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Project: The Transformation of Our Urban Home
Posted by Rob Avis over 10 years ago
Wicking beds are a unique and increasingly popular way to grow vegetables. They are self-contained raised beds with built-in reservoirs that supply water from the bottom up - changing how, and how much, you water your beds. In this article, we'll talk about how wicking beds work and why we love them. We'll also show you some great examples and leave you with ideas and instructions for creating your own.
How Wicking Beds Work
A wick works through capillary action – the same force you observe when you dip a piece of tissue paper partially into a glass of water and watch the water climb the paper. Wicking occurs in many materials; cotton, wool, geo-textile, soil, gravel and even wood to some degree. Every material has different wicking properties which you can test by placing that material into a glass of water and watching the water “climb” up. When one end of the wick is saturated and the other end is dry, it creates a moisture gradient, which drives the wick until the gradient no longer exists or you run out of water. With the earth box, one of the more popular examples in North America, the soil is suspended above the reservoir with wicks dangling into the reservoir pulling up moisture. As the plants use the moisture in the soil, it creates a moisture gradient (the soil is drier than the reservoir) which drives moister through the wick into the soil.
Advantages of Wicking Beds
Wicking beds have a lot of advantages over standard raised beds and in-grown swale-based gardens:
- They are water-efficient! Watering from the bottom up prevents evaporation of surface water (which occurs when you water beds from the top).
- They are self-watering! Wicking beds are an especially great system to use in community gardens because they save people from driving every day during hot weeks to water their beds. A full wicking bed should irrigate itself for about a week.
- They can be placed close to the house without risking flooding your basement, since the water is contained in the bed. This makes wicking beds a great alternative to swales on properties with sump pumps or basement water issues.
- No evaporation means no salting of soil. If you are watering your soils from the top with hard water, you risk accumulating salts, because the water evaporates and leaves the minerals behind. Eventually your soil will struggle to support plant life.
- They provide a lot of drainage in the event of a large downpour.
- Since they're raised, they will warm up quicker in the spring.
- You can easily attach cold frames to them.
- They are great for people with less mobility and strength as you don't have to haul heavy water containers.
- By using an intermediary tank, you can automate the watering process... but more on that in a future blog.
Disadvantages of Wicking Beds
Wicking beds do have some disadvantages as well:
- They cost more to install than in-ground swales and standard raised beds.
- They will freeze sooner in the fall than non-raised beds.
- There are additional freeze/thaw considerations that need to be taken into account, which is not required for conventional gardens.
Types of Wicking Beds
Reservoirs with Media
Most of the DIY sites for wicking beds focus on building beds that use media, a layer in between the soil and the water reservoir, as their wick. This is an easy and cheap way of supporting the soil on top of the reservoir. Gravel is the most common medium, but there are a number of materials that do the trick. Here's a good DIY blog on media wicking beds.
Beds without media require a false bottom that will allow the soil to be suspended above the water reservoir. Again, this wick system can be made from a variety of materials. Here are some examples of media-less wicking beds:
And a video example...
Design Considerations for Media-filled Reservoirs
When designing your wicking bed, it is important to keep the depth of the media-filled water reservoir at or below 300mm as the capillary action struggles to lift the water higher than that. The soil above the reservoir acts as a wick as well, so it is important that the soil layer stay between 300 – 320mm. The soil could technically be deeper than this, however, the soil at the top will likely be much drier than the lower soil, so you'd want to make sure to that the plants you use can access this deeper soil moisture, like tomatoes, which can buried deep into the bed.
Once you have determined how deep your soil and media is going to be, you need to create a containment device. There are many ways of doing this, for example, in our last blitz we experimented with four different techniques: the global bucket, a food-grade plastic tote, above-grade planter boxes, and an in-ground wicking bed.
A key element of the containment device is the overflow pipe. This pipe allows water to escape once it reaches the top of the media, ensuring you don't drown your plants with too much water.
Above-Ground Planter Box
Because saturated soil is so heavy, we designed the beds the same way that concrete forms are designed. A carpenter friend of mine recommended that we use “whalers”, which are 2x4s with the thick end perpendicular to the plywood, and bound together using a lap joint. See the photo below. These whalers were spaced around the box to resist bending and bowing of the heavy soil. Pressure treated plywood was used to prevent rotting on the inside of the whalers and increase longevity, and then the whole box was clad with cedar fence boards. Ordinarily I don't like to use pressure-treated wood in gardens, but since this bed was to be lined with plastic, the wood will not come in contact with the growing medium. Originally I was going to use pond liner, but it was cost-prohibitive at $0.90/sqft, so I decided to use 6 mil builder's poly instead.
Layers in The Bed
1. Landscape fabric stapled to the wood which protects the poly from sharp edges.
2. Poly liner.
3. Landscape fabric on bottom of bed to protect poly from punctures from the gravel.
4. Weeping tile to increase rate of water communication in bed as well as reservoir capacity. If you have a long enough weeping tile you can bend it up the side of the bed and use it as a water fill pipe.
