Here’s our ever expanding wicking bed science repository – we’ll continue to add to this as we find more strong science to support and explain the magic of wicking beds.
Wicking bed science
Understanding capillary action (aka wicking)
The principle upon which these beds derive their name, capillary action or wicking movement is water passing through the small spaces in-between other particles. This can be best seen by dipping the corner of some (dry) paper towel in water and watching what happens… the water moves through the towel, even against gravity; another favourite is the coloured food-dye and celery experiment. This action also happens through our veggie bed soil, over the surface of the wicking bed reservoir material, and any number of places and applications.
The forces at play in capillary action are a combination of water being attracted to an external substance (adhesion) and to itself (cohesion), happening simultaneously. A great example of the difference between those two forces is a water droplet on a leaf tip: cohesion holds the water as a droplet, adhesion holds the droplet to the leaf.
There is one more element regarding water and plants worth understanding here and that’s transpiration, or how water moves through plants. When a molecule of water evaporates from a leaf surface it ‘pulls’ on the molecule behind it (remember cohesion?) which it turn pulls the one behind it, which in turn… well, you get the picture. When combined with adhesion inside the plants’ H2O plumbing (aka xylem), they once again produce capillary action, the primary force responsible for water movement through plants (and a key contributor to ‘creation’ of rain, but we digress!). See this lovely graphic for more on transpiration.
A robust road-test of the anecdotal claims wicking bed owners have been making for quite some time:
“Evaluating the Efficiency of Wicking Bed Irrigation Systems for Small-Scale Urban Agriculture” Niranjani P. K. Semananda, James D. Ward and Baden R. Myers; School of Natural and Built Environments, University of South Australia, Mawson Lakes Campus, Adelaide 5095, Australia
Excerpt from the conclusion: “Wicking beds (WBs) have been widely promoted and adopted as an efficient irrigation system for urban agriculture. However, there has been little published research to support popular claims about the effectiveness of WBs. This study rigorously tested the performance of WBs relative to best-practice, precision surface irrigation systems in terms of total water use, marketable yield, fruit quality, water use efficiency (WUE) and irrigation frequency. Overall, the results of this study indicated that WBs matched (or exceeded) WUE and yield achieved with best-practice surface irrigation, and offered a potentially substantial labour saving for gardeners. Moreover, given that surface irrigation in urban agriculture is likely to fall well short of the precision irrigation method used for comparison in this study, the relative WUE improvement of WBs in practice is probably greater. This study therefore provides a much-needed scientific basis for the widespread adoption of WBs in urban agriculture.”
What we love: there’s a robust process exampled here, in the pursuit of evidence around claims made of wicking bed performance – the results offer support for the anecdotal evidence we’ve experienced and had fed back to us about reduced water usage and healthier veggies; this quote: “Most importantly, the WB’s (wicking beds) eliminated the problem of deciding when and how much to irrigate”
What we’d love more of: larger sample sizes across different crop types; testing over multiple seasons and across a few years;
To further understand how water moves through plants, and thus supporting how wicking beds work (through similar processes):
“Water Uptake and Transport in Vascular Plants” By: Andrew J. McElrone – U.S. Department of Agriculture, Agricultural Research Service, University of California, Davis, Brendan Choat – University of Western Sydney, Greg A. Gambetta – University of California, Davis & Craig R. Brodersen – University of Florida, © 2013 Nature Education
Excerpt from the introduction: “How does water move through plants to get to the top of tall trees? Here we describe the pathways and mechanisms driving water uptake and transport through plants, and causes of flow disruption”; and: “Water’s importance to plants stems from its central role in growth and photosynthesis, and the distribution of organic and inorganic molecules. Despite this dependence, plants retain less than 5% of the water absorbed by roots for cell expansion and plant growth. The remainder passes through plants directly into the atmosphere, a process referred to as transpiration.”
What we love: a good science-y explanation of how water moves up through plants (or more accurately how plants move it!); this handy graphic: