Introduction: Passive Gravity Irrigated Planter
Home gardening is a great method to reduce both your monthly food budget and environmental impact. In this guide, you will be shown how to build a planter, capable of growing a variety of different types of produce and/or herbs, along with a drip irrigation system to deter disease and pests and passively keep the soil moist. Furthermore, the drip system is designed with a leak rate (0.05 gal/hr) a full order of magnitude below the leak rates obtained from commercial drip irrigation products (0.5 gal/hr), along with reduced cost. Simply pour a few glasses of water into the bucket and allow gravity to slowly feed the water to your plants for hours. This step-by-step guide was written for even the most inexperienced builder or gardener and teaches the basics of gravity irrigation design, as well as how the system could be improved in the future.
Step 1: Materials List
Below is a list of the consumable materials needed for the project. Highlighted prices are estimates, and all prices were gathered in May 2012. While the listed total price is approximately $140, the project could be completed for less than $100 with a few modifications. A few examples are: 1) Substituting cedar for redwood boards (save $20), 2) Eliminating the angle brackets (save $5), 3) Eliminating the copper tape (save $10), 4) Supplying your own soil instead of purchasing (save $5).
Step 2: Tool List
Along with the list of consumable materials, a number of tools are required. Estimated costs are included for small tools that are not particularly common.
Please note that this project was completed with a few expensive tools that are common for home projects. The first is a #80 drill bit; the smallest commercial bit available, which has a diameter of 0.3 mm. These bits are very easy to break and a CNC machine was used for higher stability and control when using them. Most people do not have access to a CNC machine, but you may be able to use a drill press, as long as it is stable enough and you are careful. Before you start this project, please make sure you can complete this step. Also, independent of your method, please purchase several of these bits; changes are high that you will break a few.
The second tool that is not common for most home projects is a heat gun capable of softening Polyethylene tubing. This was the tool used for this project; however, any controllable heat source would also work (preferably non-fuming), such as a propane camping stove (though this was not attempted). A simple open flame from a lighter is not recommended.
Step 3: Container Construction - Prepare the Boards
The first step of the project is to construct the planner which will hold the plants and soil. Wood is a strong, low-cost, sustainable, easily available and workable material. Specifically, redwood or cedar is recommended because both of these varieties are resistant to rot without chemical treating and should last for approximately a decade before replacement is necessary. Cedar has additional benefits of being a natural pest deterrent and a lower cost than redwood. Chemically treated, potentially cheaper wood is not recommended, especially if you have the desire to grow organically.
While cedar was the originally planned building material, it was not available at the time of construction. The most ideal redwood boards found locally had dimensions of 8’ x 11.25” x 1.5”. Two boards were purchased and both were cut in half into 4’ segments. Three segments were used for the base and the two long sides of the planter, with the remaining two short ends of the planter being cut from the remaining 4’ segment. These segments were cut with a length of 14.25”, leaving 33.75” remaining.
Step 4: Container Construction - Screw Into Place
The boards were held in place while four angle brackets were installed at the center of each junction, where the sides attach to the base board. The sides were then joined together in a similar manner with four additional angle brackets while the boards were held square. Glue was not used to allow water to leak out at the seams, if need be.
Once the initial shape was formed and held steady with the brackets, pilot holes were drilled along the parameter of the container at the joints. Along both long sides of the planter, pilot holes were drilled using a 1/8” drill bit every 6”, starting 3” in from the side. If you are using a size of screw other than a #10, you may need to use a different sized drill bit for the pilot holes. As a rule of them, the drill bit should never be larger than the inner diameter of the screw. Nine additional pilot holes were drilled in each of the two shorter sides of the container as well; three along each joint. They should be somewhat equally spaced. The only major concerns are that you do not hit any of the existing screws you already placed or drill too close to the edge of each board. Once all the pilot holes have been drilled, fill in each with a 3” wood screw. When dealing with screws this long, pilot holes are a necessity.
Step 5: Container Construction - Shelf (1)
Once the container is constructed, an additional shelf is needed for placing the bucket that will hold the water reservoir. It is important that the shelf is not as wide as the planter, as you will need to later connect the spout from the bottom. This component does not require a lot of wood, so the left over 33.75” segment of redwood is more than enough. Also, do not worry about messing up, as this is the final woodworking step and any remaining wood is not needed.
Step 6: Container Construction - Shelf (2)
Cut 7” of length from the remaining wood. Take this piece, and cut another 7” from it to from a 7” square and save it. Cut the remaining 4.25” x 7” segment down the middle, to from two segments, roughly 2” x 7”. Make two, 45° cuts on the ends of each segment to from two braces, as shown in the figure above.
