Thorny Vegetation Is Naturally Repellent

Black locust is painful to handle.

Roses might be more fun to grow and prune without their thorns. Blackberries are easier to pick from thornless canes. Thorny vegetation is simply unpleasant to work with. Some very desirable plants, such as roses and most blackberries, are innately thorny. The only alternative to contending with their thorny condition is to grow something totally different.

Thorns and similar structures are as diverse as foliage, with a few distinct classifications. True thorns are simply modified stems, like those of hawthorn. Spines are modified foliar structures or leaves, such as those of cacti. Prickles, such as those of rose, are modified epidermal structures. Spinose foliar margins, like those of holly, serve the same purpose. 

The purpose that thorny vegetation serves is defense. It intends to inhibit consumption of foliage, fruit or bloom. That is why some trees are thorny only while young and low to the ground, then almost thornless as they grow beyond the reach of grazing animals. Thorny trunks of honeylocust may protect seed pods from bears, so birds can disperse the seed. 

It is therefore no mystery that many of the thorniest plants are endemic to desert regions. The scarcity of edible vegetation in such regions increases the need for protection. Also, it is no mystery that most grazing animals are not too deterred by thorny vegetation to get enough to eat. Otherwise, roses and firethorn would be exempt from the ravages of deer.

Despite its intentions of deterrence, thorny vegetation inhabits home gardens for various reasons. Some produces desirable bloom or fruit, like roses and blackberries. Cacti and agaves develop remarkably striking forms. Such plants should be situated appropriately. Rose canes can be bothersome close to walkway. Agaves can be downright dangerous. 

Thorny shrubbery, such as firethorn, barberry, Natal plum and English holly, is a practical deterrent for unwanted traffic. Firethorn espaliered on the top of a fence is as effective as barbed wire, and is more appealing. Maintenance of thorny plants is more of a challenge than for thornless plants. Otherwise, thorny plants should be more popular than they are.

Toxic Plants Are Diversely Enigmatic

Smoketree is related to poison oak.

There is no single reason for toxic plants to be as potentially dangerous as some of them are. They are toxic by various means, and to various degrees. Some actually seem to be incidentally toxic. Many are intentionally and justifiably toxic. A few live in home gardens and landscapes. Of these, a few surprisingly produce safely edible fruits and vegetables! 

Immobility is a major disadvantage for plants. Those that begin to grow where resources are inadequate are unable to relocate to more accommodating situations. Those that live within ecosystems that periodically burn must either regenerate efficiently after fire, or be resilient to fire. Flowers of all plants must rely on other organisms or wind for pollination.

Since plants are immobile, they can not evade other organisms that eat them. Therefore, some do what they can to be unappetizing for the organisms that are most threatening to them. Some use thorns or similar protective devices. Unpalatable tomentum (fuzz) works for others. Many use unappealing flavor. Some are unappetizing because they are toxic.

Toxic plants are generally not toxic to a broad range or organisms. Insects eat plants that are very toxic to mammals. Onions, although commonly consumed by humans, are toxic to canines. Fortunately, most organisms instinctively know what plants are toxic to them, and avoid eating them. Unfortunately, humans and canines are occasionally exceptions.

Because young children put random items into their mouths, and puppies chew anything that they can get their teeth around, they should not have access to toxic plants. Morning glory, foxglove, yew, oleander and castor bean are some of the more poisonous of plants that are common in home gardens. Dieffenbachia is a potentially dangerous houseplant. 

African sumac and smoketree are related to poison oak. Although not so poisonous, they are incidentally toxic allergens to those who are sensitive to them. (Incidental toxins may not be intentionally deterrent to consumptive organisms.) Hellebores are both poisonous and very allergenic. Poinsettias and their relatives are toxic because of their caustic sap. 


Overwintering birds can not miss firethorn.

The most abundant bright red berries of autumn are those of firethorn, Pyracantha coccinea. They resemble English hawthorn, cotoneaster and toyon berries, but are shinier and more abundant. Their weight causes their limbs to sag. Most are either rich deep red, or brighter slightly orangy red. Orange berries are uncommon locally. Cultivars with yellow berries are less vigorous and rare.

Berry production may seem to be more variable than it actually is. Healthy plants typically produce a profusion of berries. They only seem to be more profuse if they ripen late, after migratory birds fly south. Such birds are voracious with early berries. Overwintering birds are less numerous, and consume berries slower. Regardless, few berries last long enough to deteriorate and get messy.

Firethorn is thorny enough to function as an impenetrable hedge. However, it is unpleasant to prune with all those thorns. Also, pruning without removing the stems that bloom and produce berries takes effort. Upright cultivars can get higher than downstairs eaves. Some cultivars get only a few feet tall as they sprawl over the ground. Even without berries, the evergreen foliage is appealing.

