Edge Effects

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Edges in the landscape are important because they are interfaces between two different types of environment or habitat. They share characteristics of both adjacent areas but have a unique character of their own. We see edge environments along the ocean/land interface, in the connection of field to forest and at the edges of streams and rivers. Edges also occur in broad surfaces as when two air masses meet or at the interface of the crown of a tree and the air around it. At the microscale there is an edge around the soil particles to which water will bond. These thin, edge ecosystems are known for their diversity and intense activity.

At the regional scale we often find human settlements in edge environments. Cities have naturally accumulated at the coasts of continents and along rivers. Along the east coast of the United States there is a series of cities along the 'fall line' where the piedmont meets the coastal plain. These 'fall line' cities originally took advantage of the fall of water for power in running mills. Urban areas on edges are able to make use of the resources of the two areas between which they sit.

The planner, Kevin Lynch, identifies edges as one of the major components of the 'image of the city.' The other components are districts, paths, nodes and landmarks. Edges frequently occur between districts, which are large areas of similar land use, and are often reinforced by paths, which are usually roads or railroads. Edges also occur along rivers or other major topographic features. Lynch claims that these edge patterns help us visualize and understand our urban areas. His concept of analysis for the 'image of the city' can apply to smaller scale design.

Mollison and Slay (Introduction to Permaculture, pg. 26-30) examine the importance of edges in landscape design. One important function they identify is that the edge is an accumulator. Soil collects along fence lines; the 'tide line' along a beach is easy to find because of the accumulation of shells and debris; sticks, silt and vegetation are caught at the edges formed by trees that have fallen into a stream. We can use this 'trapping' characteristic in design to collect materials we may wish to use. Edges may also be used in design to define and control areas by breaking the design up into manageable areas. Fences, trellises, chicken runs, walks, roads, and barrier hedges are all good edge features.

Thinking of edge as 'design pattern' we can begin to experiment with the shape of the edge. Some patterns are quite common. Mollison and Slay identify spirals, crenelations, chinampas and edge cropping as approaches observable in many cultures. Beds spiraling up or down provide a series of edges with different orientations. Crenelating an edge increases edge distance considerably without increasing the surface of an area. Chinampas are ditch and bank systems used in Mexico and Thailand to create gardens that are almost all edge. Edge cropping has been used extensively in many parts of the world by planting various crops in strips and varied patterns.

Mollison and Slay identify a number of other common patterns of edges. "Edge patterns can be zigzag (zigzag fences stand up to wind better than straight fences); lobular (keyhole beds create different microclimates); elevated (mounds and banks provide wind protection, greater growing surface, and good drainage); pitted or "waffle iron" (for garden beds in dry climates, and to trap mulch and debris blowing across the landscape); gently curved (paths cut on the contour along hillsides allow access for planting, mulching and watering); and sharply curved (suntrap design to enhance heat and protect from cold winds).



 from: Mollison and Slay, Introduction to Permaculture, pg. 31


We can select appropriate edge patterns for our designs to take advantage of climate, topography, site size, orientation and slope aspect. Small gardens may allow great complexity of edge whereas larger areas may require more simple patterns.