How to build a wicking bed (AKA sub-irrigated plant box)
A ‘wicking bed’ is simply a planter box with a low-tech irrigation system. So, why choose it for your garden or urban farm?
The combination of sub-irrigation and raised-beds offer a number of advantages. For one, the passive irrigation system enables consistent and efficient watering without the need for electrical pumps or complicated piping. This makes sub-irrigated planters an easy and relatively affordable solution if you’re looking to decrease water usage and save time.
If that’s not compelling enough, ‘sub-irrigation’ also ensures that your soil/substrate never dries out, thereby creating an environment that is beneficial for both plants and their microbial symbionts. We know that healthy microbial activity is key for creating regenerative grow systems, ultimately contributing to increased plant health, productivity and resilience to pests and other environmental factors such as heat/water stress.
While it’s useful to understand the mechanics of this system so that you can make your own, you may want to jump straight ahead to PART 2 to see how one gets assembled!
PART 1 — Principles
As mentioned, a wicking bed is simply a planter bed with a simple irrigation mechanism. The ‘planter box’ could be any non-permeable container such as a raised bed or a large pot. The idea is to keep the good stuff in (water, nutrients, microbes, etc.) and the bad stuff out (pests, weeds, etc.)
What makes the wicking bed really useful though is its simple-yet-effective irrigation mechanism.
Sub-Irrigation: Water is drawn up from a reservoir through capillary action (hence the term ‘wicking’) — i.e. no pumps are used!
The reservoir sits at the bottom of the bed and is topped-up manually through an inlet. Alternatively, if filling the reservoir once per week is too much admin, you can automate this process entirely by installing a simple valve mechanism leading from a water source such as a rainwater tank. We’ll cover this in the guide below.
Advantages of sub-irrigation:
- Improved water management (‘self-watering’).
- Increased watering efficiency (plants draw exactly what they need, minimal surface evaporation).
- Optimum conditions for microorganisms (the substrate never dries out).
- Reduced salt build-up in the soil.
- Weed prevention (weeds have difficulty taking root due to the dry surface soil)
PART 2 — DIY Wicking Bed (Assembly Guide)
The following guide runs through the construction of a (2.7 x 0.7m) wicking bed with an optionally added automated-refill mechanism.
Step 1: Constructing the frame
Materials:
- Treated-timber [ 8 x (2700mm), 8 x (700mm)]
- 64 x Threaded-bolts + nuts
- 4 x Right-angled bracing (700mm)
Equipment: Hand drill, saw, ratchet-spanner, clamps, sandpaper, spirit-level
In case you were wondering, it’s safe to use treated-timber with this grow system since an impermeable barrier is made between the soil and the wood by the pond-liner/plastic sheeting (see Step 2)
- Prepare your planks by cutting them to your desired lengths (remember to take the overlap into account — see image on the left).
- Start with the base of the frame and use clamps to position the planks together and to the right-angled bracket (that vertical aluminum bracket in the left image).
- Line-up your drill and punch the holes for the bolts, inserting the bolts as you go (this will keep all the holes are all lined-up).
- Attach all the nuts and tighten everything fairly loosely (too much and we’ll warp the bracket).
At this point, you might want to move the base of the frame to the desired location due to weight considerations.
5. Make sure the base is on a level plain (this is important for adequate wicking).
6. Finish punching the holes and attaching the rest of the planks one level at a time (remember to keep adjusting your level & alignment) until you reach the top.
7. If your bolts protrude too much (see left image) they could puncture the plastic, creating unwanted leaks. File them down or attach a protective covering.
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Step 2: Constructing the reservoir
Materials used: builders plastic (cheaper option) / pond-liner (durable option), stones/gravel, ‘builders’ fabric, weeping tile, threaded pipe, (optional: Toilet cistern with flush-valve, PVC piping, irrigation pipes and the appropriate joins)
Equipment used: drill, clamps, scissors/blade, staple-gun
Design considerations:
- Ensure that the length and breadth of your plastic is sufficient to cover the bottom and sides (all the way to the top) without requiring any joins (we want to avoid leaks at all cost).
- Lay weeping tiles (AKA those black, ribbed drain pipes with slits in them) on the bottom of the bed before adding the stones to increase your reservoir capacity.
- Flatten and level the ground inside the frame. Remove all objects (twigs, roots, etc.) that could penetrate the plastic when the stones are put down.
- If you have doubts about the strength of your plastic, put down a layer of ‘builders’ fabric to buffer any stones that could pierce the plastic.
- Lay the plastic down inside and along the sides of the frame, smoothing out the wrinkles as much as you can manage. A good strategy for removing the slack is to make vertical folds along the corner vertices. Use a staple-gun and/or clamps along the top (we don’t want leaks, remember!) to keep the plastic in position.
- Lay down the weeping tiles (‘drain pipes’) and — if not using the automated top-up component — make a hole at the end of one of the weeping tiles (preferably in an accessible corner) and insert a PVC pipe vertically into it. This will serve as your inlet, allowing you to top-up the reservoir — so make sure it’s long enough to protrude above your soil.
- Once everything is in position, carefully add the stones until you reach the desired reservoir depth (Important: if you’re installing the automated top-up component, proceed with that before continuing with 5–8).
