Here were my two primary information sources when I was trying to puzzle out my design:
Step 1: A Bit of Background
In reading about Square Foot Gardening and the wicking beds, both sources indicated that having uncompacted soil was desirable. In my mind, not adding the majority of the water on top of the soil seems like a very good start in keeping the soil uncompacted.
Another tidbit that came up was that of nutrient leeching. One of the claims regarding wicking beds is that because the water is largely contained within the raised bed. Because the overall direction of flow is upward, the watering action does not flush nutrients away from the garden and out of the soil (as run-off to nearby streams or else just out of the cultivated area). I'm not sure that this would be a significant effect within the time frame that I am expecting to use this system, but it does seem like a realistic concern, especially in the case where the installer has to spend significant time with soil ammendment to achieve good productive soil.
Step 2: Bill of Materials
Here's a list of my purchased parts:
(2) 1"x12'x10ft pine boards $19 each at Lowes
(1) roll 10ft x 25ft 6mil plastic $24
(1) roll 3ft x 25ft shade cloth $5
(1) box 1.5" deck screws $7
(1) bag of peat moss $10
(10) bags pea gravel $3.50 each (0.5 cubic feet) each
(1) bag pine mulch $4
(2) 10ft x 1.5" PVC pipe $4 each
(2) 1.5" PVC elbows $~1 each
(1) 1.5" PVC tee $~1
(1) PVC glue (4 oz) $4 (buy as little as you can - it goes bad after a few months)
Things I had
(1) can PVC primer
(1) staple gun
(1) cordless drill
(1) set of drill bits
(1) hand saw
(1) 25ft tape measure
(1) utility knife
(1) 24" Level
(1) 3/4" Auger bit drill
*Fun Fact: 10 bags of gravel (5.0cubic feet) weighs quite a bit (around 500 lbs). My Prius was pretty much freighted! The cargo capacity is somewhere around 800 lbs total, so between me, the rocks and whatever other stuff I had in there, I think I hit that limit.
Step 3: Build the Frame
I cut each board into a 6ft section and a 'a bit more than 4ft" section. The goal is to make a 4ft x 6ft box.
Into each end of each 6ft section, I drilled 3 holes. One in the center, and one each about 1.5" from the ends.
About 3/8" from each end of the 4ft boards, I drilled 3 holes to match the above pattern.
If I hadn't pre-drilled these holes, boards this thin would definitely have split when I drove the screws in.
I set the torque limiter on my DeWalt cordless to about the '5' setting to prevent over tightening the screws and splitting the wood.
Assembly was pretty straightforward - screws got started on the 4ft sections, and screwed into the 6ft section.
Next, I laid out the plastic over the frame. The 10ft width of the plastic roll was just right for the entire length of my frame, so I ended up cutting only about an 8ft piece of the plastic. Plenty left! I centered my sheet of plastic over the frame and carefully fitted it ( sort of like making a bed with a flat sheet). I put one staple in each board, in the center, tucked the plastic to make sure that everything fit and then put staples into the corner area. Once I Was sure that it still fit without wrinkling, I stapled the crap out of it. Staples are cheap. I did not trim the plastic hanging over the edges - I'm not sure if I want to keep it to keep the frame relatively dry. I can always cut it later if it is ugly.
I didn't take any dedicated pictures of just the frame, but you can see all it quite nicely in this shot:
Step 4: Build the PVC Header
The purpose of adding water through the standpipe instead of watering the plot in the traditional manner is twofold. First, it helps to prevent compaction of the soil (though that will still occur whenever it rains). Second, it prevents the formation of an oxygen starved waterlogged zone at the bottom because fresh water is added directly to the bottom. Plant roots do not survive well in oxygen starved water.
It's just a minor note, but I probably should have had my two long pipes 24" apart to get even distribution width-wise. I messed up my math and had them 32" apart. Ultimately it really doesn't matter - the whole point is that you add water to the bottom of the bed so that it does not compact the soil during watering, and so that nutrients don't leach downward over time.
To construct, I measured the inside area of my frame - I want my PVC header to fit as snug as it can in here so that I don't have to install caps on the ends of the pipes to keep out rocks! It is pretty straightforward to build.
