Plants with Prickly Personalities

As a long-time avid gardener, I take pride in being able to identify most ornamental plants on sight. But weeds are another matter. Identifying the usual cast of weedy characters, like chickweed, henbit, and crabgrass, is no problem.  As for more exotic or less common weed species, I confess my knowledge is somewhat limited.  This shortcoming became apparent this summer when I failed to recognize a weed with small yellow flowers that popped up in my perennial garden.  It looked harmless enough, so I chose to ignore it.

As the days started to grow cooler, the yellow flowers on this interloper gave way to clusters of spiky-looking seeds with tiny barbs on one end.   I discovered the seeds when I accidentally brushed against the plant.  To my annoyance, dozens of those little barbs latched onto my shoes, socks, and jeans.

My curiosity about the barbs on those seeds kicked into high gear as I contemplated their form and function.  Lots of plant species worldwide are armed with sharp, pointed outgrowths.  But why?  What purpose do they serve?  Clearly, this called for a little sleuthing to find some answers.

Delving into the subject, I learned that these outgrowths vary depending on which part of the plant they occur.  I’ll leave it up to the botanists and forensics experts to figure out how and when these structures evolved.  Why they evolved seems intuitive. Basically, some of them serve as defense mechanisms to deter nibbling by herbivores, whereas others evolved as seed dispersal mechanisms.

PRICKLY PLANT STRUCTURES DEFINED

There’s a tendency to refer to any sharp pokey thing on a plant as a thorn, a spine, or a prickle.  While the three terms are used interchangeably in casual conversation, botanically they are not the same. Each is an adaptation of a separate plant part.

THORNS

Thorns occur in the axil of a leaf where a branch would normally develop, but instead, terminate in a sharp, hard, woody point. They grow from a plant’s stem tissue and contain internal vascular tissue (phloem and xylem).   A number of native tree species, such as Washington hawthorn (Crataegus phaenopyrum), honey locust (Gleditsia triacanthos), and black locust (Robinia pseudoacacia), evolved with thorns.  Many non-native species have thorns as well, such as firethorn (Pyracantha) and Japanese flowering quince (Chaenomeles spp.).

Here’s a little known fact:   Most citrus trees such as lemons, limes, grapefruits, and oranges have thorns.  More surprising, apple, plum, and pear trees also evolved with thorns, although the varieties grown today have been bred to be thornless.

SPINES 

Spines are derived from a plant’s leaf tissue.  They are firm, sharp, slender, pointed structures that represent a modified leaf or stipule (appendage found at the base of a leaf’s stalk). Like thorns, spines also have internal vascular tissue.

In general, spines are found on most cactus species and on some succulents.  A flat, wide leaf can lose a lot of moisture, but when the leaf is transformed into a narrow spine with a restricted surface area, the plant is able to reduce water loss.  Spines help provide a little shade to the plant and that is certainly a useful function in hot, parched environments. Spines also appear to have evolved as a defense mechanism against birds and animals that would otherwise eat the plant or attempt to extract water from its cells.

Some plants other than cacti species have evolved with spiny outgrowths that are located on the edges or tips of their leaves.  For example:

Spinose leaf margin – Some trees and shrubs such as holly (Ilex) species, false holly (Osmanthus), and Oregon grape holly (Mahonia aquifolium) have spiny edges along the margins of their leaves.   These marginal spines are extensions of the leaf’s major veins.

Spinose apical processes – Yet other plants, such as Agave, Sansevieria, and Yucca species, have sharp tips (terminal spines) on the ends of their leaves.

PRICKLES

Prickles are short, sharp, woody projections that are derived from a plant’s skin (epidermis) tissue, which is the single layer of cells covering the leaves, flowers, stems, and roots of certain plants. Think of it this way:  If you encounter a sharp, pointed outgrowth anywhere on a plant where a branch or a leaf would NOT normally be, then it’s very likely to be a prickle. Unlike thorns and spines, prickles don’t have any vascular tissues connecting them to the main body of the plant.

