A primer on the cactus family, and growing mountain ball cactus, Pediocactus simpsonii, from seed

Cacti make up one of the most diverse and rapidly-evolving plant families in the world, native exclusively to the Americas.

First appearing about 35 million years ago (fairly recent for such a large group of plants), the first cacti were thorny tropical shrubs with woody stems and lush leaves. They’re hardly recognizable as cacti, but you can still see these ancestral plants in the genus Pereskia.

Nature’s high-tech survivors

Cacti employ an advanced type of photosynthesis, in which pores only open at night in cool temperatures to absorb carbon dioxide. They bind carbon as malic acid, a very weak acid found in many life forms, in special cell organs called vacuoles. They close their pores at dawn, before the air heats up, then draw four-carbon malic acid molecules into chloroplasts during the day to use it in photosynthesis without losing water to evaporation. Only a few plant families in the world can do this, allowing them to make glucose with minimal water loss. The same process arose independently in bromeliads, orchids and a few other groups.

Many of these carbon-storing plants are succulent, since thick fleshy tissues create more space to store carbon. That trait provided an added benefit of reducing the mass-to-surface area ratio which further limits evaporation, and storing lots of water, for a second level of drought-resilience.

With these evolutionary tools in tow, cacti quickly colonized areas other plants couldn’t. They lost their leaves, climbed into trees in the rainforest where they could grow without soil, advanced up dry rocky cliffsides and mountains, and eventually developed the familiar spherical, paddle and columnar shapes widely recognized as cacti. They then spread into extremely hot and dry areas west of the Andes and across what is now Mexico and the American Southwest.

The cactus family’s evolutionary diversification has been remarkably fast, with new cacti species appearing every few thousand years to give us some 2,000 documented kinds today. But because of the tropical origin—to the chagrin of northern gardeners—most cacti are still unable to survive cold winters.

The genus Opuntia, or prickly pears, are one exception, a large genus containing a lineage that rose with the Rocky Mountains and eventually spread to the plains, eastern U.S. and Canada. Another is the genus Pediocactus.

The Mountain Ball Cactus of the Mountain West

Pediocactus simpsonii, or mountain ball cactus, is one of the hardiest cacti outside Opuntia. Native to the high plains, Rocky Mountains and sagebrush steppe valleys of the western U.S. up into Idaho, Oregon and Montana, the cactus clings to shallow gritty or powdery soils among thirsty pine tree roots and nestled under tufts of bunchgrass or between rocks.

This hardy cactus can reach 11,500 feet in altitude and survive temperatures of 35 degrees below zero (-35°F). It grows to around 3 inches tall by 3 inches wide as single plants or small clusters. Unlike some other globular cacti, it is tolerant of part shade, and in hot dry weather it partially dehydrates and shrinks downward to hide from the sun or herbivores. The small globes plump up with snowmelt or rains in early spring, when they bloom and set fruit, spread by birds.

For a long time I thought these cacti were rare, though perhaps I wasn’t looking close. During the spring of 2021, when most of the Western U.S. was gripped in drought, the grass and wildflowers grew unusually thin, exposing the little cacti in their multitudes.

On a family member’s undeveloped 5-acre property in the foothills west of Berthoud, CO, I came across dozens of them around the bases of planted pinyon pines where they seemed to thrive despite the competition of thirsty tree roots. With permission from the owner (my grandma, who until then had no idea the cacti were there) I picked a few fruits, smashed and strained them and collected the seeds. There were dozens in each crambeery-sized fruit, each one a very hard black sphere about the size of a poppy seed.

Weeks later on a camping trip on BLM land near Radium Hot Springs, a dry forest of pinyon, juniper and sagebrush between higher Rocky Mountain ranges, I found literally thousands of mountain ball cacti—a cluster every few feet among the sagebrush and around the drip lines of pinyon trees. At around 8,000 feet, they were early in the growing season and just starting to bloom.

Growing Pediocactus from seed

Pediocactus seeds are reputedly difficult to sprout. Like many plants that thrive in harsh climates, they have internal barriers that ensure their progeny hide away in the soil for years or decades until an opportunity arises. This is commonly referred to as seed dormancy which prevents seeds from sprouting even when they’re moist.

Triggers could include mild spring weather after freezing, a wildfire that clears the ground of competitors, a flash flood that shifts the soil, a dry spell that leaves gaps in the grassland or just the advance of time. Thar way, droughts or events that injure the established plants or leave them unable to bloom for decades won’t wipe out the population as a whole.

The best way to grow the seed, according to most sources, is to plant them in containers outside in a suitable climate and simply wait for the best conditions to occur naturally. That could take six months to a year and even then may only get you a moderate germination rate. With a few contradicting, anecdotal accounts about the best way to grow the seed, I thought I’d subject my seed collection to a science experiment to improve the odds and come to a more objective answer.

Testing various methods of germinating Pediocactus seeds

To test my seeds, I the collection into plastic bags with paper towels and introduced them to a variety of conditions. One bag was not given any treatment, two were scoured with fine sandpaper, one sanded batch and one unsanded batch was moistened and frozen and thawed several times over about two weeks, and one was subjected to the same freeze-thaw treatment dry. Another was wetted with hydrogen peroxide and left at room temperature.

After the treatment, the seeds were planted in a seed tray with mixed sand (50 percent), potting media (25 percent) and vermiculite (25 percent), packed down and covered with an additional 1/8 inch of sand and potting media. The seeds then went into a clear plastic bin and set outdoors for light and warmth to germinate.

At about three weeks in, I’ve spotted my first seedling—a tiny green sphere no bigger than a grain of sand, in the hydrogen peroxide row. That surprised me! I was expecting the sanded+frozen+thawed seeds to emerge first if any did at all.

I’ll keep updating to follow the progress of this batch.