Natal Plum

Lush foliage hides oddly branched thorns.

The very glossy and richly deep green foliage seem to be so soft and luxuriant. Simple bright white flowers enhance the appeal. However, closer examination of Natal plum, Carissa macrocarpa, reveals impressively nasty thorn structures that are doubly branched into pairs of paired rigid and sharp thorns. That is four thorns each!

To make matters worse, all parts of Natal plum that are not fruit are toxic. But hey, they all work together to be visually appealing, even if really unfriendly. The one or two inch long evergreen leaves are quite round and stiff. The star shaped flowers that bloom as long as the weather is warm are somewhat fragrant in the evening.

Some varieties of Natal plum can reach the eaves, but most stay lower. They do not need much water, and can tolerate a bit of shade. The rosy red fruit would be delightfully colorful if it were not so rare. Manual pollination, preferably with a different pollinator, produces more of the two inch long berries that are something like mildly flavored cranberries.

Roses Have Thorns And Thorns Have Roses

Roses certainly look innocent enough.

Thorns, spines and prickles are not often considered to be assets. The weirdly stout prickles on the distended trunks of floss silk tree are more of an oddity than an ornamental attribute. The striking spinose foliage of agaves and yuccas can be more trouble than it is worth. After all, thorns, spines and prickles are intended to repel grazing animals.

They all work the same way, but are physiologically quite different. Thorns are modified stems, like those of bougainvillea. Spines are modified leaves, like those of cacti. Prickles are modified epidermis, like those of roses. Then there are all sorts of plants with spinose leaf margins, like English holly. Such defenses can be a nuisance to those of us who must work with them.

However, they can be useful deterrents. Natal plum that seem to be so innocent can be grown as low hedges that no one would go through more than once. Firethorn (Pyracantha) makes a larger and comparably impenetrable hedge. Their potentially dangerous thorns are almost always adequately effective with their first few pokes, before they cause too much injury.

Some plants are so dangerously thorny that they should be kept at a safe distance. Cacti, agaves and some yuccas can be strategically placed near the perimeters of large landscapes to deter trespassers. They are scary enough to be visually deterrent remotely. Mediterranean fan palm looks friendlier, but is just as mean up close.

Japanese barberry, roses and the spinose foliage of English holly are relatively safe deterrents to inhibit traffic in smaller spaces. They will keep people away from windows without necessarily blocking either access for washing the windows, or views from within. In case of fire, they are not too dangerous to escape through; although roses should not be allowed to get overgrown.

Because thorns, spines, prickles and spinose foliage are unpleasant to handle, the plants outfitted with them often get less maintenance than then should. When they get overgrown from a lack of pruning, they are even more difficult to handle. It is probably better to put nasty debris into greenwaste instead of compost. Thorns and spines are hard, so linger as foliage decays.


91023It is easy to mistake various cultivars of Dyckia for diminutive relatives of Agave or Yucca. They form stout rosettes of rigid leaves with wickedly sharp terminal spines. Their comparably nasty but incurved marginal teeth resemble those of most species of Agave, but are generally more abundant. Surprisingly though, Dyckia are instead related to bromeliads. They just happen to be xeric.

Dyckia do not bloom much, but when they do, the tall and arching floral stalks really are more typical of bromeliads. As they bloom late in spring or in summer, the many small individual flowers on each stalk are collectively colorful enough to be delightfully popular with hummingbirds, butterflies and bees. Bloom is most commonly rich reddish orange, but some cultivars bloom red or yellow.

No one seems to mind that Dyckia does not often bloom. The foliage is remarkably striking alone. Many cultivars form dense mounding colonies of pups that, although unpleasant to handle, can be divided for propagation. A few toothless cultivars, armed only with terminal spines, are easier to handle. Individual rosettes can be as tall and broad as a foot. Dyckia do not need much water.


60817It may not look too nasty, but barberry, Berberis thunbergii, is the sort of small hedge that one goes through only once. It does not have big strong branches to hold anyone back. In fact, the limber branches are quite twiggy. The tiny spines are not impressive either, and might go unnoticed by cursory observation. Yet, they are sharp enough and plentiful enough to make quite an impression!

Because it is so unpleasant to prune, barberry should probably be planted where it has room to grow as big as it wants to without bothering anyone. If it is too close to walkways, it will either offend whomever bumps into it, or whomever needs to prune it to keep it out of the way. Mature plants will unfortunately need to be pruned eventually, so that old deteriorating stems can be groomed out.

The most popular cultivars of barberry have dark reddish or purplish foliage. A few are variegated with white; and a few have golden foliage. Green barberries are now uncommon. The tiny leaves turn bright orange in autumn before winter defoliation. Densely dwarf cultivars may not get much taller than two feet. Taller cultivars might get taller than six feet. Some barberries are very vertical.

