5 Golden Rules for Managing Irrigation of a Container Crop

Making the switch to soilless substrate production from a soil-based system takes some fine tuning. Here we are going to look at the 5 Golden Rules that are crucial for managing the frequency and volume of a substrate irrigation system.

1) Apply water using the 'little and often' mentality to avoid over watering and accidental dry down periods

Unlike a soil-grown crop where the roots have access to vast amounts of soil holding water and nutrients, a containerize crop is quite limited in the resources it has access to. As a result, irrigation needs to be applied many times a day to prevent the media from drying out, and ensuring the plant can remain productive. Growing containers are only able to hold a certain amount of water, so it doesn't make sense to try and over-saturate the media 4 times a day, for example. This also creates a stress on the plant, which is constantly battling either too wet conditions, or much too dry. What DOES make sense is setting up your system to receive short, regular intervals of water throughout the day. The volume of water delivered is going to vary depending on the age of the plant and the crop itself.

• Advice on salad crops in rockwool is to apply 3-6% of container volume

Figure 1. An example of a tomato root system colonizing a coco coir growing media. Note the restricted volume that the roots have to sustain the plant throughout the growing season. Photo credit: Talia Plaskett

2)   Start dripping 1-2 hours after sunrise and finish 1-3 hours before sunset

The crop's demand for water is significantly lower at night, so it is important to introduce a 'dry down' period through this time. By preventing inactive roots from sitting in a moist growing matrix for many hours a day, you are significantly improving the health of the root system, and helping establish a strong base that will carry you through the growing season. 

3)  When there is little/no transpiration happening, the substrate is allowed to dry back a little

      While this is a similar concept to rule number one, it is important to highlight that dry-back applies during cloudy periods or low-light days as well. The volume of water delivered to the plant is highly dependent on what is happening in the growing environment. Periods of high light intensity and warm temperatures require a lot more water than a mild, overcast day. 

      4) Leachate is a necessary evil! 

      Monitoring run-off is going to be a big part of managing a soilless substrate. Looking at the volume, as well as the EC and pH, is going to give you a lot of feedback on how you are doing so far, and gives you room to make necessary adjustments. 

       Here are a few guiding principles:

•        if you get run-off after your first irrigation event, either the timing of your first irrigation event was too early in the day
•       there was too much water given the previous day, and the media was not given the opportunity to dry-down

      The goal is to see drainage coming out of your growing containers during the second or third drip of the day. If you still aren't seeing any drainage by this point, you need to make adjustments to your watering volume and frequency. 

      5) On hot days, plan for a high frequency of irrigation events around peak water use (mid/late morning to late afternoon)

       

      This ensures that the crop is being provided with the water when it needs it most. There will be no benefit to the producer or the crop to cluster the majority of the irrigation events in the morning or late in the afternoon. Take a look at when your first and last cycle should be, as well as figure out how frequently you need to irrigate in between that 

Figure 2. Here is a representation of a watering strategy for a mature tomato crop in the heat of the summer. Notice that the first irrigation event happens a few hours after sunrise, and the last irrigation event takes place well before night fall. The peak times for solar radiation (10 am - 4 pm) show a high frequency of watering to sustain the plant through periods of high intensity production. Photo credit: Delphy U.K. 

     One final note on irrigation: while we focused a lot on the flow of water that takes place throughout the day of a container crop, its important to remember that there is also the over-arching cycle relating to crop production. Younger plants are not going to need as much water as a fully loaded plant in the middle of harvest. It is important to adjust your irrigation strategy based on the life stage of the crop. 

   

      Small canopy/young plants: infrequent drip cycles to avoid excess run-off

Large canopy/high fruit load: frequent drip cycles required, cluster around peak use period. Additional rounds may be required in small pots in the afternoon

For more information on anything you saw here, or on irrigation management in general, check out some of our resources:

• 2020 - 2021 Getting Into the Weeds Session on Irrigation Management
• Contact Talia Plaskett, our Protected Crop Specialist