Monday, September 26, 2011

Slug Moat: Pond, Rainwater Catchment, & Protected Nursery

Watch a video of our contraption thanks to Paul Wheaton of

Background & Objectives

We had lots of scrap pond liner left over from the ecoroof projects, so I decided to glue them all together to make a big pond, which can hold more water with less surface evaporation than a bunch of small ponds like our current array of 4 50 gallon bath tub ponds.

We've had lots of trouble with slugs attacking young seedlings in our nursery pots, especially legumes and brassicas, so I wanted a way to protect them.

Our front porch ecoroof receives rainwater from an area twice the area of the actual ecoroof, discharges a lot of runoff.  I wanted a way to store some of that water.

I wanted a way to conveniently water our nursery pots with rainwater.

We think the property should integrate ducks into the front yard to help with slug control and to provide a diversified egg and meat source, so we wanted some sort of pond area for future ducks.

Put all that together, and you get our slug moat!


First I had Tulsey dig a giant pit for the pond.  The hole wound up about 30" deep, 5' wide, and 18' long.  Closest to the house, we made a slope of about 45 degrees entering into the pond, to enable ducks and other animals easy access and escape.  We tried to make the bottom more or less uniformly deep, and to keep the banks level. 

I placed cardboard in the bottom of the pit to help protect the pond liner.

Next Jonathan and I placed the giant sheet of pond liner into the hole.  I had created the giant sheet from all the small scrap sheets by overlapping them and gluing them together with P&L Roof & Flashing sealant from Home Depot.  I used a double line of caulk, about 2-3" apart, for extra protection.  The manufacturer doesn't guarantee it for underwater applications, but many aquarium & fish enthusiasts have used it successfully as a fish-safe, underwater-proof sealant.

I was concerned about the possibility of a shovel, cattail rhizomes, or duck bills eventually prying the glued pond liner joints apart.  We had some 6 mil visqueen plastic lying around, so I put a single solid sheet of that on top of the pond liner.  The visqueen will probably wear through and leak sooner or later, but will still function to reduce the danger of something popping open the main pond liner seams.

Next I laid out the footings for the slug moat: concrete blocks leveled on a thin layer of sand & gravel, stacked high enough to keep the posts out of the water.

I covered the bottom of the pond with a 2-3" layer of river rock, accumulated during the excavation of the pit.  The rocks provide more protection for the visqueen and the pond liner.

I tucked the pond liner and the visqueen underneath a layer of soil to anchor it all around the edges, using a long 2x4 and a 4' level to get the banks roughly level.

I built a wooden frame by anchoring four 4x4s into the concrete post footings at the top of each pile of blocks.  The two posts closest to the house are about 10' tall, to provide support for the drainpipe coming from the ecoroof.  With some additional cross-pieces, they could act as a trellis for a vine.

The back two posts are about 3' tall.  I connected all the posts with 2x4s.  I considered installing diagonal cross-braces for more rigidity, but that would make it harder to access the area underneath.

I attached some pallets to the frame at a height convenient for watering and other nursery access needs.  The pallets also add stability to the structure.  When our friend Jasmine expressed doubts, I climbed onto the pallets and shook everything around as a test, to my satisfaction at least, if not to hers!

At last, I moved our nursery plants onto the pallets.  Since the posts are sitting on concrete blocks in the middle of the pond, slugs should have a difficult time gaining access as long as vegetation is kept clear along the banks.  We ran our graywater into the pond for a few weeks, and got a few hundred gallons of roof runoff from our last decent rain before the summer dry season.  This kept the pond reasonably full through the summer until rains resumed at the end of September, despite use of the water for nursery and some yard irrigation.


Rainwater catchment

The pond should hold somewhere around 1000-1200 gallons of water, so can act as a significant source of irrigation water in spring and early summer, again in fall, and possibly in the middle of the summer in years with good rainstorms.  The water from the roof falls about 10' into the pond, creating a mini waterfall fun to watch during rains, and aerating the water in the pond.

I added a small cross-piece board from which to hang our watering cans.  Now I water the nursery pots by dipping the cans into the pond to fill them.  The excess water from the pots falls right back into the pond, re-aerating the water and keeping all nutrients contained in the system. Keeping the cans right next to the pond also makes it fairly convenient to dip in and go spot water plants throughout the yard.


From our other small ponds, I transplanted some cattails (eventual starch, vegetable, and pollen source), duckweed (accumulates excess nutrients in a form easily removable from the pond as much appreciated duck food) and wapato (nice potato substitute.)  They're all thriving.

We bought about a dozen tiny goldfish to eat mosquito larvae and eventually provide duck and/or human food.  They doubled in size within a few months, and I recently counted 9 all at once, which could very well mean they've all survived. (Oops: on October 1st I found one floating on his or her side, dead for long enough to smell funny. I couldn't tell what happened.)


We've had tons of new life attracted to the pond: mud dauber wasps gathering mud for their homes from the edges; a red dragonfly we've never seen before laying eggs in the duckweed; damselflies; drone flies; and lots of little aquatic critters we haven't identified.

Future Potential


Ducks thrash ponds, unless you have a really big pond and not many ducks.  Someone could build a fence to section off 1/3 or 1/2 of this pond for ducks, leaving the remainder to grow plants, provide refuge for the goldfish, and filter and clean all the duck manure and the muck they stir up.  The pond edges will also need a small fence to keep the ducks from entering the off limits portion directly from the banks.  The ducks won't have a huge area, but it'll be enough to swim around, dive down, forage some food, and get laid.

Temperature moderation & humidity

I haven't tried this yet, but I envision some sort of plastic enclosure coming down to the edges of the pond to create a greenhouse with thermal mass and high humidity.  This might help nursery plants get off to a quicker start in the spring, and especially help cuttings as they root.  Building a second "deck" of nursery pot shelves underneath the existing set would also help with rooting semi-hardwood and softwood cuttings in the summer time, by providing shade but still enough brightness from the sides, along with the humidity boost.

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