Horridculture – What’s The Point?

P90403In this situation, the point is that all those pointed tips of the leaves of this awkwardly floppy century plant, Agave americana, are extremely sharp, extremely rigid and EXTREMELY dangerous. Those shorter teeth on the margins of the leaves are just as sharp and rigid, and are curved inward to maximize injury to anyone trying to get away from an initial jab. With tips that impale, and marginal teeth that slash, this is one very hateful perennial!

Another point is that this big and awkwardly obtrusive century plant is on a patio at a Mexican restaurant. Yes, it is in a public place where people get dangerously close to it. On Friday and Saturday nights, this restaurant can get quite crowded. Some within such crowds are inebriated, so are more likely to stumble about and bump into things that are best avoided. Those concrete slabs to the left are benches where people are often seated.

The third point is that the only remedy for this ridiculously bad situation is to remove the century plant. Chopping the leaves like those that were over the bench on the left only removes a few tips and teeth, but does not make the rest of the foliage significantly safer. Nor does folding the leaves inward, like those that are next to those that were chopped. Such abuse only makes the whole mess uglier. Now it is both dangerous AND ugly.

Now, who thought that putting the most dangerous of all perennials available into this public situation was a good idea?! (Cacti with inward curving spines and other plants that are more dangerous are not even available in nurseries.) Century plants are dangerously nasty even when small and young, so even someone who knows nothing about landscape design should have known better than this!

Horridculture – Miss Congeniality

p90109Now, before I commence with my rant and long list of problems with this picture, I should mention that this seemingly abused rose tree does seem to be appreciated. All the roses in this landscape seem to be very healthy, and they bloom constantly between spring and autumn. Their performance suggests that they are regularly fertilized and deadheaded.
The unusually brutal pruning may be an attempt to keep this particular rose tree as compact as possible, within very limited space. It is not how I would do it, but perhaps it helps. The size of the burl suggest that this rose tree has been pruned effectively like this for a few years, although the lack of weathering of the labels indicate that it is not more than several years old. Older canes really do seem to be getting pruned off annually as they should. Even though the remaining canes are stubbed much too short, the end cuts are done properly. I can not help but wonder of pollarding back to the main knuckle would be just as effective, and neater.
The labels seem to be retained intentionally. In fact, the smaller white label to the left is attached to a new cane. Either the label was removed from an older cane that was pruned away, and attached to a new cane intentionally, or the rose tree was planted only last year rather than a few years ago, as mentioned earlier. I do not know why this uninformative label would have been retained; but the other larger label might be there for anyone who wants to know the name of the rose when they see it blooming in season. It happens to be in a very trafficked spot, where people walk by it constantly.
Rather than snivel about the (seemingly) very bad pruning, and the retention of the (trashy looking) labels, I should just say that this apparently appreciated rose tree should have been planted somewhere else in the garden, or not at all. That wheel in the background really is in a parking spot that is bordered by the red curb. A tiny bit of another red curb on the opposite side of this very narrow space is visible in the very lower right corner of the picture. (I can not explain why the curbs are red.) This really is a very narrow spot between a parking space and a walkway! Those mutilated stems to the right are another shrubbier rose. Thorny rose canes could really be a bother for those getting out of or into a parked car, or walking by on the walkway. The seemingly useless stake might be there so that the rose tree does not get yanked over when it grabs onto someone. To make matters worse, this rose is a grandiflora, which wants to grow bigger and wider than most other types of roses. Defoliation during winter dormancy is no asset either. The pathetic marigold on the ground really does not help much.
The point of all this is that more thought should have gone into planting this rose tree here.
Even Miss Congeniality, who so proficiently adapts to the most unfamiliar of situations, has certain limitations; and this situation demonstrates the worst of them.

English Hawthorn

81226The first specie to be imported to North America from Europe were utilitarian plants that produced fruits, vegetables or other horticultural products. English hawthorn, Crataegus laevigata, was likely one of the first ornamental specie to be imported merely because those who were familiar with it appreciated the delightful white spring bloom and the rich red berries in autumn and winter.

The biggest trees should not get much higher than second story eaves, but the handsomely coarse texture of the bark, furrowed trunks and almost oaken foliar texture make these relatively small trees seem rather grand. Unfortunately, the stems, particularly the most vigorous, are quite thorny. The lobed leaves are about one or two inches long and wide, and might turn orange in autumn.

The trusses of small white flowers that bloom in spring resemble those of pear, and like pear bloom, produce a musky fragrance that some might find objectionable. Some cultivars and hybrids bloom with less fragrant pink, double pink or double red flowers. Some produce orange or yellow berries. If birds to not take them too quickly, the pendulous trusses of berries can last into winter.