5. Drainage pipe the length of the bed to encourage even drainage from the bed. This pipe is connected to the bulk head fitting and has holes drilled in one side facing down.
6. 300 mm or less of gravel. Note you want to make sure that you have enough gravel to cover the weeping tile as you want to make sure that the gravel is in contact with the soil, not the weeping tile.
7. Landscape fabric to segregate the soil from the gravel and preserve the pore space in the bed.
8. High carbon soil
To account for the freeze/thaw issue in this climate, I set my drainage hole to the bottom of the beds so that I can drain the bed before winter. The amount of water held in the bed is determined by an elbow and stand pipe which can rotate on the outside of the bed. I like this method of water control as it allows me to infinitely control how much water the bed can store.
When it comes to raised wicking beds, cheaper alternatives to the raised wood boxes are food grade 1000 L totes cut in half. We decided to use this method for our passive solar greenhouse as these tanks were inexpensive (they cost me roughly $100 each, and one tank cut in half can make 2 wicking beds). This is far more affordable than the wood variety, which are roughly $600 each, not including the time they take to build.
Layers in the ICU
1. Weeping tile to increase the rate of water communication in the bed as well as reservoir capacity. If you have a long enough weeping tile you can bend it up the side of the bed and use it as a water-fill pipe.
2. 300 mm or less of gravel. You want to make sure that you have enough gravel to cover the weeping tile so that the gravel is in contact with the soil and not the weeping tile.
3. Landscape fabric to segregate the soil from the gravel and preserve the pore space in the bed.
4. High carbon soil
In-Ground Wicking Beds
For our blitz we built one in-ground bed. The in-ground bed is cheaper than the other two because you use the earth as the support for the water reservoir. This means you only need a containment device for the soil above grade. For our bed we chose to use cedar planks to build the above-ground bed and we made a small dugout to contain the gravel. To allow the excess water to spill out, you need to make sure that the water can leave the bed either with a designated spillway as we did, or just raise the bed up on shims so the water can leave the periphery.
Layers For The In-Ground Bed
1. Landscape fabric on the soil to protect the poly from holes.
2. Poly liner.
3. Landscape fabric on the bottom of bed to protect poly from punctures from the gravel.
4. Weeping tile to increase both the rate of water communication in the bed, as well as the reservoir capacity. If you have a long enough weeping tile you can bend it up the side of the bed and use it as a water fill pipe.
5. 300 mm or less of gravel. You want to make sure that you have enough gravel to cover the weeping tile so that the gravel is in contact with the soil and not the weeping tile.
6. Landscape fabric to segregate the soil from the gravel and preserve the pore space in the bed.
7. High carbon soil.
A Neat Blog on In-Ground Beds
Milkwood Permaculture has pioneered an in-ground wicking bed using builder's plastic and a round galvanized culvert ring. We have also seen people use stock watering tanks.
We chose to use a garden mix from Western Canadian Compost which is combination of loam, compost and peat. I was really impressed with the quality of the soil and I will give updates on the results over the course of the summer.
We selected gravel as our resevoir/wicking material. Generally speaking, gravel has about a 33% pore space which means that 1 cubic meter of gravel in a container will only have enough room between the gravel to hold 333 litres of water.
We also used 4-inch weeping tile in the bottom, which increases the amount of water that the bed can hold (because it is hollow) and increases the rate at which the gravel bed disperses water. The dispersement action of the weeping tiles ensures that one side of the bed does not get initially over-saturated. The weeping tile also doubles as the watering pipe.
There are all sorts of design elements that can be added or modified to change how the beds work. One popular tweak is to insert a worm composting tube into the soil portion of the bed. Food scraps can be added to the tube for the worms to process, and the resulting vermipost and worm juice will be distributed throughout the bed keeping the nutrient levels high in your soil. The worms also help to keep the system aerated and therefore prevent the system from going anaerobic. I do not think that the red wriggler can survive our cold winters, so I would recommend having an indoor worm system ready when it starts getting colder out, so that you can keep them alive until the next growing season.
As my students know, I am a big fan of cover crop systems. Typically I recommend nitrogen fixing legumes that build carbon and nitrogen into the soil through their root systems. Since these wicking beds are segregated from the subsoil, strategies are needed to keep up soil fertility. This could include cover cropping or the addition of compost, blood and bone, and rock dust. Cover cropping also reduces weeds, shades the soil, and provides a built-in mulch system.
As you can see, the sky (or the soil) is the limit when it comes to wicking beds. They are an effective and water-efficient DIY gardening implement that can be created and adjusted according to your budget, materials, space, and garden plan.
Even though we've built our fair share of wicking beds now and have gained a great deal of knowledge from other people's designs and experiences, we are still learning as we go. Stay tuned throughout the summer for updates on our wicking beds, and hopefully you can learn with us!
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