A few years ago ECHO published a technical note on low-cost wicking beds (Edwards and ECHO staff, 2019). This topic came up recently in a conversation about growing food plants in 19-L (5 gal) buckets. This approach to gardening makes use of capillary rise of water through the soil from a reservoir of water at the bottom of the bed or container.
Research by Semandanda et al. (2016) is mentioned in the aforementioned technical note. They approached the topic from the perspective of small-scale urban gardens, using containers with an inside diameter of 56 cm and up to 100 cm tall (some were cut to shorter lengths depending on the depth of the soil and water reservoir used for each treatment). They evaluated water efficiency and tomato fruit production with a gravel reservoir depth of 15 or 30 cm and with a soil bed height of 30 or 60 cm. Below is a diagram, adapted from a figure in their paper, showing the main elements of their wicking bed containers.
They concluded that, of the treatments tested, a wicking bed with a reservoir depth of 15 cm and a soil depth of 30 cm was the most efficient design combination. Below are a few factors they discussed for optimizing the performance of wicking gardens.
Wicking beds versus surface watering
Before going into design aspects of wicking beds, let’s consider the rationale for a wicking bed approach to watering, versus just filling a container with planting media and watering from the surface. Semandanda et al (2016) compared water use efficiency between containers with and without a reservoir at the bottom. Those with a reservoir (wicking beds) were watered through a standpipe to direct water to a reservoir that consisted of gravel. Containers without a reservoir were watered at the top.
For both watering approaches, watering was done whenever the volumetric moisture content declined to 75% of field capacity moisture. Water use efficiency, based on the ratio of total marketable tomato fruit yield to water used by the crop or applied via watering, was higher (23-24 g fruit/L of water) with wicking beds (those with a soil depth of 30 cm) than surface watering (13 to 15 g fruit/L of water). Whereas the surface-watered containers needed to be watered 40 to 50 times (every 2 to 3 days), the wicking beds needed less than 26 waterings (every 1 to 2 weeks with mature plants). Fruit yield was statistically similar between surface watering and most of the wicking bed treatments. Thus, wicking bed gardens showed promise for being just as productive as carefully controlled surface watering with less water and labor associated with watering.
Soil and water reservoir depth of wicking beds
Of these two parameters, soil depth showed the most promise for optimization. In their study, a soil depth of 30 cm outperformed that of 60 cm. Reasons for this were not clear. The researchers initially theorized that this was due to less water reaching the surface with 60 than 30 cm; however, even though soil evaporation was less with 60 than 30 cm of soil, total water loss (from the soil and through plant leaves) in planted wicking bed containers was similar at both depths, suggesting that the amount of water reaching plant roots was similar at both depths. The authors mentioned that the 60 cm depth might be better for crops with deeper roots than the tomato plants grown. As far as reservoir depth, there was no difference in performance between a depth of 15 and 30 cm.
Adding a soil column to the reservoir
One of the wicking bed treatments incorporated a soil column in the reservoir (30 cm deep) to better connect the reservoir to the soil (30 cm deep). The researchers’ approach to doing this is shown in the graphic below. They used a perforated large-diameter (30 cm) PVC pipe to contain the soil column in the reservoir. Perhaps a cheaper option would be a tin can. This treatment yielded the most tomato fruit per plant, but it lost more water to evapotranspiration than other wicking bed treatments. Nevertheless, this design feature may be worth adding if water savings is not an overriding factor. One could experiment with different diameters. Some wicking designs use cloth or thick rope instead of soil to improve wicking of water into the soil.
Reservoir and growing media
Coarse gravel (10 mm) was used to contain water in these research trials. Curtis (2020) found that sand worked better than gravel in the reservoir—for maintaining moisture of the soil above. Wicking is influenced by the material used in both the reservoir and growing space.
As mentioned earlier, the distance over which water can move upward in the soil through capillary action will vary with soil texture. Water will generally wick further, but slower, with fine- than coarse-textured material. Curtis (2020) discussed other factors that influence capillary rise such as the size/width of the wick and factors (e.g., air pockets) that slow or block water movement.
Mulch for reduced water loss and salt buildup
Semanda et al. (2016) mentioned mulch as a potential consideration. In a subsequent study the same authors, Semananda et al. (2021), observed a benefit in terms of water use efficiency of a 5-cm thick layer of gravel as mulch on the surface of the soil. They pointed out that a potential downside of wicking beds is salt buildup in the upper layers of soil as water wicks upwards and evaporates. This was observed in their 2021 study with recycled water. They theorized that mulch could help preserve water and minimize salt buildup, and that people could experiment with different types of mulches. Curtis (2020) added that most roots with sub-irrigation are below the surface levels of the soil where most salt buildup occurs.
Final thoughts
An online search for wicking bed designs will yield other approaches to the reservoir and other aspects of design. A few other questions to think about are:
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Will a bottom layer of growing media, saturated with water due to close proximity to the reservoir, become anerobic? Has anyone observed what happens in this regard with different growing media? Has anyone found this to be an issue in growing root crops?
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In the event that nutrients already in the growing medium are insufficient to sustain the plants, what is the best way to add fertility? Should liquid fertilizer be added to the reservoir or at the top?
Input on the conversation is much appreciated.
References
Curtis, C. 2020. Wicking bed design: the effects of different reservoir media on plant growth, water use and soil moisture in wicking beds using capillary watering. Dissertation in fulfillment of a Bachelor of Science degree at Charles Stuart University. https://www.researchgate.net/publication/353917033_Wicking_bed_design_The_effects_of_different_reservoir_media_on_plant_growth_water_use_and_soil_moisture_in_wicking_beds_using_capillary_watering (see Roogulli Farm - Wicking beds for related information)
Semananda, N.P.K., J.D. Ward, and B.R. Myers. 2016. Evaluating the efficiency of wicking bed irrigation systems for small-scale urban agriculture. Horticulturae 2(4) Horticulturae | Free Full-Text | Evaluating the Efficiency of Wicking Bed Irrigation Systems for Small-Scale Urban Agriculture
Semananda, N.P.K., J.D. Ward, and B.R. Myers. 2021. Assessing reliability of recycled water in wicking beds for sustainable urban agriculture. Earth 2:468-484. https://doi.org/ 10.3390/earth2030028