Healthy saguaros can ward off small infections by walling off infected parts. The bacterium is carried by a moth whose maggot-like caterpillar burrows into the cactus and feeds on the rot it introduces. Many living saguaros have round, half-inch scabs on their surface. These are the caps of contorted tunnels left behind by the caterpillar.
Back in the 1990’s some biologists published a paper about the “brown decline” and impeding doom of saguaros. These scientists were unfamiliar with saguaro biology. There was a great freeze in December, 1978, which affected some of the old saguaros and this is what the visiting biologists focused on. They ignored the large population of smaller, younger saguaros.
For many generations the Tohono O’odham people have harvested saguaro fruit using long poles made of saguaro ribs. The fruit may be eaten raw or it may be cooked down to a sweet syrup. In the O’odham tradition, some of the syrup was fermented into wine used in a ceremony to herald in the summer monsoon. Dried seeds from the fruit, up to 5,000 per fruit, are rich in protein and fat and can be ground into meal.
Reference: A Natural History of the Sonoran Desert by Arizona-Sonora Desert Museum press.
The Creosote Bush
Yuccas provide food, fiber, and soap
The Jojoba bush and its valuable oil
Arizona Christmas Cactus
Mesquite trees provide food, fuel, medicine, and more
Cactus water will make you sick
Palo Verde trees about to turn the desert golden
- Zone 7 Cactus: Choosing Cactus Plants For Zone 7 Gardens
- Cold Hardy Cactus
- Types of Cactus Plants for Zone 7
- Growing Cactus Plants in Cold-Winter Climates
- How to Grow Cacti in Cold-Winter Climates
- Selecting Cold-Hardy Cactus Plants
- CHOOSING SUCCULENTS FOR ZONE 7 & 8 – TEXAS, GEORGIA, VIRGINIA AND NORTH CAROLINA
- ZONE 7
- WHAT TYPES OF SUCCULENT THRIVE IN ZONE 7?
- ZONE 8
- WHAT TYPES OF SUCCULENT THRIVE IN ZONE 8?
Zone 7 Cactus: Choosing Cactus Plants For Zone 7 Gardens
We tend to think of cacti as strictly desert plants but there are also cactus that are native to rainforest regions. Zone 7 is actually a perfect climate and temperature range for many types of cactus. The biggest problem for zone 7 cactus is usually soil type. Soil must be well draining and, in most species, slightly gritty. There are many cactus plants for zone 7 that will grow successfully and give your landscape desert-like panache.
Cold Hardy Cactus
Desert cacti experience tremendous ranges of temperature. During the day temperatures soar over 100 degrees Fahrenheit (38 C.) but at night the cold can approach freezing. This makes hardy cactus plants one of the most adaptive types in the plant kingdom. Many plants in the group are not only suitable for zone 7 but will thrive in those regions.
Hardy cactus plants are found in the mountains of northern Mexico into the western United States. These plants are adapted to the high, cool temperatures of mountainous regions. They are particularly well suited to exposed sites where cold winds and dry soils are prevalent. These plants can even tolerate temperatures of 0 degrees Fahrenheit (-18 C.). There are even cacti that can survive in zone 4 or below.
Growing cactus in zone 7 outside year round is, therefore, not only possible but there are plenty of choices regarding plants. One thing to note regarding cold hardy cacti is the type of medium in which they grow. They are often squeezed in between rocks, in crevasses or on soil that is liberally peppered with small rocks and pebbles. This keeps the plant’s roots from sitting in boggy soil even where rain is prevalent.
When growing cactus in zone 7, choose your site well and ensure the soil is well draining. Most cactus require some grit in the soil, so add some coarse sand or other gritty material to a depth of at least 8 inches (20 cm.) before installing the plant. The ideal mix is ½ grit to soil.
Full sun is preferred for most cacti but some can tolerate partial sun locations. Take care not to plant in a depression where moisture can collect. Many cactus do very well in containers too. Because the root zones can be exposed in cold, windy conditions, wrap the container in winter and use a protective mulch over the top of the soil.
Types of Cactus Plants for Zone 7
Some of the most hardy cactus plants are in the genus Echinocereus. Other cold tolerant genus are Opuntia, Escorbaria, and Pediocactus. Each is suitable as a zone 7 cactus species.
- Echinocereus are commonly called hedgehog cactus and have chubby, appealing rounded bodies covered with spines and forming clumps.
- The most common Opuntia is prickly pear but several other forms are also cold tolerant such as rat tail Cholla.
- The Pediocactus are a small group of plants that are sub-alpine. They can bloom in spring but have also been seen in full flower when snow is on the ground.
- Escobaria are small clumping forms with names like pincushion cactus and spiny star. These would perform well in containers or at the edges of borders where their bright flowers can lighten the area.
- If you want maximum punch in the garden, the compass barrel cactus in the Ferocactus genus, can grow 2 to 7 feet (.6-2 m.) with a 2-foot (.6 m.) diameter.
Some other wonderful zone 7 specimens might be:
- Golden Barrel
- Tree Cholla
- Whale’s Tongue Agave
- Claret Cup Hedgehog
- Beavertail Prickly Pear
- Fendler’s Cactus
- Bailey’s Lace Cactus
- Devil’s Tongue
- King’s Crown Cactus
Growing Cactus Plants in Cold-Winter Climates
For gardeners in Northern regions, growing cactus plants can be a prickly subject. Cacti thrive on light, heat, and excellent drainage. Though you may think they’re limited to the Desert Southwest, many are hardy enough to grow deep into Canada. In fact, cactus plants are native only to North and South America.
To successfully grow cacti in cold-winter climates, it’s important to understand how they grow. All cacti are succulents—plants that can store water in their roots, stems, and leaves. But not all succulents are cacti.
