Thorny Business: The Cactus
by Rob Sproule

Masters of Evolution
Cactus as Houseplants
To Water or Not to Water

I have a confession to make. Plant geek though I am, the cactus world baffles me. With strange names and bewildering physiology, cacti never fail to remind me how much I still have to learn about plants.

When a topic baffles me, I dive into it head first. I discovered that the 2,000 species strong cactus family can teach us a lot about how to evolve to live in harmony with our environment.

Masters of Evolution

All cacti are succulents (although not all succulents are cacti), in that they hold moisture in specially adapted fleshy tissue. Every species (except for 1 in Africa), hail from the Western hemisphere and they range from parched deserts to steamy jungles.

Living in an environment too extreme for almost any other plants, desert dwelling cacti have become virtuosos of integrating form and function. With evaporation as enemy #1, cacti have evolved away from every unnecessary exposed inch, including leaves.

While cactus thorns protect them from most predators, they also have another purpose which may surprise you. Each thorn provides a minuscule amount of shade from the desiccating sun. Multiply that amount by thousands and they’re an effective way to further reduce evaporation. Since the thorns are, except for the growing tips, comprised of dead tissue, they aren’t at risk for losing moisture.

With extensive shallow root systems, cacti can suck rainwater up quickly as it hits the ground. It pulls every drop into its body, 90% of which is devoted solely to storing water.

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Many cacti have vertical ribs, or flutes, which give them an accordion-like appearance when dry. When full of water, the ribs expand as the fleshy tissue fills. A single Saguaro cactus can store over a ton of water.

Cactus as Houseplants

While cactus are easy to grow, don’t fall for the myth that they don’t need water. The amount of water they need varies with the season.

Cactus grow actively from spring into fall and rest during the winter. While they’re growing, they like bright light (although up against a south or west window might burn them), and household temperatures. Try to fertilize them every few weeks.

In the winter, give your cactus as much cool air as possible (without freezing it). It gets downright chilly in the desert, so leave it in a cool sun room or near an open window and cut back on the watering so it can rest. After its beauty sleep, if it’s had cool nights and depending on the species, it should flower.

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Plant your cactus in a shallow pot with a sandy, well draining soil. Nothing kills a cactus faster than soggy roots, so make sure to empty the saucer regularly.

To Water or not to Water

That is the question! Cactus may thrive in arid climates, but they also have vast spreading root systems which collect a surprising amount of water. In your home, with the minimal root system it’s allowed, you’ll need to water more often.

Cactus roots need to pull in water or they will dry up and die, so don’t just throw a sprinkle on the soil. Water until it’s flowing from the drainage holes; that shouldn’t take long if you have sandy soil.

You’ll want to water again when the soil is dry but not bone dry. How to tell? Place a flat, round rock on the soil surface. When the soil around begins to dry, check under it. If it’s dark and moist, hold off. If it’s dry, it’s time to water.

Make sure to water less in the winter. With less light and the cooler temperatures, your cactus will need a rest.