Rooting for CACTUSES
Sonoran desert plants have surprising underground systems
I love obscure books on cactus by really knowledgeable authors. One penned in 1994 by Park S. Nobel, Professor of Biology at UCLA calls me back time and again. What I learned the other day from Remarkable Agaves and Cacti was about water uptake through cactus roots. The data is based upon Sonoran Desert cactus and agaves, so the information is local to our microclimate and soils. ● As a highly visual thinker, I have to imagine what’s going on underground without actually seeing it. This is important because what’s on the surface has very little to do with conditions deeper down. Nobel notes that the saguaro that reaches 33 feet in height is supported by a feeder root system just 10 inches deep. The central tap root does feed but is largely for anchorage.
This is an average rooting depth shared by native cactus Ferocactus acanthodes (barrel) and Echinocereus engelmannii (hedgehog). Conversely, roots do not form within the top one inch of soil. Nobel explains, “This upper region of the soil can become extremely hot during the summer. The surface can reach 158 degrees F, which is lethal to root cells.” This explains why fine gravel mineral mulches help to keep root zones cooler than plants growing in open ground.
This information tells us that native cactus are sensitive to loss of surface soil above their root zone. Gradually runoff and irrigation carries away loose particles until the roots are on top again. Unless covered, these will die from the heat, then your plant must start over again deeper down. Perhaps this is the cause of old cactus decline with no clear cause and the importance of mineral mulches right on the soil surface.
This demonstrates why the depth of water penetration beyond ten inches is a waste with native cactus. They simply can’t access much deeper down. Too much water in the surface soils at night doesn’t let them dry out enough in the day time to avoid steam as it evaporates in the afternoon.
Noble also explains a curious fact about Agave deserti, which can be seen in the roadside cliffs high on Highway 74. “The number of lateral roots per length of main roots is roughly 11 times higher under rocks and 6 times higher alongside rocks than in rock free regions.” Though less dramatic, Echinocereus engelmannii root systems are “three times more profuse” at a boulder’s edge than just 2 inches away from them.
This may explain why some folks have difficulty growing local cactus and succulents on the sandy valley floor, formerly an inland sea. Hike the canyons and foothills to discover their preferences for just these rocky locations. The author explains that residual moisture under rocks in spring is often the only place certain seeds can germinate at the edges. Irregular rocky soil density allows water to move underground more readily along seams, penetrating much deeper into the root zone.
This is the kind of detail you rarely find outside of academic papers. University presses often publish some really great books, particularly if written before the green revolution. The older ones give this kind of readable detail that is rooted in studies validated by our own native plant distribution and behavior. It also tells us what happens to great old specimens dubbed “suicide cactus.” These are so big and heavy they fall over from erosion caused root death.
Now you have enough information to visualize how some desert plants root for water and for anchorage. When a saguaro is transplanted, the tap root is cut and unscrupulous rustlers will plant them too deep in the new location for stability (like a fence post). This puts the 9” ring of feeder roots at least 2 feet underground, denying it rain and irrigation while slowly dehydrating to death over many year’s time.
To understand drought and how it impacts plants of all kinds you must know what’s going on in the dark netherworld. It is here that lies the soul of the plant rooted within our lean soils, thriving in our summer blast furnace and perfectly adapted to a nearly rainless lifestyle.