Some plants, like rose bushes, have prickles on the stems only.  Oh, by the way, the “thorns” on rose bushes are not really thorns at all.  Because they grow at random spots along the stems of the plant, that technically makes them prickles.

Greenbrier (Smilax) is an example of a plant armed with prickles on its stems.  Despite its weediness and unfriendly prickles, this low-growing woody vine is native throughout North American and is not without merit.  The fruits are a food source for many bird species.  They eat the fruits and then pass them randomly throughout our landscape, which explains why the plants seem to appear out of nowhere.

Other plants, like horsenettle (Solanum carolinense), have prickles on both stems and leaves.  Horsenettle is a bothersome weed that occurs both in pasture fields and in home landscapes.  All parts of this plant — stems, petioles, leaf veins, and even the flower stalks — are covered with prickles.  Because the plant is so prickly, animals are deterred from grazing.  As it turns out, this is a good thing because the plants and their fruits are capable of poisoning livestock.

For yet other plant species, the prickles are attached at the base of the leaves only.  Spiny amaranth (Amaranthus spinosus) is one such example.  The very sharp, stiff prickles on this plant can be as long as a quarter of an inch. As an aside, a mature spiny amaranth plant can produce about 235,000 seeds, making it a formidable weed to deal with.

A few plants have reflexed or recurved prickles, meaning that the tips point downward.  This design hinders small animals from climbing stems to reach leaves on some plants.  On other plants, particularly ones with long stems, the prickle design allows the plant to hook onto other branches or plants for support.    Mile-a-minute vine (Persicaria perfoliata), which is highly invasive and difficult to eradicate, is an example of a plant with reflexed prickles.

MORE PRICKLY PLANT STRUCTURES

The world of plants with prickly personalities is filled with many that don’t fit neatly into the three categories described above.  A few variations on a theme include the following.

Prickly protrusions on flower heads, seeds, or fruits — From an evolutionary standpoint, such structures make a lot of sense.  To ensure the continued survival of their species, plants need to disperse their seeds to new areas where they will have less competition for light, water, and nutrients.  One highly effective dispersal method involves the use of special barb-like mechanisms designed to equip seeds to stick or hitchhike elsewhere.  According to Botany for Gardeners by Brian Capon, “The presence of hooks and barbs is a reliable indication that a seed or fruit is waiting to hitch a ride on a passing animal.”

Here are a few examples of weeds equipped for hitchhiking:

• Beggarticks (Bidens bipinnata) – The dark brown, elongated seeds are equipped with tiny barbs designed to latch onto animal fur and clothing. This is the weed that I eventually identified as the culprit in my perennial garden.
• Cleavers or Catchweed bedstraw (Galium aparine) – Tiny, round fruits are covered with very short, curved hooks and are dispersed by latching onto animals or floating in water.
• Prickly Sida or Spiny Sida (Sida Spinosa) – This weed has very small prickles at the base of each leaf and branch. The circular seedpods have five segments that break apart. Each segment has two spikes or beak-like protrusions on the tip.
• Sticktights or Seed Tick (Desmodium species) – As its common name of sticktight suggests, the flat triangular-shaped seeds latch onto animal fur or clothing via small, barbed hairs that give the seeds a fuzzy appearance.

Fine hairs or bristles — A great many plants are covered in fine hairs or bristles that serve a variety of purposes, including protection from heat or cold and protection from herbivores. Prickly lettuce (Latuca serriola) is such a plant.  The leaves on this weed have prickly edges accompanied by a row of stiff, prickly bristles on the lower midvein.  Between the prickly leaves and milky sap, this is not a pleasant plant.  It is also quite prolific. According to Weeds of North America, a large, mature plant can produce up to 87,000 seeds!

Burs – A bur (also spelled burr) is a seed or dry fruit with hooks or teeth that are designed to repel herbivores and to disperse seeds by latching onto fur or clothing.  Depending on the plant species, burs can be a mild annoyance or a serious hazard.  On the annoying side, they can snag or tear one’s clothing.  On the serious hazardous side, some burs can puncture tires, hook onto the legs or feet of animals causing injuries, and damage agricultural harvesting equipment.

