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.

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.