Saguaro Cactus Flower Bloom

Saguaro Cactus flower.

I know I have been posting a lot about cacti lately, but hey, cacti are some of the coolest plants around.  And one of the coolest cacti around is of course the Saguaro.   Currently this cactus icon of the Sonoran Desert is in full bloom and causing quite a commotion with desert life.  Being cacti are quite the stingy conservatives when it comes to water it is quite unusual to examine a cactus in bloom.  Cacti flowers are very liberal when it comes to water use for flowering.  Much of this water use goes towards producing nectar, which attracts all kinds of desert dwelling organisms.  Saguaros begin flowering at night time and attract Lesser Long-Nosed Bats which feed upon the nectar and transport pollen from one flower to the next.  Once daylight arrives, bees and doves feed on the nectar and pollinate the flowers until afternoon or so when the flowers shrivel-up.   Buzzing swarms of bees and other flying insects quite commonly surround Saguaro blooms in the morning.  These swarms often attract insect eating birds that actively dart around catching their dinner. The simple act of blooming supports several different levels of the food chain.  Once Saguaro blooming is finished up in the next couple of weeks though, this activity will cease and the desert will become quiet until the next, and even bigger annual Sonoran Desert event takes place.  This event is the ripening of the Saguaro fruit which takes place at the end of June.

Blooming Saguaro Cactus next to a blooming Ironwood Tree.

Life of a Cactus Part 3: Rain

Cacti don’t need a lot of water, but sufficient rainfall at least a few times a year is of course desirable.  When soil is soaked by recent rainfall, cacti binge on the soil water.  Water is absorbed faster than the cacti can utilize it and the excess water is stored in succulent tissue.  This stored water ensures the plant remains healthy through periods of drought but generally is utilized within a few seasons of drought.  This is different from some desert shrubs such as creosote that can go three years without any rainfall.  Some cacti are better than others at surviving dry periods.  For example, the eastern prickly pear which grows in the central to eastern United States needs quite a bit of rainfall every year to survive, 25 and upwards to 40 inches annually.  Beaver Tail Prickly Pear and the Teddy Bear Cholla of very hot and dry deserts of the southwestern U.S. on the other hand require only a few inches of rain annually and can go up to a few years without water.  These cacti are the extremes though in North America, most species occupy the slightly wetter deserts were say six to 12 inches of rain fall annually.

In general, cacti also require rain to fall during certain times of the year to ensure success.  Precipitation that falls as snow cannot be used.  Freezing temperatures first of all kill most cacti and cold moisture is not absorbed well.  The Great Basin Desert commonly having snowfall and dry summers, therefore has few cacti.  Cool weather precipitation above freezing can be absorbed and utilized though.  Being warm to hot weather plants, summer precipitation is also well utilized.  As mentioned previously, cacti typically do not endure extended seasons of drought though.  The Mojave Desert of southern California, northwestern Arizona, and southern Nevada has winter rainfall but long hot dry summers and therefore supports few cacti.  The Sonoran Desert of Southern Arizona and northwest Mexico however, has both a winter and summer rainy season and therefore supports an abundance of cacti.  Of course cacti can survive many types of desert conditions but they do generally require evenly dispersed precipitation above freezing to proliferate. 

Once water is absorbed into the tissue of a cactus, the plant holds onto it like gold.  Contrary to Hollywood myth, cacti don’t contain a swimming pool of water swishing around inside.  Water absorbed by roots is carried to internal succulent tissues where the cells are filled with and store the water.   Cells swelling with water causes the entire cactus to swell as mentioned before.  Once water is in the cells through it is not easily removed by anything other than the cactus itself.  To remove the water one would literally have to burst billions of microscopic cells.  So extract a glass of water from a cactus would take a lot of work.  It would be much easier simply to eat the cactus and obtain the moisture that way.  Unfortunately, cacti contain oxalic acid, a chemical that can cause kidney stones and tastes bad, and for that reason is a major deterrent to any creature that may want to consume a cactus.  

Life of a Cactus Part 2: Roots

Cactus Roots

Again, as might be expected, the cactus is a highly tuned water preserving organism.  The problem is that first these highly tuned machines must access water they need.  Everything you see above ground on a cacti is designed to hold water in.  The “skin” is thick and contains a waxy layer on top and spines prevent animals from eating the plant to obtain water.  No water enters the cactus from above ground and only the minimal amount necessary is allowed to escape out of the cacti.  Only roots do the job of zealously drinking the latest rainfall.  Nearly all cacti have very shallow root systems, most of which only penetrate no more than one foot underground.  A few cacti such as the saguaro have taproots that penetrate as deep as three feet into the soil.  Even that isn’t very deep compared to some desert shrubs such as creosote brush with roots extending over six feet in depth and Ironwood over 30 feet deep.   But while cacti lack depth of roots they have a vast breadth of shallow roots.  I once examined the root system of a saguaro cactus where most of the soil had been eroded away on one side of it yet left the most of the root system intact and free from soil.  The saguaro’s roots extended twenty plus feet away from the cactus and I couldn’t find a single root deeper than 24 inches into the soil, most were about one foot deep.  Others have traced the saguaro’s root system as far as 50 feet from the trunk.  Most other cacti also have extensively shallow root systems.

Initially, it seems counter intuitive for desert plants to have shallow roots until you realize that only rarely does moisture from rain in the desert penetrate more than two or three feet deep.  Then, once it does rain slumbering cacti roots awaken within hours, sending out tiny rootlets that quickly absorb water.  And when a cactus drinks, it doesn’t just sip, it binges by drinking as much as it can as fast as it can.  In hot deserts roots can’t grow too close to the surface simply because extreme soil temperatures kill the roots, so roots begin about an inch below the surface.  In cooler regions, such as in Colorado, the soil surface doesn’t get extremely hot so cacti roots can grow just millimeters below the surface, greatly increasing their ability to take advantage of every little rainfall.  A scientist once found prickly pear cacti in Colorado with roots only two millimeters below the surface.  This cactus was able to survive on only two millimeters of rain a year owed to the fact of its extremely shallow roots!

Once wet soil activates binge drinking, the cactus absorbs way more water than it is currently using so it must store it somewhere.  Tissue within the stems, pads, or joints absorbs this water and stores it for later use.  As more water is absorbed the stem, pad or joint swells.  Simply by looking at a cactus you can tell how well watered it is.  If it is swollen it is well watered and good to go for months without rain, even in hot weather.  If it is skinny or looking shriveled the cactus needs water.  Cacti are well adapted to swelling and shrinking.  Saguaros and other columnar cacti have palliated or accordion shaped stems that readily expand when swollen with water or compress when water is depleted.  A mature Saguaro can hold thousands of pounds of water by simply expanding its accordion shaped skin.  Pads of prickly pears and joints of cholla, also swell and contract readily, easily doubling or more in size when well watered.

After the desert soil dries out, roots begin to go dormant.  Tiny rootlets that developed in the wet soil dry out and die.  Wet mucilage around the roots also dries out and forms a sheath surrounding the roots preventing the roots from directly contacting the soil.  When this happens, the cactus quits drinking or even attempting to drink water from the soil.  In-fact, the sheath functions as a barrier to prevent any water from leaving the cactus.  This is actually quite unique to the plant world being most plants require their roots to be in contact with at least a little moisture in the soil at all times.  Once the soil dries out however, the cactus must draw from the reserves of water it has stored up in its tissue in order to carry out photosynthesis.  Even so, after a good rainstorm a cactus can absorb enough water to carry it through many months of intense drought.