First I cut my two 10ft sections of pipe to the length of the inside of the frame, minus the installed length of the elbow fitting ( the pipe fits in by about 3/4" - there are multiple types of fittings out there. Dry fit, and the measure to be sure).
Next I glued the elbows to the long pipes ( one elbow on each!).
(Here's the procedure I used to glue the PVC pipe - if you know how to make a decent joint, go ahead and skip this bit)
Bevel the edge of the pipe before you start, this is just good practice - it helps to ensure that the glue is not scraped off the fitting by the pipe during insertion. I used PVC primer on the fitting and the pipe. This is not really necessary since the whole contraption is designed to leak! Mostly it was for practice for when it matters. When you insert the pipe into the fitting, it may try to pop back out a bit. Apply constant pressure and twist the pipe a bit to form a good bond. Hold it for a few seconds, and the glue will set enough to hold the fitting on its own.
Next, I glued the tee to one of the scrapped ~4ft pieces of pipe. When that had set, I cut a section out of the other scrap piece of pipe to connect an elbow to the tee. The center to center distance from the long pipe to the standpipe should be about 12". Again - measure, dry fit, measure again to verify , then take it apart and glue.
When gluing the standpipe to the first long pipe, make sure that when the assembly is sitting on a level floor, the standpipe sticks straight up. Use a level. Repeat for joining the 2nd long pipe to the assembly.
Step 5: Add Lots of Holes
There are pipes out there that you can buy with these holes already in them, but I was too lazy to look. And I imagine that they are more expensive.
You want these holes to be in the bottom of the pipes so that rocks are less likely to pop through them or get wedged in them. It'll still happen, but just not as often.
I used a 1/4" drill bit and drilled 3 rows of holes in the bottom of each long pipe, ignoring the short sections connecting to the standpipe.
Step 6: Dig a Hole and Drop in the Contraption
I dug down about 8" at the fill end and about 4" on the other. My goal was to have my drain hole be just above the ground level in the back. I graded the bottom of the hole with a flat backed rake and a spade (doesn't have to be perfect).
To test for level, I put the frame assembly in the hole and used a level on each side. Mine slopes down slightly to the back corner, just like the lot, so the drain hole I will punch in this frame (to relieve excess rain water) will drain in the right place. I should have spent a bit more time ensuring that it was level, but I think it is close enough. Make sure that there are no sharp rocks on the bottom that will puncture the plastic lining, and that the lining lays flat most everywhere. I did not walk on the liner during this step, to make sure I didn't tear it.
I put the PVC assembly in with the standpipe at the high end. Notice how nice and snug the ends of the pipes are at the end of the frame! Try to center the PVC assembly.
Step 7: Fill With Gravel
I calculated how much gravel I would need for the 4 x 8ft bed at (2/12) ft x 4ft x 8ft = 5.33 cubic feet, so 11 bags. I bought 10, thinking that it'd be fine. I built the smaller bed, and it turns out that it was just enough rock. My advice: calculate how much gravel you need, and buy 1 extra bag. It can't hurt.
This part is easy (though the bags are heavy). Open bags. Dump rocks. Use rake to level.
I did walk carefully on the gravel at this point to uniformly compact it into the frame. I am thinking that I should not have - I can't say for certain that I didn't puncture the plastic
Step 8: Add Garden Cloth`
The garden cloth was only 3ft wide, so I overlapped two sections. I made sure to have extra on all sides and used handfulls of dirt to hold the cloth into the edges of the frame.
Step 9: Add Soil and Peat Moss
I raked the top level to make sure I had enough. Turns out I had dirt left over - it was fairly heavily compacted in this garden.
Cover the standpipe with a 'rock or something' to keep dirt out. I chose a rock.
Step 10: Add Mulch
All in all, from when I started digging to when I mulched, It took about 2 hours of outside work, maybe another 2 hours of construction of the frame. And that's WITH a 20 minute break to put my kid to bed. Not bad for an evening's work!
Step 11: Add the Overflow Hole
Multiple holes should be drilled at about the same elevation around the planter - that way if one gets clogged by debris, water will not back up and cause issues.
I added two holes for now - I may add more in the future, but for now I like this number.