Once you have the three remaining pieces, start by attaching the horizontal shelf by itself. It is important that you do not make the shelf level with the top of the box, unless you plan on filling the box completely with soil (with no lip of wood remaining around your plants). The height of this intended lip is how low the top of the shelf should be. Ideally, you will want the bottom of the bucket to be level with the soil. During construction, I chose to place it 1” lower from the top of the box to make sure the plants will have enough soil. (However, I would recommend installing it 2-3" below the top of the planter, now that my planter is complete)
Carefully measure this distance below the height of the container and mark it. Align the top of the shelf with this mark and offset it to the back of the planter using the dimensions in the figure above. Hold the shelf still and drill one of three pilot holes, equally spaced along the shelf and immediately fill it with a 3” #10 wood screw. This should hold the shelf, for the most part, but you will still need to hold it when drilling and inserting the remaining two screws.
Next, turn the container vertically so the shelf is raised. Set one of the wood braces even with the side of the shelf and drill a pilot hole at a 45° angle into the end so that the 2” screw will come into contact with the supporting structure. Careful placement of these screws is necessary: too far towards the center of the brace and the screw will not come in contact with the support, two close to the edge and the wood may split. Once the first screw is in place, repeat the procedure for the second screw, then the entire procedure for the second brace.
Step 7: Container Construction - Drainage Holes
The final step for the container construction is to drill drainage holes in the base board. The hole does not have to be very large, but not so large that rocks or dirt will flow out the bottom. A 1/4” drill bit was used for this purpose and 24 holes were drilled, equally spaced every 6”. These holes were drilled while the container was upside down so that the bit would drill into free space once it was through the wood; however, the only danger to this is that you skip the holes what would conflict with the angle brackets attached to the bottom. The figure above shows the configuration used. Notice that the holes on the center edge were skipped.
Step 8: Irrigation Design
If you do not wish to learn about the physics and simply make something that works, all you need to do is make sure that the height of the water level in bucket at zero flow is as close to the level of dirt in the planter as possible. Completely level is ideal, but placing the bucket too high will increase your flow rate, and placing the bucket too low will leave more stagnant water in the reservoir. It is up to you to choose on which side you would prefer to error. Also keep in mind that if you place the bucket too high, it can be an easy fix by simply placing more soil in the planter to raise the level.
If you are content with knowing this, please skip to the next section, otherwise read on.
There is really only one equation you need to know to design a gravity irrigation system: Bernoulli’s equation, as shown below.
This equation is basically a restatement of the principle of conservation of energy, only applied to an incompressible continuum. Water can be assumed as such for a majority of applications, including this one. The equation states that for any location, or node, within a closed system, the dynamic pressure plus the gravitational field effect plus the static pressure will be equal to an unchanging system constant. The dynamic pressure is equal to the density of the water multiplied by the velocity at the node, divided by 2. The gravitational field effect is the density of the water, multiplied by the acceleration due to gravity (9.8 m/s2), multiplied by the height of the node, taken from an arbitrary, but constant, reference point. The static pressure is simply the force divided by the area at the node. Finally, the system constant is a potential constant offset of the entire system, which can be zero.
As a simple example of how to use this equation, let the elevation of the soil be our vertical height reference and let the system be built so that the bottom of the water reservoir is level with the soil. When the bucket it filled with 5 inches of water, what will be the instantaneous velocity of the water at the emitters? By examining a node at the top of the water level in the bucket, the velocity the height is moving down is negligible, so there is zero dynamic pressure. For the static pressure, we know that Earth has an innate pressure level of about 1 atmosphere, and at the top of the water in the reservoir, this is the only pressure acting on that node. However, because we know that the final state of the water will be to return to atmosphere in the planter, we can choose to ignore this value, as it will simply be a static offset for the entire system. This leaves the gravitation field effect equal to the system constant at this node, and defines the system constant for the entire system and is equal to the density of the water, multiplied by the acceleration due to gravity, multiplied by 5 inches.
We can now examine the node of one of the emitters at the location the water is released into the planter. Here, we are interested in knowing the velocity of the water, therefore we will be solving for the dynamic pressure. For the gravitational field effect, we defined this height as the reference point, therefore the height and gravitational field effect is zero. The static pressure is once again the atmospheric pressure of Earth, which we already canceled out from earlier; therefore we must do the same here. And finally, we already calculated the system constant from the first node. This leaves the dynamic pressure at node 2 equal to the gravitational field effect at node 1. The velocity at node 2 can then be solved, as shown below.
