Hoophouse on the Slug Moat

In fall of 2011 I built our slug moat to protect nursery pots from slugs while providing rainwater catchment and other typical aquatic benefits. A few months later, to prepare for spring and make the most of the contraption, I added a hoophouse over the nursery table to provide a warmer, protected microclimate for the plants, without interfering with the use of the pond as an irrigation source.

Duck Integration

I wanted ducks to use the pond, but knew that with unrestricted access they would trash such a tiny area, destroying all plants and fish and filling the pond with poop and stirred up muck. To exclude ducks from enough of the pond to allow sheltering of fish and growth of poop-filtering cattails, I put a fence across the middle of the pond and around the excluded perimeter. Ducks could still be allowed into the protected area periodically, under close supervision.

I constructed a little ramp to make it easier for the ducks to get in and of the water.

Skirting

Using thin plastic which had wrapped our lumber deliveries, I constructed a skirting sandwiched onto the table sides and posts with lathe and hanging to the water below. I hoped this would create a lower seal preventing air from coming up into the nursery table.

Hoophouse

Next, I built a ventable upper enclosure for the plants from clear vapor barrier plastic left over from the house project. I created the end "wall" closest to the house by rolling a piece of lathe into a short piece of plastic, clamping it closed with three strong binder clips. I secured the lower half of this plastic to the vertical posts with more lathe, and tacked a couple of nails into the posts to allow the wall to be "hung" up for closure, or unhooked from the nails to half-open that end of the hoop house.

Since I didn't have posts to work with at the far end of the table, I used a single large piece of plastic for the rest of the hoop house, supported by plastic hoops screwed to the sides of the table, and draping down as the far wall. I used more binder clips to seal the junction between the large piece of plastic and the end wall, unclipping as necessary to move the sheets for ventilation.

I secured the sides by creating a small gap between the existing pallets on top and new wood I attached to the table sides just below. Tucking the plastic into the gap and wedging in some lathe created a strong seal, while allowing easy access or ventilation of the sides by pulling out the lathe.

Successes

On the whole, it worked. It definitely created a warmer microclimate and allowed faster early spring growth of the plants. Using all scrap materials, it had a distinctly amateur look to it, but was fairly easy to use for ventilating and accessing plants. I'm especially pleased with the side attachments of wedged lathe, which worked surprisingly well.

Problems

Despite lathe sandwiching it to the posts as low as possible, the lowest part of the skirting, pushed outwards by the concrete pier blocks, tended to blow around, failing to make a reliable seal. Often the skirting got stuck on the perimeter fence, potentially negating the slug-proofing function of the slug moat. The plastic scrap I used was just barely long enough to reach the water surface; with more material to work with I could have wrapped the bottom with some weights to ensure a straight drop to the pond.

The cross fence within the pond was of very flimsy construction and even flimsier attachment. A fence could have been secured to two of the table legs and anchored against their concrete piers stacked in the water below, but the placement of the table within the pond didn't make sense for that. It would have created either too tiny a pond for the ducks to use, or too small an area for plants to grow without duck disturbance.

The perimeter fencing, secured very loosely with metal stakes salvaged from political yard signs, also failed to inspire confidence. Neither the perimeter nor the internal fence were tested long-term against inquisitive ducks, and I suspect both would have required some adjustment and fiddling.

The binder clips came loose easily from slippery plastic, both where I wrapped lathe into the plastic to create the end wall and especially where I tried to clip the two upper sheets together. Again, I was using pieces just barely long enough to meet, so any significant wind put a lot of force on the joints. Slightly longer pieces with more overlap and slack would have helped. Ultimately the system needed a better connector - perhaps snap-on grommets.

Slug Moat: Pond, Rainwater Catchment, & Protected Nursery

Watch a video of our contraption thanks to Paul Wheaton of permies.com

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!

Construction

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.

Functions

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.

Aquaculture

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.)

Habitat

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

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.