Some plants with burs include the following:

• Common Burdock (Arctium) –On the positive side, the flowers on this large, coarse looking biennial weed provide pollen and nectar for bees and some Lepidoptera species in late summer. Another plus is that this plant inspired the invention of the hook and loop fastener known as Velcro. On the negative side, the plant produces lots of burs that catch on fur or clothing and can be painful to extract.
• Common Cocklebur (Xanthium strumarium) – The hard woody burs on this plant are covered in hooked prickles and have two long beaks that project from the tip of the bur. The burs can float in water as well as latch onto animal fur or clothing.
• Jimson Weed (Datura stramonium) – The funnel-shaped blossoms on this large weedy plant usually open at night and provide pollen for nocturnal moths. The leaves are spiny looking and the large seed pods have a spiky appearance.  All plant parts are poisonous.
• Teasel (Dipsacus fullonum) – This non-native plant is aggressively competitive with other plant species for space and nutrients and is considered to be invasive. The wicked-looking oval-shaped spiny seed heads develop on long prickly stalks.  Often found in disturbed sites, pastures and along interstate highways, teasel produces about 2,000 seeds per plant.

ET TU, VEGGIES?

Yes, even vegetables have their share of prickly personalities. Some varieties of eggplant, for example, have small prickles on their stems and on the calyx where the fruit joins the stem. Grasping the stem at that point with bare hands can be painful.  Yellow squash plants have prickly stems and leaves that can be uncomfortable to the touch when handled with bare hands.  Even the fruits are covered with fine, nearly transparent hair-like bristles.  Some, but not all, cucumber varieties have slightly spiny fruits as well as prickly stems. Artichokes have prickly leaves and beautiful violet-purple thistle-like flowers that attract pollinating insects.  The edible flower bud is made up of scale-like leaves with short inedible needle-like thorns on the tips, which must be snipped off.

PREVENTION/CONTROL OF PRICKLY PLANTS

Except for the vegetables, many of the prickly plants described herein are opportunists. They appear most often in disturbed soil, such as alongside roadways, construction sites, or garden beds.  In addition to being blown about on the wind or carried by water, seeds may also be dispersed by wild animals, birds, humans, and our pets. Regardless of how they are dispersed, it’s important to remove the plants at the seedling stage for several reasons:

• A seedling is less prickly to handle when just emerging from the soil than when the plant has reached maturity.
• Many of these plants have tap roots that are easier to pull from the soil when they are very small.
• Many of these plants produce prodigious quantities of seeds and should be eliminated before the seeds mature.

THE TAKEAWAY…. 

A thorn is a modified short branch, a spine is a modified leaf or leaf part, and a prickle is an outgrowth from the epidermal tissue of stems, leaves and some fruits.  Knowing the distinctions among the three is not essential unless you want to use that knowledge to help identify plant species.  Keep in mind that not all plants with sharp or prickly outgrowths are weeds or even undesirable.  Many of these plant species are valuable sources of pollen, nectar or shelter for pollinators and other wildlife. However, it is important to identify those undesirable plant species that should be eliminated before they become a nuisance.  Preventing them from setting seed in the first place or eliminating them at the seedling stage are your best options.

As I contemplate the beggarticks weed that started me on this path to discovery, I now understand and even admire its survival strategy. The plant is rooted in the soil and cannot move. So, many millennia ago, its ancient ancestors figured out how to use mobile creatures to disperse their seeds for them.  There’s even a term for this strategy:  zoochory.   Pretty clever, don’t you think?

Featured photo:  Common thistle armed with formidable prickles on stems, leaves and flowers. Photo:  Pat Chadwick

SOURCES: 

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 (vt.edu)

University of Missouri Weed ID guide, https://weedid.missouri.edu/

“Seed Dispersal,” ivycreekfoundation.org/docs/9.Seed_Dispersal

“Identification of Virginia’s Noxious Weeds,” Virginia Cooperative Extension Publication SPES-244NP https://www.vdacs.virginia.gov/pdf/va_noxious_weeds.pdf

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.