Using various species of cactus to create security has great benefit beyond no requiring the exploitation of hard to come by resources such as iron.
Once created, the living fence will provide excellent security for crops and homes, fruit while allowing natural species to migrate environs. Deer and its main predators can jump it or crawl through natural tunnels.
Cactus are also excellent users of nutrients and water - perhaps the most efficient plants on earth: Cactus have extremely efficient processing systems which create life where almost nothing else exists. If you think about it, it’s almost a miracle. A human being would not last two days in the same environment.
Also , given even reasonable amounts of water, cactus can grow quickly – and since most farms and homes have water - a fence can be established in a year. The closer you plant pads, the quicker the fence will fill in, but one will face a difficult pruning job somewhere down the road.
Once established, a cactus border will provide security, beautiful flowers (bees are drawn to them along with other insects), and fruit known as prickly pear and tuna. Each variety should be sampled for ease of handling and flavor. Some are simply too seedy. The leaf / pad has medicinal qualities (diabetes, lotion for burns…) and is fried and eaten (nopal, nopalitos). And there’s no doubt that if one's family were in a survival mode, this fence is a dream come true – security and food.
Eventually, the cactus fence will need to be pruned. This can be roughly done with a shovel. Pads are handled with a pitchfork and hauled to a mulching hole or pile. I scar them with a shovel blade, water to create moisture, cover with mulch of some sort (a mixture of dirt and cuttings would be fine), soak the pile well, and then allow the sun do its job. If one is concerned pads will start sprouting, cover the pile with plastic and secure edges with dirt / mulch to keep it in place. If using plastic, you only have to water once. Without plastic, depending on weather, you may have to rewater deeply – but this is the exception if you make the mulch cover thick enough – 8-12 inches.
The standard prickly pear Opuntia makes a good fence if you use a spinned variety. The spineless varieties cannot keep humans away unless they only know it from a distance.
The Cereus variety (columnar) grown about twice as slow but offer fruit as good as dragon fruit. Cereus is grown in the deserts of Israel as a commercial fruit in extreme environs.
The bluish cactus variety seen here is Opuntia violacea which makes an excellent fence. The fruit is beet red packed with seeds and almost no edible pulp. It also has thick spines clustered on the fruit.
The common prickly pear has a nasty fuzz of spines which can be hard to see. These are available commercially and appear to have been tumbled when hard – I cannot imagine how else they removed the spines – but then the flavor suffers when harvested this early. The orange fleshed Prickly pear are very good. You know when they’re in prime eating condition – the birds will let you know!
To make a fence
One severes the thickest pads from a plant, allows the scar to heal a bit - a week, then plant them on 4-foot spacings.
If you have some tim one can place them on the ground, flat, the same day and forget about them for a year or two. They will form a cup-shape and thus retain rainwater. If making a fence in this way, choose very thick if not woody pads.