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Natural Access Control
Natural access control guides people entering and leaving a space through the placement of entrances, exits, fences, landscaping and lighting. Access control can decrease opportunities for criminal activity by denying criminals access to potential targets and creating a perception of risk for would-be offenders.   (Virginia CPTED Committee, 2002)

Perhaps it's a bit extreme, but the image on the right provides excellent access control. The homes of these ancient Native Americans  were accessible only by ladders and entrances could be sealed.

Physical and Psychological (Symbolic) Barriers
Natural access control and natural surveillance work closely together. In addition to restricting access, the environment (landscape) can be designed to create the perception of risk in those individuals considering illegal activities.

One important goal of access control is to inform the public of boundaries between public, semi-private, and private spaces. For example, a low growing hedge can effectively define an area and direct traffic on a property as well as a fence yet still allow for natural surveillance. Thanks to the increased visibility, visitors to an area will probably feel more welcomed walking along, and between, hedges than they would with fences. Here are some general guidelines to follow in order to provide some access control with landscaping:

• Design, select (plant material), install, and prune to create a row of vegetation to guide visitors to a formal entrance, or area, that has natural surveillance.
• Avoid trees and shrubs that may become climbing aids to the roof or upper story windows.
• Hedges can serve as a physical barrier but keep in mind that they can grow up to block natural surveillance and provide opportunities for concealment.
  • Hedges don't always have to be obvious and massive in order to indicate to the public how to enter and exit a space. Even a bed of flowers can serve as a symbolic barrier.
    • Symbolic barriers are still effective because violating them makes the individual conspicuous.
  • Access control can still be achieved using non-painful shrubs so long as the selected plant material has a dense/compact habit and can be planted close together to physically block access.
• Cement planters and other hardscaping materials make excellent access control devices.

• Hawthorn Thorns

  Painful plant material (i.e. plants with thorns, sharp points on foliage, irritating chemicals, foul smell, etc) can be almost as effective as a barb wire fence but without the perception of target hardening. 
• Establish a dense planting of painful plants beneath windows and other areas of access.
• Time over distance is another form of access control. This technique can only be implemented when a home is still on the drawing table. The farther away a home is from the street or place of concealment the less inviting the house seems to a potential burglar.
• Traffic calming devices, including planting trees along the sides of a street, aid in access control by slowing down traffic.

Container Trees

Trees growing in containers, in my opinion, are underused in urban areas. A tremendous advantage to trees growing in containers is the ability to move them and to grow trees in a variety of areas where soil is lacking (i.e. on sidewalks). 

Plant Selection
Trees and shrubs selected for access control should provide two things: a compact form and a painful anatomical feature to reinforce pedestrian compliance with staying on the intended path. One drawback to working with plants that fit the above description is the challenges associated with pruning and maintaining these plants. Without proper maintenance, trees and shrubs installed to encourage access control may actually end up blocking natural surveillance and working against territoriality. [click on the heading for additional information for this topic]

Landscape Lighting
Lighting can influence the intended user’s emotional state. Landscape lighting can be used to make us feel welcomed to a space. On the other hand, landscape lighting can discourage unintended users from using a space.   Lighting can create a light barrier to keep bad guys away or to direct and control the attention of people to a particular area. Bollards (w/lights) add even more access control with their physical presence. [click on the heading for additional information for this topic]