In my last blog, I talked about Permaculture Principle 11 - Use Edge and Value the Marginal. Yesterday I was reminded of this principle when I watched permaculture teacher, Tom Kendall, discuss the importance of defining edge in order to collectively maintain systems more easily and effectively.

Tom argues that defining edge early on can save alot of work and second-guessing. Logs make for a good edge between a garden plot and grass, weeds, and path area.

On my walk today I reflected on edge design and thought this picture of a log suspended over a shallow river offered another avenue to think about edge design.

Here we have a large hemlock tree that has recently fallen across the river. The branches and remaining root attachments prevented it from falling into the river and creating an edge perpendicular to the flow pattern of the river. If the tree had fallen cleanly into the river it may have significantly impacted upon the flow pattern and distribution of plants and animals on each side of the log.

This photo suggests that edges are dynamic forces in the landscape. When an edge is put down it can affect flow patterns in a way that can significantly change a landscape/waterscape. Some edges have little effects, others, such as a big log across a river, can have more significant effects on the local ecosystem.

What constitutes an edge is something worth thinking about because it is not an easy question to answer. In the video above the log is presumably the edge, but if there was only one log separating the garden area from the grass would it still be an edge? Does the existence of the edge depend upon the system that it is presumed to belong to? A series of logs around a garden constitutes an edge better than one log strewn between a garden area and the grass. Perhaps the students can see the edge because they can more clearly see the mulch gardening system it contains and defines.

Permaculture co-founder Bill Mollison approaches the concept of edge from many different directions from very practical to very metaphysical. In the metaphysical version an edge arises from an event or series of events that takes place when two different types of "media" interface with each other. If you are looking for an abstract definition of what an edge is this is actually a pretty useful one. On either side of an edge are two different media - water & grass, air & water, gravel and grass, trees and grass, etc... These edges arose as a result of an event or series of events. Edges can be designed by human interventions and/or by nature. Edges are dynamic as their persistence in an event sequence can result in the edge changing gradually or catastrophically. If you want to go deep into what edges are then listen to Bill's lecture on the Fundamentals of Pattern.

As you walk through nature an interesting exercise is to look for the edges that nature has created in the landscape, how those edges came about, and how they might evolve over time. You can then compare how nature creates edges with some of the beautiful designs that humans create by the masterful use of edge. This young circular herb garden is also very fragrant adding to the delight it offers.

Designs that give the sense of a multi-story forest garden along a meandering path are quite nice. The edge design around the local Dalhousie Agriculture campus is simply exquisite with incredible lushness, color and variety on display.

The alpine garden is also spectacular. Here is a small fragment. A stone planter with a playful use of line, color and textural edges.

The last photo shows (to the discerning eye) some natural edge along a riverbank . We have ferns to the left, purple and white wildflowers (of the same species) in the middle, and young willow trees to the right (next to the river below). Each type of plant is neatly separated from each other as if by an invisible edge that arose over time as the result of flood events, patterns of sun and shade, and mutualistic and competitive interactions.

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