Step 12: Just Add Water
I topped off the standpipe each day for the next few days and it did seem to need quite a bit of water each time. I'm not sure that I didn't accidentally puncture my plastic liner! Perhaps I should not have walked on the gravel to compact it.
I was hoping that after filling I would be able to see water in the bottom of the standpipe. Unfortunately due to the slope of the bed, when I fill the system to the overflow (which is a bit above the geotextile), there is only a little standing water visible. I have no good way of determining whether the system is leaking, or if this is indicative of natural evaporation and startup water absorbtion by the soil. This does highlight the importance of starting with a good, level spot for your system. Spend extra time ensuring that this detail is correct.
Step 13: Just Add Plants
Of course, I have not yet installed my grid, so I had to wing it: :) I did use a tape measure though, to make sure that I am somewhat adhering to the guidelines of the technique.
For the few plants that I grew from seed this year (and managed not to kill by not watering a few weeks ago), I pushed some of the mulch aside, pulled the plants out of the cardboard planters and placed them into well watered holes. I then back covered with mulch being careful not to cover the tiny plants with mulch.
For the seeds, I cleared each square foot that I was planting of mulch. I then planted the seeds according to the packet instructions, in a grid pattern in line with the square foot gardening guidelines. Once the plants are large enough, I'll backfill with mulch around them - otherwise I think that the mulch will prevent germination of my seedlings.
What I planted:
(3) tomato seedlings - I eat tomatoes with my tomatoes
(1) row of sugar snap peas - they're like candy
(1) square of arugula - my favorite leafy thing
(1) square of cilantro - great for guacamole
(1) square of parsley - for pesto?
(1) square of basil - just good times.
(1) square of bell peppers - I eat them with almost every meal
I still have a couple of free squares that I haven't decided on yet - I figure I'll go to a garden center at some point and buy some seedlings of something that I have forgotten. It's nice to have options.
More pictures to follow as everything grows! I'll also be adding some support structure for the peas and tomatoes that I planted along the north side of the garden (north side is used for tall plants so that they do not shade the rest of the garden).
Step 14: Adding the Trelace
The next step was to find trelace netting - none at my home depot, so I went to a local Agway. Success!. I got a 5ft x 10ft section. By trimming the top horizontal thread from the top edge, I was left with a nice 4" piece of string with which to tie to the conduit. I did the same with the two sides, cutting the full piece to size. This method is taken straight out of 'Square Foot Gardening' and it worked quite nicely.
Step 15: Next Time...
1.) Proponents of wicking beds seem to lean in the direction of using organic media (not hippy-organic, I'm talking carbon-based!) instead of gravel. The idea is that having a porous material in the lower section of the bed helps to wick water upward more efficiently. To me this makes sense. The downside is that the media will decay, requiring eventual replacement. Also, through the process of decay, there will be a negative impact on nitrogen levels which must be added through fertilizer or nitrogen rich compost (such as blood and bone meal).
1a.) another possibility is to use lava rock, which is porous - it may wick better than gravel does. I would lay down a bed of sand first though, to prevent puncturing of the liner, I imagine that lava rock is not as smooth as gravel is.
2.) In my next version, I will add a hard piped overflow outlet. A 1/2" PVC pipe would be connected to the end of the perforated PVC pipes. It would protrude through the frame of the planter. Any water flowing out of this overflow would be filtered by the geotextile, reducing the possiblility of clogging. Alignment of the pipe header system would be a bit trickier, but I would prefer this to the somewhat hokey method that I used. One other consideration would be that a screen should be installed to prevent insects from crawling in through this overflow - they could clog the pipe.
3.) Do a better job leveling the bottom of my excavation. Also, I should have added 1/2" to 1" of sand to ensure a nice soft, non-pokey bed for the plastic liner to sit on. This would probably be a bit expensive, since that'd be another 7-10 bags of heavy stuff to go buy at the store, but in the end I think it'd be worth it. According to Lowes, a bag of sand is 0.5 cubic feet is about 50 lbs. Depending on what you buy, the cost is $3 to $4 per bag.
4.) Only have the gravel below grade - my overflow is slightly below ground level, I don't like that I had to dig a little hole to see it.