As a passionate gardener, I take pride in my ability to identify most ornamental plants instantly. However, when it comes to weeds, I must admit that my knowledge is somewhat limited. This gap in my expertise became evident when I failed to recognize a weed with small yellow flowers that appeared in my perennial garden this summer. Ignoring its presence seemed harmless enough until the cooler days arrived, and those yellow flowers transformed into clusters of spiky-looking seeds with tiny barbs.
Intrigued by the barbs on these seeds, I couldn’t help but wonder about their purpose and function. Many plant species worldwide are armed with sharp outgrowths, but why? What role do they play? To satisfy my curiosity, I embarked on a journey to find answers.
Exploring Prickly Plant Structures
The world of plant defenses is filled with intriguing structures, and it’s important to differentiate between thorns, spines, and prickles. While we often use these terms interchangeably in casual conversation, they refer to distinct adaptations of different plant parts.
Thorns are sharp, woody points that grow from a plant’s stem tissue. They typically appear in the axil of a leaf, where a branch would normally develop. Native tree species such as Washington hawthorn, honey locust, and black locust have evolved with thorns. Surprisingly, most citrus trees, including lemon, lime, grapefruit, and orange, also boast thorns. Even apple, plum, and pear trees originally had thorns, although modern cultivars have been bred to be thornless.
Spines, on the other hand, are derived from a plant’s leaf tissue. These firm, sharp, slender structures can be found on most cactus species and some succulents. They serve multiple purposes, such as reducing water loss by transforming wide leaves into narrow spines with restricted surface areas. Spines also provide shade and act as a defense against animals and birds.
Some plants, like holly species and false holly, have spiny edges along the margins of their leaves, while others, such as Agave, Sansevieria, and Yucca, have sharp tips on their leaves.
Prickles are short, sharp, woody projections that arise from a plant’s epidermal tissue. Unlike thorns and spines, prickles lack any vascular tissues connecting them to the main body of the plant. For instance, rose bushes have prickles on their stems, and horsenettle has prickles on both stems and leaves. Spiny amaranth, with its sharp, stiff prickles, demonstrates how these outgrowths can be attached to the base of leaves. Some plants even have reflexed or recurved prickles, hindering climbing animals or providing support to long-stemmed plants.
Beyond the Basics
The realm of prickly plant structures extends much further than the aforementioned categories. Plants have developed various adaptations to ensure their survival and the dispersal of their seeds. Some employ barb-like mechanisms to hitchhike on passing animals, ensuring their seeds find new areas with less competition. Beggarticks, cleavers, prickly Sida, and sticktights are among the weeds equipped for hitchhiking.
Additionally, many plants boast fine hairs or bristles for protection from heat, cold, and herbivores. Prickly lettuce, a weed with prickly leaves and milky sap, exemplifies this adaptation. Burs, seed or dry fruits with hooks or teeth, are designed to repel herbivores and latch onto fur or clothing for dispersal. Common burdock, common cocklebur, jimson weed, and teasel are notable examples of plants with burs.
Even Veggies Have Personality
Surprisingly, even vegetables can possess prickly personalities. Some varieties of eggplant, yellow squash, and certain cucumber types have prickles on their stems and fruits. Artichokes, with their prickly leaves and violet-purple thistle-like flowers, showcase both beauty and defensive adaptations to attract pollinators while deterring potential threats.
Preventing and Controlling Prickly Plants
For the most part, many prickly plants mentioned above are opportunistic and often appear in disturbed soil or gardens. It’s crucial to remove them at the seedling stage for several reasons. Seedlings are easier to handle and eliminate before they mature into more formidable, prickly plants. Additionally, many of these plants produce copious amounts of seeds, emphasizing the importance of preventing them from dispersing. By addressing these issues early on, you can prevent the nuisances associated with prickly plant invasions.
While understanding the distinctions between thorns, spines, and prickles might not be essential for every gardener, it can aid in identifying plant species. It’s worth noting that not all plants with sharp or prickly outgrowths are weeds or undesirable. Many of these species provide valuable resources for pollinators and wildlife. However, it is crucial to identify and eliminate undesirable plant species before they become a nuisance. Preventing seed production or eradicating them at the seedling stage are effective strategies.
Reflecting on the beggarticks weed that sparked my exploration, I now appreciate its survival strategy. Unable to move, it cleverly relies on mobile creatures to disperse its seeds. This strategy, known as zoochory, demonstrates the resourcefulness of plant evolution.
Featured photo: Common thistle armed with formidable prickles on stems, leaves, and flowers. Photo: Pat Chadwick
- Botany for Gardeners, Third edition (Capon, Brian, 2010)
- Weeds of the Northeast (Uva, Richard, Neal, Joseph C., and DiTomaso, Joseph M., 1997)
- Weeds of North America (Dickinson, Richard and Royer, France, 2014), The University of Chicago Press
- Virginia Weed Identification, Virginia Tech website Start – Weed Identification
- University of Missouri Weed ID guide
- “Seed Dispersal,” ivycreekfoundation.org/docs/9.Seed_Dispersal
- “Identification of Virginia’s Noxious Weeds,” Virginia Cooperative Extension Publication SPES-244NP
- Botany 115 Vegetative Terminology, Modified Roots, Stems, and Leaves
- Of Thorns, Spines, and Prickles, University of Missouri Integrated Pest Management article by David Trinklein, published January 3, 2013.