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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).
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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.