To discover the total leak rate of the system, you then just need to multiply this velocity by the cross-sectional area of the emitter and number of emitters in the system. But keep in mind this is only the instantaneous leak rate at the current height of water in the reservoir; as the water height decreases as time goes on, the leak rate will also decrease. If you with to know exactly how long your planter will be watered and at what rate, you will need to use a write a simple script with the actual dimensions of your system. This can be accomplished by discretizing time (with each step being on the order of seconds) and continually calculating the leak rate and volume loss at each step. This was done in MATLAB for the system shown here so you can see an example of the expected results.
One factor that has been ignored above is resistance in the system. Because the supply lines are short and relatively wide, we can calculate the resistance to be relatively low. This is also a more complicated equation and somewhat more difficult to calculate. Because of the difficulty and that my experimental observations are that it is negligible, resistance will not be discussed further.
Step 9: Irrigation Construction - Water Reservoir (1)
The first step to creating the drip system will be to create the water reservoir. A standard polyethylene bucket will be used; however, because gravity will be supplying the pressure and the hose line will not be primed with water when filled, a hole will need to be drilled in the bottom of the bucket and connected to a hose with sufficient leak mitigation. This was accomplished with a stepping drill bit with a final diameter of 3/4", a brass barbed flange with pipe threads, a corresponding face busing, and two gaskets. Slowly drill a hole into a flat portion of the bucket, close to the edge. Roll one of the gaskets onto the threads of the brass barbed flange and screw it into the bucket from the bottom. When completing this step, the connection was already very tight and required pliers. Do not simply push the threads into the connection. You want the connection to remain tight to prevent leaks.
Step 10: Irrigation Construction - Water Reservoir (2)
Once attached, turn the bucket over and roll the second gasket over the threads inside the bucket. Finish by screwing the brass face bushing over the threads to complete the connection. Plug the brass hole with your thumb and fill the bucket with water to test for leaks in the seal. If the connection is leaking, you may want to adjust how tightly you screwed the brass barbed flange into the bucket.
Step 11: Irrigation Construction - Water Reservoir (3)
Now that the pipe interface has been included on the bottom of the bucket, a mechanical filter needs to be added to prevent particles in your water supply from clogging the emitters along the pipe. For this project, an 80 Gauge drill bit will be used, which has a diameter of 300 microns (0.3 mm). The filter you use must be a few sizes smaller than the hole you intend to keep free, so 50 micron filter paper was used. Cut approximately a 6 inch square of this paper and fold it over the brass face bushing on the inside of the bucket. While holding the paper, wrap a rubber band around the base a few times to hold it in place.
Step 12: PVC Emitters (1)
The first step to creating the emitter system is to assemble the PVC piping. Connect the two 2 foot segments of PVC with the coupler and connect the two 90 degree corners to the ends of the now 4 foot length of pipe. Due to the connections, it will be a few inches longer than 4 feet. Attempt to place the length of pipe in the wood container horizontally and measure the additional length you will need to remove. Pull one of the 90 degree corners off of the length of pipe and cut the measured length off of the PVC pipe. Reattach the 90 degree corner and confirm the length of pipe can now fit horizontally in the container.
Next, you will need to measure and mark the locations of the emitters. The system shown here was constructed with eight emitters, each placed 6 inches from each other. Measure 3 inches from one side, mark the location, then continue measuring and marking every 6 inches. There should be 3 inches left from the far side when you are complete.
With the locations marked, it is time to drill the holes. If you are using a micro drill bit (gauge #80 through #61), do not use a hand drill. Because the bits are so small, you can not use them to hold the weight of the drill and you will snap the bits easily. You can probably use a drill press, as long as it stabile enough. I attempted this on a cheap drill press, but it continually broke the bit. I finally used a CNC machine with a controlled feed rate and it worked perfectly. Once you have found a system that works, drill the eight emitters along the length of pipe at the marked location.
Step 13: PVC Emitters (2)
With the emitters drilled, you can now create a simple test to determine if you are achieving a uniform drip rate. Cut two segments of 0.5” diameter polyethylene tubing and attach each to one of the Nylon adapters you purchased. The Nylon adapters must also be screwed into the threads of the 90 degree PVC corners. Attach the opposite end of one of the tubing to the bucket you created earlier and suspend it slightly above the PVC pipe. Make sure the emitters are facing down and you can then suspend the PVC piping above a number of cups, each oriented below one of the emitters, as shown in the figure above. If you only care about the uniformity, put a little bit of water in the bucket, then leave it until it runs out (NOTE: Make sure you see water in the far tubing that is exposed to the air, otherwise you will need raise the bucket for a moment to purge the air from the system). After a certain amount of time, you can then remove the system and measure the volume in the cups.
