I'm typically a pod person, but I had a question that I have had a tough time researching. I have an on-contour swale project that I will be starting soon, and I am trying to figure out what to fill the trenches with. Toby Hemenway talks about filling up the trenches halfway with mulch, and he mentions straw with sand on top. I've also thought about filling the trenches with river rock. Some possible options:
Straw: I think this would work, but I don't like the idea of packing 2000 linear feet of trench with straw by hand.
River Rock: I also think this would work OK, but it would be very expensive 112 cubic yards.
Compost: I'm not sure this is a good option. Is it a bad idea for growies to grow in the trench. Or maybe this would be nice for water loving plants?
Wood Mulch: This might be OK to fertilize the berms, but I wonder about sow bugs and slugs going to town on my berm plantings.
I dug several contour ditches last year and they filled with vegetation, which competed with the roots of my tree plantings and camouflaged the 2ft trenches making it hazardous for the unwary. I now fill my contour ditches with tree trunks for a number of reasons - will prevent weed growth, provide nutrition and fungi, absorb rainwater and prevent falls. I will know in a year's time whether sow bugs, slugs etc. are a problem. In the meantime I will continue to pile tree trunks and limbs into the ditches and have their elements return to the soil rather than dry out dissipate into the air.
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You don't put anything in them, particularly if they are more ditches than swales (less than three feet, top to trough). If you do put stuff in them, especially big things like logs, you defeat your own purpose of digging them in the first place, the better to retain water. Putting straw in the bottom does work for larger swales, which act as intermittent ponds, and hold water for long enough periods for the straw to decompose and act as fertilizer; it also keeps weeds down. But you don't pack anything in by hand -- save that sort of work for zone 1.
About logs: their virtue is that they don't decompose fast, so use that virtue rather than fight it. Put them on the bottom of the mound, hugelkultur style, burying them deeply enough so that they don't decompose fast, along with any big rocks. Otherwise, if you want them to rot, keep all stumps and logs covered with mulch, and wet, as Scott Nearing mentioned in 'Living the Good Life.' They will rot in their own good time, you need do no more, although throwing part of a rotting log on top of them will introduce the right fungus to them. A low spot would be good for that, but not a swale!
For even lower swales, plant vetiver grass on contour instead.
By the way, according to a Lawton video on ponds and swales, you should have a flat section at one end of the swale to allow overspill into a gravel spillway. It is a good idea to combine them with a pond that they can feed into.
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