More information about the "broken window" concept.
(Background: When Good Spaces Go Bad)

Trees & Graffiti (bottom of page)

Territoriality - promotes a sense of ownership
Physical design can create a sphere of territorial influence that can be perceived by, and may deter, potential offenders. Defined property lines and clear distinctions between private and public spaces are examples of the application of territorial reinforcement. Territorial reinforcement can be created using landscaping, pavement designs, decorative gateways, signs, and fences.   (Virginia CPTED Committee, 2002)

For territoriality to be successful, it's important that intended users, as well as potential offenders, recognize the boundary and purpose for a particular space.

Maintenance is a very important part of territoriality because it helps keep the space the way it was intended. A well maintained landscape sends the message that the homeowner really cares about his property. A landscape that is in disrepair is a “broken window.” Maintenance is a chore so it’s important that the inexperienced homeowner plan and plant a low maintenance landscape. In addition to hiring an inspector to perform a careful inspection of a home, the potential buyer should also have an arborist inspect the landscape and provide a realistic estimate on how much maintenance will be required to keep the landscape performing at a high level.

There are variety of ways to express ownership/territoriality. 

• Maintenance    
• Display of addresses
• Flags or banners
• Sculpture or yard art
• Other amenities to humanize the physical environment

A homeowner will usually purchase a security system when he has a lot of money invested in his property. Installing, or improving, a landscape can allow low income homeowners an opportunity to invest sweat equity to improve their property value and to make a territorial statement. Many resources (see Putting Theory to Practice) are available to help train willing homeowners.

Barriers, physical (fence) and symbolic (row of flowers), can be installed to express territoriality and access control. Planting rows of trees have been used for thousands of years to help identify borders and to reinforce desired behaviors (i.e. entering and exiting defined areas).

Members from the neighborhood can join together to construct pocket parks in highly visible neighborhood easements. Ideal locations would be at entrances to neighborhoods.  This sends a message, similar to the Neighborhood Watch sign, that the residents are claiming and defending their space. Garden clubs may be interested in assisting in the design and installation phases of local neighborhood pocket parks. It’s important to get the residents to “buy-in” and have sweat equity in the landscape initiatives or the landscapes are certain to decline and work against CPTED. A critical step in getting a community volunteers to maintain their gardens (like pocket parks) is to make sure they're involved in the planning and planting phases. Otherwise, the garden may be perceived as just another city project forced upon them. Naturally, the neighborhood group should be sensitive to community ordinances that may impact their pocket park plans.

No territoriality Image from J. Murray

Apartment dweller expressing territoriality Image from VA Crime Prevention Association

Landscaping can allow renters to have a "space."  Design in space for garden spots for residents. Doing so will better allow residents to establish territoriality and perform natural surveillance.

 Another excellent example of territoriality is the ubiquitous sign: Keep off grass!  

Graffiti problems?
It's absolutely imperative that graffiti and gang symbols be removed as soon as possible. Everyday that graffiti remains in a public space it serves as a reminder that someone else (not the public) is claiming ownership to that space. Graffiti is a broken window. 

The metro system in Washington DC has enjoyed tremendous success in preventing graffiti by enforcing a policy that includes removing all graffiti within 24 hours. Graffiti artists no longer feel they're getting enough exposure for their effort so they simply move on to the next available "canvas."

I recommend doing a little research so you can get to know the "taggers." First, take a picture of the graffiti. Contact your community's crime prevention resource officer and the officer in charge of that territory to see if they can identify the symbols/message. Dealing with a graffiti artist is different than dealing with a gang member tagging the gang's territory. Second, consult with your law enforcement representative and public works department to decide upon the removal of the graffiti. PLEASE don't take it upon yourself to remove the graffiti. People have been shot removing gang symbols.

It is possible to design physical structures to deter graffiti artists. Builders can modify surfaces to make tagging more difficult to perform and less likely to be seen. However, don't get carried away with physical modifications or you may run the risk of target hardening and then the public will fear crime and avoid that public space.

The role of trees in preventing graffiti
With respect to graffiti problems... are trees part of the problem or part of the solution? It is possible to plant and train ivy, or espalier other plants, to cover the wall or surface. Painful plant material can also be installed to provide some degree of access control, preventing the taggers from easily reaching an area. Trees can be strategically planted to block the view of areas routinely tagged. If plant material will be part of the approach to prevent graffiti, make sure your local arborist, public works superintendent, and department of transportation representative are all on the same page when it comes to removing impervious surfaces, preparing the planting site (soil pH, drainage, species selection, long term maintenance) and movement of vehicles and pedestrians. Hopefully, there is already a neighborhood located near this site that might be interested in assisting in the planting project. Make sure they're involved at the very beginning, otherwise they may not take on ownership, and then the area becomes yet another burden on the shoulders of the city arborist.

Existing Vegetation
Perhaps the problem is that the existing vegetation (planned and unplanned) is providing an area of concealment for the taggers. I strongly recommend you call in a certified arborist to help you identify options on pruning and/or removals to reduce areas of concealment and to improve natural surveillance.

Plant Selection
It’s important not to confuse the selection and training of trees and shrubs for territoriality with the creation of a “green wall” which will provide screening and privacy (and decrease natural surveillance). The goal of landscaping to support territoriality is to communicate to the public ownership of property. For example, evergreen trees can mark property lines year round. Slow growing, columnar shape trees and shrubs will remain compact and not spread over property lines or block natural surveillance. Make sure tree selection matches the property owner's desire to keep up with maintenance. For example, fruit trees require a significant more amount of time and energy to maintain (pest problems, pruning needs, dropping fruit, etc) than a small ornamental flowering tree. Usually, pest problems and poor vigor occur for three reasons: first, the individual failed to properly select an appropriate tree for a particular site (see cultural site conditions above); second, the individual failed to research the plant material and selected plants lacking insect and disease resistance; and finally, because the individual failed to take maintenance into account when purchasing plants. [click on the heading for additional information for this topic]

Landscape Lighting
Appropriate lighting decreases fear of crime and encourages the population to use a space for the intended purpose (even at night). Landscape lighting shows that someone may be at home. Motion detection lights provide an economical way of providing security without being irksome to the neighbors. Landscape lighting (spot beams) can also be used for highlighting important information like hours of operation or the presence of a security system. [click on the heading for additional information for this topic]

Personal musings.... on a research question I've been asking myself
The fact that people demonstrate ownership of "their" space. Is it an inevitable display that occurs AFTER territorial behavior has already been established? Or, is it the landscape (and other components of good design) that stimulate territorial behavior? This will have a huge impact on the success of attempting to establish territorial behavior in a neighborhood where there previously was none.

Links:
Winnipeg Police Service - Graffiti Prevention