May 15, 2013
Wireless soil moisture sensors a good option for pot-in-pot system
Experimental layout at Connon Nurseries/CBV. Soil moisture sensors are inserted to the centre of the potting substrate. A computer was located in an office, 1 km from this site for recording and displaying growing substrate moisture levels in the pot, and for wireless control of irrigation.
By Dr. Youbin Zheng, Katherine Vinson, Cody Thompson
School of Environmental Sciences, University of Guelph

Janice Carmichael
Connon Nurseries - CBV

Irrigation is one of the most important elements for the success of container nursery production. Too much water will cause drought stress, while over-watering wastes water and fertilizer, and leads to root zone oxygen deficiency and pathogen proliferation, lowered crop productivity, and negative environmental impacts.  

Currently, most nursery operations base irrigation schedules on non-scientific methods, such as touching or visual inspection. These methods will work under certain circumstances, however, it is critical that the one in charge of watering is very experienced and diligent.

Pot-in-pot production technology is becoming popular in caliper tree productions in nursery operations. When the pots are small, growers can lift them up to check the growing substrate water content in order to decide when and how much to water. However, most of the pots used in caliper tree productions are too heavy to lift and assess the growing substrate moisture content (SMC). Without technology to accurately assess the SMC, it is difficult to make the right decisions regarding when and how much to water.

To address the above issues, we have introduced a sensor-based automated irrigation system ( to the nursery industry. There were a few unanswered scientific questions related to a plant’s response to SMC and variation of the SMC with time. Therefore, we designed a research project to systematically study these uncertainties in order to develop better strategies for irrigating pot-in-pot trees.

The objectives of this research were evaluate the feasibility of using wireless soil moisture sensor-based automated irrigation technology (SMAIT) for nursery plant irrigation; investigate the responses of pot-in-pot Acer rubrum (red maple) trees to different growing SMCs; and develop recommendations for using SMAIT in pot-in-pot tree productions.

An on-farm trial was conducted at Connon Nurseries/CBV from May 2011 to Oct. 2012, covering two growing seasons. Acer rubrum trees were used as the testing species based on the fact that they will rapidly display visual signs when placed under stressful situations, or will alternately display vitality when placed under optimum conditions. This tree is very well represented in Ontario nursery operations. Two-year bare root trees (~2.5 cm caliper size) were potted in 62-litre (15-gallon) pots with growing substrate (Gro-Bark Tree Mix; the laboratory-measured container capacity was 78 per cent and the actual container capacity measured using a sensor in the field was 50 per cent) at the beginning of the 2011 growing season and irrigated at five different regimens (treatments).

The five treatments included a control and four automated irrigation regimens. For the control, the timing and amount of water that each tree received was determined at the grower’s discretion. This represents the current common practice in the majority of nurseries. For the automated irrigation regimens, trees were irrigated automatically whenever the SMC reached 36 per cent, 29 per cent, 22 per cent, or 15 per cent.

The automated irrigation system used FreeLink Wireless Smart Central, which was installed in the office at Connon Nurseries to provide the wireless communication from the field to the office for irrigation control and reporting; three FreeLink Wireless 8 channel Valve and Sensor Controllers in combination with DC Latching Solenoids and Decagon EC-5 moisture sensors; two FreeLink Wireless Flow Sensors to monitor the irrigation in the control and test areas independently and two FreeLink Wireless Flow Sensors to monitor the irrigation in the control and test areas independently. Also, one FreeLink Wireless 8 channel Environmental Sensor Controller was used to monitor both the ground temperature and moisture content in the control plots.

A soil moisture sensor was buried in the centre of each pot and the SMC was read and sent to a computer once every 10 minutes. These SMC readings were used to trigger irrigation events for the automated irrigation treatments. When the substrate reached the targeted moisture content, irrigation was delivered through two emitters (NetaFim’s Plum PCNL Single-direction spray stakes provided 3.2 gph) per pot over three cycles. A time of 90 minutes was left between each irrigation cycle to deliver a total amount of water equal to the amount the pot could hold between the target  SMC (e.g. 22 per cent) and 50 per cent (field measured container capacity) in order to reduce or eliminate leachate from the bottom of the pots.

