Walking in my neighborhood the other day, I stopped to stare at a flowering hedge that I had never noticed before. It was planted in front of a wall running east to west, so the southern sun was blocked. There were also large trees overhead, keeping it mostly in the shade. What grabbed my attention were the intriguing, foot-long violet flower clusters known as panicles hanging amidst the slender, dark green leaves.
The plant in question is the narrow-leaved palm lily (Cordyline stricta), an unusual species since it is a monocot, yet grows to a height of 15 feet. Nearly every plant that grows to this height or taller is a dicot, which would include just about every tree you see, except for palm trees, tree aloes, and Joshua trees, which are monocots. Arborescent monocots also include most bamboos, some Dracaenas and Cordylines, a few Yucca species (e.g. Joshua trees), as well as banana plants and sugarcane.
Let’s step back for a moment and recall that flowering plants are divided into monocots and dicots, based on their cotyledons or seed leaves that are the first sign of life we see when a seed sprouts. In monocots, there is a single slender seed leaf, appearing as a narrow blade. With dicots, the seed leaves are identifiable as two halves of the seed itself, as is the case with kidney beans, for example, where the cotyledons are easily recognizable as the two kidney-shaped halves of the seed that pop out of the soil.
Until the late 19th century, it was thought that monocots preceded dicots on the evolutionary timeline since the oldest fossilized pieces of plant parts were those of monocots, and, until today, that remains true. However, fossilized pollen of dicots was later discovered whose age exceeded that of any fossilized fragments of monocot leaves, stems, or seeds. DNA analysis has also solidified the theory that dicots preceded monocots.
It is an open question as to whether dicots or monocots have been more successful in their evolutionary journey. While we generally recognize species that developed later as the stronger ones, with proven ability to adapt to changing environments, this is far from clear when examining the two groups of flowering plants. While 40% of the earth’s land mass consists of grassland, with grass family members being highly prolific monocots, the ratio of dicots to monocots is roughly 3 to 1, with 200,000 dicot versus 60-70,000 monocot species in existence. When evaluating the success of individual plant families, the dicot daisy family (Asteraceae) is number one with 32,000 species, while the monocot orchid family (Orchidaceae) is number two with 28,000 species.
Getting back to the palm lily, it possesses four morphological characteristics — and one anatomical one — that account for the success of many monocot species. One of these is the presence of an underground structure that stores starch and allows the plant to spread through the soil when growing conditions are good, and to rejuvenate from environmental stress should its above-ground growth die. These underground structures are known as bulbs, tubers, rhizomes (e.g., in the case of palm lilies), and corms. I just saw two of my gladioli start to bloom, having disappeared for months, but suddenly having sent up leafy shoots and flower stalks from their corms. Virtually every plant that grows from one of these structures is immortal due to the clonal and perpetual proliferation of its underground storage structure, and it behooves thrifty gardeners to plant them.
Not only are plants that grow from bulbs and their associates virtually impossible to kill, but by planting a large variety of them, they will provide color as well as flowers for arrangements throughout the year. I am talking about every kind of lily (Lilium spp.), daffodil (Narcissus spp.), iris, hyacinth, canna, gladiolus, and tulip, as well as plants called lilies, even though they are botanically something else, namely lily-of-the-Nile (Agapanthus), and daylily (Hemerocallis). Incidentally, dahlia is the one popular flowering plant that grows from a tuber but is not a monocot, belonging to the dicot daisy family, just as the tubers we call potatoes come from dicots as well.
A second structural characteristic that imparts resilience to monocots is their ability to sprout from root crowns, where stems, shoots, or leaves meet roots. This is why lawns or turf grasses regrow after being cut. Although some of the grass blades grow back, the turf rejuvenates itself after every mowing thanks to tillers or new leaves that arise from root crowns. And if a palm lily were cut down by mistake, you could rely on its root crown regrowth to express itself as fresh shoots and leaves that would immediately burst forth. This capacity allows many monocots to regrow after their above-ground portion disappears due to grazing, fire, drought, flooding, or seasonal dieback.
Incidentally, monocots are not the only plants with root crowns packed with dormant buds. Several classic California woody dicots regenerate after fire or drought thanks to root crown structures known as lignotubers, including manzanita (Arctostaphylos spp.), California lilac (Ceanothus spp.), California buckeye (Aesculus californica), and California bay laurel (Umbellularia californica).
A third characteristic that imparts toughness to monocots is the shape of their leaves, which are nearly always long, narrow, and grow upright. This means that even in crowded situations, foliage can capture light, including shady locations where sunlight is at a premium. Many popular monocot bulb plants, such as daffodils and lilies, will bloom in partial shade. Another advantage of long and narrow leaves is their ability to withstand heavy rainfall and battering or desiccating winds. These leaves channel rainfall downwards and fold up in strong winds to minimize damage.
A fourth characteristic that gives monocots the ability to withstand environmental stress is their network of shallow roots. While we generally think of a plant with a deep tap root as most capable of withstanding drought, this is not necessarily the case. In dry climates, where rainfall is limited but dew may be common, an extensive network of shallow roots may be better equipped to maximize absorption of any moisture that touches the soil. It is also true that, in a windstorm, a vast network of shallow roots may be more capable of keeping a tree trunk stable than deeper, but more confined anchor roots, which are more easily uprooted, especially in rain that soaks the soil to a significant depth, compromising deeper roots’ attachment to the earth.
Anatomically, monocots have a series of tubes interspersed throughout their stems or trunks that take up water and minerals. These tubes serve as stabilizing cables that allow flexible fibrous trunks to withstand hurricane-force winds. Dicot trees, on the other hand, may feature gargantuan, columnar trunks meant to support an enormous canopy. Yet these same trunks are rigid and subject to breakage in a storm. While palm trees offer no resistance to the wind and may shed all of their leaves in a hurricane, as long as the growing tip at the top of the trunk remains intact, the tree will leaf out once again. A woody tree, on the other hand, once its limbs break or, worse yet, its canopy is sawed off by the wind, may never recover.
If you have a story to tell about a plant that you love, please send it to joshua@perfectplants.com. Your questions and comments, as well as horticultural conundrums or successes, are always welcome.
California native of the week: Recently, while visiting Palm Desert, I spotted a low-growing ground cover that resembled Thompson’s beardtongue (Penstemon thompsoniae). Flowers were mauve, and foliage was grey and pointed. I had never seen such a delicate penstemon. Penstemons are the friendliest of plants for the spring garden; their nodding, bugle-shaped blooms announce that the peak time for flowering in Southern California has arrived. There are 250 penstemon species, and most are native to California and other western states. They are found in every ecosystem, from desert to forest, from mountainous elevations to the plains. They are encountered in every version of violet, purple, red, and pink, but orange, yellow, and white Penstemons are occasionally seen as well. Although they seldom live more than a few years, where the soil is to their liking – extremely fast-draining, that is — their seeds will self-sow where they drop, so you will have a self-perpetuating crop of penstemons.