Barrier, or border plants, are plants that can offer growth suppressing services for other plants. Some emit chemical (allelopathic) compounds. Others have physical properties that act as a natural defence against plant competition. Below is a short list of some perennial plants that are good barriers. There are a host of shrubs and larger plants that are great for segregating large areas, and creating both privacy and protection. Consider if you need the barrier year round, or only during the growing season. There are large and small plants that are evergreen options. Some plants loose their leaves in the fall, but have thorns all year long. This makes your barrier change with the seasons.
Depending on the purpose, function, growing conditions, and size of the garden space. There is a plant that will work for to delineate the different garden sections. There are many plants that compliment a hard edge as well. Plant these plants next to a stone wall or pathway to soften those edges.
Many of the plants below have multiple benefits in the garden. If the name is a link, click on it to see some of the other benefits these plants have.
Here are some examples:
White Dutch Clover