- Add another layer of ‘builders’ fabric over the stones, making sure there is some overlap on the sides. This is to prevent soil from falling into the reservoir.
- If you haven’t already done so, cut the excess plastic around the top of the bed to neaten it up a little.
- Last (but incredibly important), we’ll want to add an overflow pipe just above the maximum water level (approximately 1–2cm above the stones). Simply drill a hole right through your frame and plastic lining, and insert a short threaded pipe. It’s a good idea to seal the hole you made in the plastic by twisting a gasket onto the threaded pipe on the inside of the bed, ‘clamping’ the plastic to the frame.
Optional (automated top-up component):
Note: In the image above, you’ll notice that there are some PVC pipes connected to a toilet cistern. Originally this was intended to evenly distribute the water throughout the reservoir — but this is unnecessary due to the slow wicking action relative to the flow of water through the stones. For the purpose of this guide, imagine that they are the weeping tiles (drain pipes).
- Position the cistern in place, making sure that the maximum water level (i.e. the top of the stones) will be approximately halfway up the cistern.
- Before placing the stones, you may want to brace the cistern with something to prevent it from buckling.
- Connect your irrigation piping to the cistern (the other end will connect to your rainwater tank/water-mains), making sure that the end is accessible once we’ve filled the bed with soil. You will likely require two elbow-joins for this.
- Bend the float arm so that the valve shuts-off at the maximum water level (just enough to allow wicking — i.e. just above the top of the stones)
- Connect the inlet to your rainwater tank or water-mains (you may want to add an extra valve to this pipe in case you need to turn off the flow at some point)
Step 3: Adding the soil
Soil mix: Your choice of soil will depend on various factors such as the types of plants, budget, and your ability to source quality organic material.
Your main consideration should be to ensure that the substrate will wick effectively. Wicking can be improved by using high quality organic compost, loamy soils, peat, etc.
- Premix your soil, or mix and churn it as you lay down the different layers (beware of puncturing the plastic with your spade/rake).
- Lay down an organic mulch layer over the surface of your soil. This will help prevent weeds, reduce water evaporation, protect the surface soil (and microbes) from direct sunlight, as well as providing nutrients to the soil below as it decomposes.
- Plant!
General Design Considerations:
- Whatever materials you decide to make the frame from, ensure that the final component has enough strength and rigidity to hold the weight of the stones and soil (shown in step 2).
- If using bolts to attach the frame, ensure that they are just long enough to protrude through your planks + right-angled bracing but short enough that they won’t stick out and puncture a hole in your reservoir.
- If you’re looking to save money, consider digging a hole in the ground instead of constructing a raised-bed. Alternatively, you can use an existing wall to form one side of the wicking bed.
- The maximum depth of your water reservoir should be around 30cm. It’s up to you to determine the optimum size based on your local rainfall, plant thirst, and how frequently you want to be topping up the reservoir. *If installing the automated top-up component, you can make the reservoir much shallower.
- It’s necessary to have an upper limit of about 40cm for the depth of the soil-substrate due to the amount of wicking that is possible. We are fighting gravity, after all. Soils higher in organic matter will improve your bed’s wicking ability, so this number will vary according to the type of soil used.
Recommended depths: Reservoir height = 20–30cm (manual top-up) / 10cm (automated top-up) | Soil height = 25–35cm
Strategies / Recommendations
No-till: It’s good idea to adopt a no-till policy with these raised-beds. By minimizing disturbance to the soil we allow the microorganisms and worms to determine the soil’s structure, resulting in increased microbial biomass and the beneficial emergent effects thereof. By actively tilling the soil, we oxidize existing humus and destroy one of the crucial aspects of the soil’s ‘energy transportation system’: the mycelium networks, leading to further dissociation of the soil-plant supraorganism.
Probiotic amendments:
These powerful inoculants provide a way to improve your soil’s microbial diversity and consequently your garden’s health and productivity. They can be added to your soil, composting systems, or, if you have a wicking bed, straight into the reservoir via the inlet (another advantage of sub-irrigation). Below are a few worth mentioning:
Lactic Acid Bacteria Serum (LABs): In probiotic and natural farming circles, these bacterium are often referred to as the ‘microbial workhorse’ of soil ecosystems, providing a number of important functions in the anaerobic decomposition process and in the production of beneficial enzymes and antibiotics. These are highly abundant organisms and can be cultivated at home with ease.
(LAB cultivation guide coming soon..)
Effective Microbes (EM): This powerful microbial inoculant typically contains a variety of lactic acid bacteria, yeasts and photosynthetic bacteria that form a particularly synergistic microbial consortium within the soil-plant ecosystem.
(You can order this in South Africa from Efficient Microbes).
Bokashi: This is an organic fertilizer produced by the fermentation of organic material by various microbes (typically EM or LAB). Bokashi bran (wheat bran inoculated with these microbes) can be used as a probiotic soil amendment (added to the mulch layer, for example) or to further ferment organic waste such as kitchen scraps. The end product of this fermentation process (Bokashi) is typically buried in the soil as an incredibly rich source of microorganisms (and their byproducts), nutrients and minerals.
(You can order this in South Africa here and here, and from most garden centres).
I hope you’ve enjoyed/made use of this guide! If you have any suggestions, feel free to contact me at my email address below.
Email: rosseyre@gmail.com
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