Sunday, April 26, 2009

Permaculture swales : Part 2
In Part 1, I explained why we needed a swale and how we surveyed it. A swale is a horizontal water-harvesting ditch that follows the contour of the land. To dig it, you excavate topsoil and deposit it on the down-slope side of the swale. During heavy rain, water tends to run over the land, rather than soaking in, but the swale arrests its progress and helps it to soak into the land, which then holds that water like a sponge: we’re effectively storing rainwater underground.

In all the information I read through in books and on the Internet, I couldn’t find any calculation or other guidance to size our single swale. What’s really important is that it never overflows: as the bank is topsoil, rather than the compacted subsoil of a dam, any breech would allow water to quickly carve a big hole and wash the topsoil away. So, it needs to be big enough to contain the heaviest rain of the year and, in time, the roots of the fruit bushes and other plants will consolidate the mound. We’ve dug ours 60 cm wide 20 cm deep (2 ft x 8 ins) on a best-guess-suck-it-and-see basis. Thus far, we’ve had 20 mm of rain (4/5 inch) in one day, with no problems.

Having surveyed the swale the next job was to actually dig it. Turf removal is one of the more frustrating tasks, especially as it’s the first thing you do. Cut it with a spade, before you slide the blade underneath and try to lift it, to give you straight edges. Invert the turf and then pile the rest of the soil on top. Our soil is has a high proportion of clay and loads of stones, which makes digging with a spade very difficult. Having seen big earth moving machinery on Geoff Lawton’s excellent DVD Harvesting Water – The Permaculture Way I thought I could justify borrowing our neighbour Michel’s rotovator (see photo) with plough attachment to literally bulldoze through the worst of the job, before shaping and levelling by hand. Don’t rush out to the hire shop it didn’t help at all. Wrestling this roaring, churning beast is backbreaking and I couldn’t get it to penetrate deep enough, then have enough traction to move forward. It got clogged up with turf, meaning that it was no help for that difficult first stage. I gave up with it and continued, much more effectively, and permaculturally, with a five-tined broad-fork, here called a une grelinette along with a convention -al spade and fork. It was hard work and the art, as far as my character is concerned is not to look at the whole job too often, don’t be impatient (ho, ho, says Gabrielle!) but just concentrate on each forkful and get into a rhythm, almost in meditative fashion. It is then amazing how much earth one person can move in a day.

The final levels are adjusted (read Part 1) and then the swale is planted up. We’ve put three fruit trees, a medlar, a crab apple and a weeping mulberry on the swale along with several soft fruit bushes. I’ve sprinkled on nitrogen-fixing white and violet clover seeds as a cover crop and Gabrielle has planted some dill in the ditch of the swale, to which we’ll add others in the umbellifer family. With their deep roots, we hope they’ll help water infiltration and have also broken up the bottom of the ditch with a fork for the same reason. It’s possible that a swale or two would benefit our largest pasture field but we will keep and eye on how our current swale performs, along with its plants and make any changes to it, or the design of further swales, as appropriate.