Place bushes with thorns around the perimeter of your fence. While an intruder might not think twice about a bush at the bottom of your fence, he will not want to tackle one that can make him bleed and leave evidence. Plants that deter trespassers include rose bushes, hawthorn, cacti, Oregon grape holly and firethorn.
Grow climbing, thorny plants such as blackberry, raspberry, and Pyracantha along the bottom of your fence. These plants don't require a lot of ground space, and the addition of spiky vines to fence posts makes it twice as hard for an intruder to scale a fence.
Install a trellis over the top of your fence to make it higher. The higher the fence, the less attractive it is to intruders. Train climbing vines or flowers with thorns to grow along the trellis. If an intruder is smart, he will see that the trellis is not only covered with spikes, it could snap under his weight, make a sound, and break into jagged points.