Plants that are thorny or prickly can be used to prevent or at least discourage illegal entry into yards and homes. When planted in the right place, these plants can provide a good barrier. A few of the plants that are most effective in this use, according to Bruce Wegner, a landscape architect and parks and recreation manager for the city of San Clemente, include:
Carissa grandiflora (natal plum)--This evergreen shrub is available in a variety of sizes--from low-growing to several feet high. It has large thorns, small white flowers and small plum-like fruits and is easy to maintain. However, it can be damaged by frost in inland areas.
Chamaerops humilis (Mediterranean fan palm)--This plant is low growing and provides a nice accent or focal point in a yard. It is very prickly.
Grevillea noellii-- This evergreen shrub has attractive foliage and mounds nicely. The foliage is needle-like and prickly.
Hakea suaveolens (Sweet Hakea)--This upright shrub has prickly, needle-like leaves and is available in many varieties. It has fragrant white flowers and can grow to 10 to 20 feet high. It does well against a house (between windows) and would be difficult and uncomfortable to hide behind. Several planted against a fence provide a good barrier.
Ilex species (holly)--This familiar shrub is available in a variety of sizes, from low-growing up to tree forms. It has prickly leaves and bright red berries.
Juniperus species (juniper)--The prickly foliage on this plant makes it a good barrier plant. A variety of sizes are available--from a 4-inch ground cover to tree-size plants. The ground cover is ideal for preventing access in pool or beach areas (where people might be barefoot), while the larger varieties make good barriers.
Mahonia species--This plant has prickly foliage and characteristics similar to holly. It is available in several varieties.
Phormium tenax (New Zealand flax)--This plant is not an upright shrub. It is similar to an agapanthus, but is larger, with long, slender leaves with sharp edges.
Pyracantha species (firethorn)--This versatile plant is available in a variety of sizes--from low-growing to tall--and has large thorns on its branches. It also has white flowers and berries.
Taxus species in shrub varieties (yew)--This conifer plant has a needle-like leaf and is available in a variety of sizes. It provides a good barrier and is drought-tolerant.
Bougainvillea--This fast-growing vine with large thorns and beautiful flowers is available as a low ground cover or as a trailing vine on fences and houses. It is good for providing a barrier over large expanses. It is often used in parks as a barrier between park property and adjacent homes. The plant does not tolerate frost well.
Macfadyena unguis-cati (cat's claw)--This vine adheres to a wall with a thorny growth that resembles a cat's claws.