The above experiment was carried out with this system, with water volume measured every 20 minutes. The results are shown above and support Bernoulli’s equation with negligible friction. In the second figure, the red lines represent each individual emitter and the blue lines represent the compete system. The final figure shows the distribution in the water volume measured from each emitter. While emitter 2 is unusually low (perhaps due to a partial clog), the other emitters appear to be somewhat uniform. If you are not satisfied with the uniformity test, you can attempt to unclog any potentially blocked emitters by flushing water through the PVC pipe and attempted to force air through the emitters with your mouth (it may look strange, but it works). If you test it again and the uniformity has remained unchanged, you can potentially drill additional emitters.
Step 14: PVC Emitters (3)
Once you are satisfied with the uniformity, the next step is to form the polyethylene piping. Polyethylene is a thermoplastic, which means you can soften it with heat, bend it, and cool it to hold the new shape. If you attempt to simply force it into place without heat, it will resist and buckle, potentially cutting off the flow of water or damaging the tubing, resulting in a leak. My first attempted was to use an open flame from a lighter; however, I could not keep a large enough section of the tubing hot long enough to bend it into shape. Also, the impurities from the flame blacked the plastic quickly. Instead, a heat gun was used and formed the segments of tubing shown above, with the pictures taken after assembly (NOTE: I suspect you could use a propane camping stove to apply the heat with perhaps some increased difficulty in forming). You will need one segment that turns 180 degrees, a segment that is turns 90 degrees, a straight segment 1.5 feet long, and a straight segment ~4 inches long. The above images should help you complete the assembly.
Both ends of the pipe need to be open to atmosphere in order to easily purge the air from the system after water is added. On both of these open ends, cut another square of the 50 micron filter paper and attach with a rubber band to prevent particles from clogging the emitters.
Step 15: Soil Layering and Plant Placement
Now that both of the container and irrigation system are constructed, place the planter at the desired location, balanced upon the two cement blocks. Once you begin filling it with soil, it is going to get heavy and may be difficult to move afterwards. Pour in a layer of drainage rock and spread it across the bottom so that you can no longer see the wood. For the rock used here, the thickness of this layer ended up being 0.75 inches. This layer of rocks has a few benefits in case the container is over-watered: 1) It will help prevent soil from running out the bottom of the container. 2) The roots of the plants will not grow into the rocks and be sitting in a pool of water as it drains out the bottom. This can help reduce mold from attacking the plants roots.
Once the layer of rocks has been placed, fill with soil and fertilize as recommended on the packages.
With the limited space available in the planter, do not be afraid to plant a few types of varieties of plants together. You will have to research to make sure the plants require similar soil conditions and are not detrimental to each other, but when done correctly, it is an efficient use of space. I am not an expert in this, but for this planter, we are planting strawberries and spinach. Both require slightly acidic soil (strawberries: 5.5 to 6.5 ph, spinach: 6.4 to 6.8 ph) and the small spinach plants can be placed around the larger strawberry plants easily.
There are plenty of websites on the internet to help you determine excellent pairings of produce, as well as the correct spacing of the plants.
Step 16: Conclusion
And the planter is complete! I also included a strip of copper tape around the base of the container to ward off snails, which has worked far better than expected. Copper is believed to react with the slime produced by both snails and slugs, inadvertently giving them a kind of electric shock when they touch it. The tape is a little expensive, so I would not include it unless you start observing a problem on your own. If you do decide to apply it, make sure you purchase tape that is at least 1.25 inches wide. Snails and slugs can stretch across distances shorter than this, so it will not be effective. The tape shown in the image below is 2 inches wide, just to make sure they are blocked. When applying the tape, I would make sure it is not directly along the bottom of the container (I placed it 1 inch above the bottom). If you do over-water, you do not want it to touch the copper tape, as it easily oxidizes when wet. I do not know if this affects the effectiveness of the material against these pests, but I would suspect it would if the oxidation is bad enough.
Applying to the Bernoulli equation to the dimensions of the planter, we can see an excellent agreement from measuring the height of the water in the bucket as shown in the figures below. An interesting note, though, is that while the model agrees exceedingly well, the model was completed with an emitter diameter of 200 microns (0.2 mm), instead of the 300 micron diameter of the #80 drill bit actually used to from the holes. This could be due to a number of factors: 1) The force due to drilling stretched the PVC pipe as it drilled, and then bounced back once the bit was removed, 2) The PVC pipe slightly swells with contact with water, or 3) The resistance in the emitters is not negligible. None of these explanations sound completely believable to me, but I am also not too concerned about it. Besides, smaller emitters without a noticeable reduction in uniformity is ideal. If I was building a larger system, I would definitely want to determine the cause of this observation, but for this small scale example, it is actually a positive effect, independent of the cause.
And there you have it! A passive, gravity-irrigated planter capable of keeping the soil moist for hours after watering.
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