It’s sometimes difficult to tell which succulent is really a cactus. There’s really only one way to know: All cacti—and only cacti—have spine cushions, called areoles. These look like small bumps from which spines, branches, leaves, and flowers grow.
Image zoom Denny Schrock
How to Grow Cacti in Cold-Winter Climates
Cold-weather cactus plants that grow in northern regions prefer much the same conditions as their southern counterparts. Place them in sunny, dry locations; they need sun to bloom.
How to Plant Cacti
Cacti require soil that drains quickly but avoid growing them in pure sand, which doesn’t hold enough nutrients. Add 40-60 percent coarse sand and up to 10 percent compost to garden soil or purchased topsoil for a nutrient-rich, fast-draining mix. Or add pea gravel in place of some of the sand. Avoid using fine-grain sand; it gums up the soil instead of adding drainage.
Raised beds are recommended to provide excellent drainage. The more rain your area gets, the more drainage you need. In super-wet regions, grow cacti in pots under shelter such as a roof overhang. Likewise, never plant cacti in regular or clay soil as they can easily get too much water and die.
Watering Cactus Plants
Cactus plants do need some water. The best practice is to simply let Mother Nature do the watering for you. However, if you go for several weeks in hot, dry weather without rain, feel free to water them. If the weather has been hot and dry and the plants look limp or are beginning to droop, they’re telling you they need water.
Avoid watering cactus in the fall or winter. Cactus plants begin to shrink and take on a wilted, off-color appearance to prepare themselves for the coming weather. If you water them then, the excess water freezes and the plant dies.
Cactus plants grown in the ground don’t need much fertilizer but they benefit from spring applications of compost or a liquid fertilizer designed for bulb or vegetable use. Avoid fertilizers with a large nitrogen component (the first number of the three shown on the package). Nitrogen causes rapid growth, but the plant may be too tender and become susceptible to winter damage, especially late in the growing season.
After planting cactus plants, avoid disturbing the soil around their shallow roots. Pea gravel or other small rock mulch prevents soil from blowing away, assists with weed prevention, and keeps the soil temperature even.
Protecting Cactus Plants
In areas with plenty of snow cover, hardy cacti easily survive. In areas with harsh winds and sun but little snow, cacti can become sunburned or frostbitten. To prevent damage, carefully cover the plants with burlap as late in the season as possible. The burlap allows the plants to breathe while protecting them from sun, ice, and wind. During warmer winters, carefully place a structure such as a canvas tent over the cactus plants to shelter them from excess moisture.
Image zoom Ryann Ford
Selecting Cold-Hardy Cactus Plants
Because of the wide variety of plants within each species, check the hardiness of all cacti before buying them for outdoor use. The champions of cold-weather cacti come from the prickly pear family, known botanically as Opuntia. Opuntia species come in a wide variety of sizes and shapes. They grow with paddlelike pads and flowers that grow in red, bright pink, or yellow.
There are many kinds of cacti in this family, but two of the toughest are Opuntia fragilis, hardy to -35 degrees F, and Opuntia poryapantha, hardy to -25°F. The eastern prickly pear (Opuntia compressa), native to most parts of the eastern United States and southern Ontario, is an easy-to-grow choice. Its juicy red fruits (the “pear” of the common name) are edible.
Not all prickly pears are hardy in cold-winter climates. Other cactus plants with cold-weather tolerance to Zones 5 or 6 include:
- Cylindropuntia, called cholla, a prickly pear relative, grows with segmented cylindrical stems that can reach 10 feet tall. Various species have a varied range of hardiness, with many tolerating temperatures to -30 degrees F.
- Echinocereus, known as the hedgehog or porcupine cactus, includes species that are usually shorter than a foot tall. Echinocereus reichenbachii is one of the most cold-hardy, to about -10 degrees F.
- Escobaria vivipara, also called pincushion or beehive cactus, features squashed spheres or short cylinders with wooly gray stems growing up to 5 inches tall. Hardy to 0 degrees F or colder.
- Corynopuntia (also called Grusonia), known as club cholla, is closely related to the prickly pear and forms low-growing mats of stems with heavy spines. Hardy to about -10 degrees F or colder.
- By Deb Wiley
CHOOSING SUCCULENTS FOR ZONE 7 & 8 – TEXAS, GEORGIA, VIRGINIA AND NORTH CAROLINA
Zone 7 spread to over 20 US states, reflecting a diversity in environments and climates. This typical feature allows a wide range of plants to be cultivated in zone 7. However, it also means that there are often other considerations to be made to address issues such as soil conditions or drought tolerance.
Typical states in zone 7: Oklahoma, Texas, New Mexico, Oregon, Virginia.
WHAT TYPES OF SUCCULENT THRIVE IN ZONE 7?
With a mild winter, the lowest temperature in zone 7 falls between 0 to 10 degree F. Many types of succulents such as Ghost plant, Sedum and most Sempervivum can perform well in such weather.
Zone 8 cover a majority of US southern coastal regions. Regarded as one of the warmest zones with a minimum degree of 10-20 degree F, states in zone 8 are cultivation lands to various types of succulents.
Typical states in zone 8: Georgia, Mississippi , Texas, Louisiana, Alabama, South and North Carolina.
WHAT TYPES OF SUCCULENT THRIVE IN ZONE 8?
Succulent hardy to zone 8 should enjoy a mild winter and hot summer. Such succulents can be Cactus, Sedum, Sempervivum and some Echeveria.
See more about Choosing Succulents for Zone 3,4,5 & 6 – New York, Pennsylvania, Ohio and Minnesota
to get all the details.
If you found this article interesting, share it with your succulent loving friends!
And get a free plant when your friends make an order. Sign up here!