The edge effect is one of the major facets in permaculture design. Indeed, it is the tenth in David Holmgren’s list of underlying principles. It is a technique that takes its cue from nature and seeks to maximize biodiversity and productivity. In essence, the edge effect refers to the zone where two distinct environments meet within an ecosystem. This area, where the zones interact, is sometimes referred to as an ‘ecotone’. It benefits from inputs from both the environments, making ecotones among the most biodiverse and fertile areas on a permaculture plot.
There are many benefits that derive from increasing the amount of edges in a permaculture garden. Variations in light, shade, temperature and moisture occur across the edge, making a greater number of microclimates that can support different species of plant. This wide variety of plant attracts a greater range of beneficial insects which in turn, attracts birds and other insect eating animals. The variety of plants also increases productivity and often plants that grow in the edge benefit from their proximity to one another, acting almost like a guild. This also makes harvesting them easier as the crops are close together in one place.
As with most permaculture design practices, the edge effect is something that we observe on nature and seek to replicate. Consider the edge of a forest or wood. The ecotone where the forest meets the grassland is typically rich in plant life and animals that typically inhabit one or other of the two environments move frequently between them at the edge. (We also see how the edge effect has impacted upon human society, with many traditional settlements located in rich, edge areas, such as the banks of rivers or between mountains and plains.) So as permaculture designers, by instituting a naturally rich ecosystem – such as a forest garden – we are increasing the number of edges in the site, but there are also ways in which we can design to create different edges across the various environments on the plot.
The interface between land and water is one of the most fecund ecotones. We see it in nature, where riparian zones next to rivers and streams boast great biodiversity, while mangroves and estuaries also host diverse plant and animal life due to their proximity to the land. A water body such as a pond on a permaculture plot provides a chance to mimic this. The first thing is not to dig a round pond. A circular pond has the least edge; it is much better to dig a pond that is irregular in shape, maximizing the amount of edge that can then be planted with a greater number of species, in turn attracting more insects. You can also add islands and spurs to further enhance the effect, which also provide niches for different species of plant. It is also advisable to vary the depth of the pond, creating more niches. Having one area close to the bank that is deeper than another will create two niches for species to inhabit.
We apply the same design approach to garden beds and the paths we use to access them. Planting crops beside a straight path is the least efficient use of space and has the least edge effect. This impacts upon the yield you can get from your plot. Consider the difference of having a wavy path, with plants located all along its edge. You can fit many more specimens on the land and have more space to access the crops. A technique that is even more efficient in terms of edges and access is the keyhole design, which offers a series of ‘nodules’ in which the gardener can stand and access a whole variety of crops arrayed around the circular beds.
Even at the level of the manner in which individual plants themselves are placed in the ground we can seek to maximize the edges between them. If you plant in straight rows you can fit less specimens into a space than if you use staggered or tessellated arrangements. This not only allows you to fit ore plants into your beds – increasing yield – it also means that each plant has more of an edge with its neighbors, particularly useful when intercropping two or more different species which have different functions that benefit both, such as planting a legume that fixes nitrogen in the soil with a taller crop that offers the legume shade in return for the increased soil nutrients.
Using trellises, walls and fences to grow climbing species of plants adds vertical edges. The heights of such plants can serve to attract different types of insect that operate higher up in the air, they can also offer microclimate niches to plants growing below them, and add to the benefit f the site as a whole by acting as windbreaks and filters of both dust and potentially harmful bacteria.
If you need to provide a windbreak, either by building a fence or planting a protective species of tall plant, increasing the edge of that border helps offer more protection to the plants on the lee side. A straight fence or line of trees cannot handle as much wind energy as a zigzag can. This is why, in nature, the edge of forests and forest clearings are not linear; they are wavy to create more stable conditions. Using curved and wavy edges is also a good way to create suntraps for plants that need a lot of light and warmth.
Spirals are another pattern we see in nature that increases the productive edge. Look at the seeds on a sunflower – their arrangement in a spiral allows the plant to grow more seeds and so increase its chances of propagating. The most common use of a spiral design in permaculture is the herb spiral, a rising spiral that has lots of different microclimates and maximum edge to allow for the cultivation of many different herbs, even in a relatively small ground space.