There are some techniques that are strongly associated with Permaculture. Swales, as the Swedish possibly called for severing ditches, is one of these. One of Europe's most renowned permakulturlärare and writers, Patrick Whitefield, recently published a post on his blog, where he argued that the swales are overestimated in a climate like the UK. His point was obviously to keep permakulturdesign should be based on needs and not from the technology itself. Posted figured on some Facebook pages in a week or so and since then I have thought to write about swales from a Nordic perspective. In the first part of the short series of swales thought I explain in general terms how a swale work and how it can create value in a forest garden.
Swales are ditches that do not normally have any inclination. Unlike ordinary ditches, used to divert water from one area to another, Swales used to retain and distribute water evenly in the landscape. The idea is that they will capture runoff during major rain event. Then the water slowly infiltrate the soil, wherein it is eventually formed a lens of water stored in the earth. The storage of water in the soil is easier if the plants planted on the embankment in front of Swalen, preferably bushes and trees with an undergrowth of deep-rooted plants that both stabilizes the ridge and increases soil fertility. Although the trench should be covered with organic material or a cover crop such as clover, to facilitate the infiltration of water. Swales has been used successfully in very dry areas of the tropics and on steeply sloping, rainy areas in the temperate parts of the world to slow down and smooth out the large water, replenish groundwater reservoirs and improve farming conditions locally. In addition, prisoners trenches up any particles that water brings and can thus reduce erosion problems.
In our northern climate has swales not quite the same function as in dry areas or in the tropics. First of all surface runoff occurs on vegetation-covered surfaces very often in our latitudes. However, it can be run off during the thaw, especially when there is still frost in the ground. This runoff can cause erosion problems here on earth is bare, and the trench can then catch any particles that water brings. Whether the slowdown in the water do any good for plant growth, which of course has not started when snowmelt is not clear, because I have not been able to find any evidence in the literature for the water lens which is said to be formed during Swalen.
Yet swales a role to play even in our circumstances. Plants that are planted on the embankment in front of a swale get an extended growing season, dike warms up earlier in the year and the plants are lifted away from the ground, which also reduces the risk of frost damage in spring. Furthermore, the tree's root system to be drained more, because they get farther from the water table, while they still can access the water in the deeper soil layers. This is especially valuable in areas with high groundwater, as fruit trees and bushes would rather have two feet or more into the ground water. The swale-dike will never be able to be water saturated. Swales can also be used to allocate water between locations with a surplus of water and places with a deficit. Finally create themselves Swalen an unevenness in the terrain that forms more microclimates than it had been on the site or else. This may be of benefit to biodiversity and increase pest balance of the forest garden. One drawback that we have observed with our swales in Putt Myra Forest Garden is to the plants on the embankment requires more irrigation in the early years and to the dike seems to be a good habitat for voles, which in turn require specific actions in the form of sorknät at planting on the embankment.
In the next post I will go into more detail on how to dimension a swale and how the plant itself goes to.
P.S. David Roxendal who took the nice air photo on Putt Myra Forest Garden has begun offering the service to take similar images (both motion and still images) to interested around the country. Han tar 1000 SEK plus travel expenses for a documentation visit – for me it has been an incredibly helpful planning tool to have this aerial photo. David reached in david@ihop.nu.