Wicking Bed Design and Construction

What is a Wicking Bed?

A wicking bed has been described as a ‘water-well pot on steroids’. With a water reservoir at the bottom of the bed, the growing medium is kept moist as the water rises against gravity via capillary action in the tiny spaces between soil particles. This is essentially how water gets to the top of trees. We have several now at LPCG, with more planned. We have made a couple in our herb garden and the author of this has just completed one in her plot. Another plot was converted earlier in the year.

Benefits

  • Soil is more evenly watered, so plants can grow with the minimum of stress.
  • Higher moisture levels at the bottom of the bed encourages roots to go down, making the most of the nutrients and helping plants survive heat waves better.
  • There is a barrier to tree roots, so nutrients and water are not lost from productive gardens, reducing the need for water and fertilizers.
  • Filling the reservoir is usually needed only weekly, and even this can be automated if desired
  • The very top of the soil remains relatively dry (unless mulched) so weed seeds are less likely to germinate

And disadvantages…

  • Construction costs can be higher, depending on what you have available and can recycle
  • Soil depth is limited to about 300mm (water will not wick higher than this) so deep rooted plants may not be suitable. This is ample for most vegetables
  • Drier soil at the top means hand watering of seeds may be needed until they have established a deeper root system.
  • Salts can build up, especially if non-organic fertilizers are used (we don’t encourage these anyhow), and with the use of hard water.

Picture Describing Wicking Bed Design

Key Features

  1. A water holding vessel around 450mm-650mm deep (allowing ~50mm for mulch). This will also hold the soil, so needs to be strong enough. It can be a large container, a raised garden bed lined with heavy-duty plastic or pond liner. It can be partly dug into the ground to the depth of the reservoir, or built on a platform or on top of fill in a higher raised bed. The bed needs to be level. Smaller versions can be made with polystyrene boxes or other recycled containers. We used orange builders plastic to line raised beds – widely available and up to 4 m wide
  2. The water reservoir is filled with scoria or gravel, (some use sand) around a length of slotted (agricultural) hose (used for drainage at the bottom of retaining walls etc). This provides extra void space to hold more water and allows water to quickly flow throughout the reservoir. We had delivered from Garden Grove (10mm) and had some 20mm scoria donated for herb area beds.
  3. The slotted hose is connected to an upright pipe for filling the reservoir. We have used both 50mm and 100 mm slotted hose – we had them from other projects. Comes in 10 m rolls
  4. On overflow pipe at the top of the reservoir, so excess water or rainfall can drain. This is particularly important in areas with high rainfall. If the soil profile fills up with water your precious plants will drown. This pipe (and the inlet pipe) should be protected so mosquitos do not get into the reservoir. We used tank fittings, available at Newton’s, and Masters and probably other stores. These have a couple of plates that sit either side of the orange plastic to seal it.
  5. A membrane so that the fine particles from the soil cannot clog up the reservoir. We have used geotextile fabric, which we would recommend over shade-cloth or other similar materials as doing the job much better. Available from Glynde Mitre 10 and Newtons
  6. Good quality organic soil. Wicking into the soil is more effective with higher organic content – better for your plants also! We added mushroom compost to the soil we removed and replaced in the beds.

Order of work

  1. Dig old soil out of bed, or put up edges to at least 450mm (up to 650mm – some can be below ground level, but only up to the level of the top of the reservoir). Level as well as you can.
  2. Spread sand (10-20mm deep). Helps level the area. (Pictures 1)
  3. Spread outlining over bottom and up sides. Be careful not to puncture this (it’s pretty tough!)
  4. Put in Ag pipe, elbow and upright. (Picture 2)
  5. Start filling with water to about ½ way up the Ag pipe
  6. Add gravel/scoria. Take care not to puncture plastic
  7. Level scoria. This is easy to do if you add water to the level of the scoria. (Picture 3)
  8. Place geotextile fabric on top of scoria. (Picture 4)
  9. Drill a hole in the side of your walls for outlet. The bottom of the outlet pipe should be at the level of the geotextile (ie top of the reservoir).  You can put this lower down and add an elbow on the outside to get the right level if you want. This has the advantage that you can drain the reservoir – essential in areas where it freezes in winter, but probably not necessary in Adelaide!
  10. Cut a small hole in the liner and put outlet pipe in place. (Picture 5)
  11. Fill with soil. We made a ‘lasagne’ of soil and compost. Leave some space for mulch on top
  12. Trim liner.(Picture 7) Top up reservoir over the next few days until wicking wets soil profile
  13. Check out what you are will plant!

Some other considerations

  1. It will take a few days for the wicking to get going. You might like to water from the top to assist (just this once!) Check the level in the reservoir (you can see down the inlet pipe) and top up frequently especially if your soil is dry.
  2. I added Wettasoil as the old soil was hydrophobic, also some GOGO Juice and rock minerals for good measure, to get good microbial activity started. Thinking of adding some worms also as the bed is now disconnected from the soil. I also added a couple of buckets of Bokashi scraps which we had.
  3. Place your outlet pipe where you can see it from the inlet pipe, so you know when the reservoir is full (you can see it overflow!) (Picture 6) You can also check the level by noting where the water comes up to in the inlet pipe.
  4. You might like to add some posts to the corners (outside the liner but inside the wall) to support shade-cloth.
  5. Some have added a couple of larger (capped) pipes vertically into the soil to use as mini worm farms. These have several holes drilled below soil level, and veggie scraps can be added to the top. The idea is that the worms will break down the veggie scraps and keep the bed fertilized at the same time.

Instructional Images regarding making a wicking bed

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