The irrigation system and tree performance were checked daily. Tree caliper, circumference, height, and visual appearance were measured or assessed regularly during the growing seasons. Soil moisture sensors remained in the pots and left in the field to overwinter with the trees to test whether the sensors could withstand harsh Canadian winters.  

Measurements at the middle and end of the growing season in 2011 indicated that the caliper, circumference, height and visual appearance of the trees were not affected by the irrigation treatments. There were also no differences in leaf colour and the amount of leaves that remained on the trees on Oct. 4, 2011 (end of the 2011 growing season). Temporary wilting events were observed for some of the trees in the 15 per cent treatment during the growing season. Trees in the 22 per cent treatment were not significantly different in performance (caliper, circumference, height, and visual appearance) from trees in the control, 29 per cent, and 36 per cent treatments, but used 55 per cent less water than the control.  

Final measurements at the end of the 2012 growing season showed that trees did not differ in performance (circumference and height, visual appearance, leaf colour, and the amount of leaves remaining on the trees on Sept. 28, 2012), however, the SMCs were more consistent and well-controlled by using the sensor-based automated irrigation system than by using a grower’s judgment. Without using sensors, it is difficult, if not impossible, to judge the water content of the growing substrate, and this can therefore lead to either over- or under-watering.

Results suggest that pot-in-pot Acer rubrum caliper trees can be irrigated using a sensor-based automated irrigation system, and irrigation events can be triggered at a SMC as low as 22 per cent to save irrigation water and prevent over- or under-watering. The wireless soil automated system worked reliably and controlled the SMC in the designed levels. The Decagon soil moisture sensors can be left in the pots in container production over the winter.

The sensor-based irrigation system needs homogeneous growing substrate and plant size. If the plant size variation is too big, it may cause the smaller plants to receive too much water, while the larger ones do not receive enough water. Also, when inserting the sensor in to the substrate, it is very important to avoid large chunks, such as pine bark or large roots around the sensors, and to make sure the sensors are inserted where most of the roots are located. It is important to ensure the pump, plumbing lines, and emitters are working properly, so that the automated system delivers the required amount of water.

Based on our research, we strongly suggest growers adopt some of the sensor and control technologies to guide their day-to-day irrigation practices.

For pot-in-pot Acer rubrum trees, the suggested minimum SMC used to trigger the irrigation events was about 22 per cent. More research is needed to determine optimal irrigation schedules for other crops and production systems, especially container shrubs and perennials, which commonly use overhead irrigation. The sensor-based automated system has great potential to save water, reduce root diseases, and improve crop production.

When there is no available information on the irrigation schedule for your specific crop, substrate, and irrigation combination, it is recommended that a soil moisture sensor be used to check the SMC before irrigating. For example, several experienced personnel in charge of irrigation on a specific nursery can get together to come up with a conclusion on when to water, based on their experiences and observations. A sensor can be inserted into a pot to measure the SMC at the point when the growers would decide to irrigate. From there, everyone can use this moisture content as a guide to decide when to water in the future. Substrate container capacity can be measured to calculate how much water needs to be added to reach container capacity from the irrigation triggering point.

This project was financially supported by Landscape Ontario, Agriculture and Agri-Food Canada’s Growing Forward Program and the Agricultural Adaptation Council. We thank FreeLink Wireless Irrigation Systems for providing sensors and wireless automated irrigation systems and providing other necessary material and technical supports. We also thank Connon Nurseries/CBV for providing plumbing, land, plant materials and technical support; and Rob Vanderkruk’s valuable inputs during the entire process. Finally, thanks to Linping Wang for her technical support.

For more information contact Dr. Youbin Zheng (, an associate professor of the University of Guelph, and the Environmental Horticulture Chair for both University of Guelph and the Vineland Research and Innovation Centre.