Drybacks 101

To be sure they’re making informed decisions when steering crops toward certain outcomes, growers must take cues from their plants. But knowing what to look out for can be tough to gauge. Using sensors removes much of the mystery behind what’s happening in the substrate and overall environment, but putting time series data to work can be challenging without some understanding of plant behavior. One topic we regularly cover on Office Hours is drybacks, a technique commonly practiced in indoor and greenhouse environments.

In this post, we break down what drybacks are, why they’re important, and how getting a handle on them can help maximize crop yields and product quality in the long run.

What are drybacks, and why are they important?

To understand drybacks, it helps to understand volumetric water content, or VWC, which refers to the amount of water held in coco, Rockwool, or other substrates. Expressed as a percentage, VWC is the volume of water per volume of substrate. 

After the last irrigation event of the day occurs, typically the plant doesn’t get watered again until the following day. Plants transpire and water evaporates between those irrigation events, changing the volume of water in the substrate. This decrease in VWC% is known as a dryback, which measures the reduction in moisture content from the peak level achieved after watering to a lower level before the next irrigation cycle. 

Maintaining proper drybacks is important for…

Root health: Ensuring roots receive enough oxygen promotes healthy development while preventing issues like root suffocation. 

Nutrient uptake: Optimal moisture levels facilitate the roots’ ability to take in essential nutrients, supporting overall plant growth and vitality.

Disease prevention: Consistent dryback monitoring keeps conditions from being overly wet, which can lead to root rot and other diseases.

Efficient water use: Honing in on drybacks allows growers to be more intentional – and less wasteful – with their irrigations while also mitigating the risk of water-related plant stress. 

How to calculate drybacks

Here’s a step-by-step breakdown of how to calculate drybacks:

Step 1: Use a substrate sensor to measure the moisture content of the grow medium prior to watering. 

Step 2: Water plants thoroughly until runoff is achieved. 

Step 3: After allowing excess water to drain, then measure moisture content again using the same method.

Step 4: Calculate the difference between pre-watering and post-watering moisture content to determine the amount of water that was absorbed or evaporated during the dryback period.

Example: If moisture content before watering was 60% and after watering was 80%, 80% - 60% = a 20% dryback.

To calculate drybacks without using a substrate sensor, manually weigh the plant before and after watering, then calculate the difference as shown in the example.

Factoring drybacks into crop steering

Knowing how to calculate drybacks properly can help growers develop a more reliable and efficient irrigation strategy. This is especially crucial when crop steering, as growers can use drybacks to adjust irrigations according to plant response at each growth phase. 

For example, cultivators who choose to steer vegetatively to promote rapid plant growth must find a balance between providing adequate nutrition and removing any plant stressors. Implementing smaller drybacks and more frequent irrigations during the first 7-10 days of flower is a strategy that can lead to upward growth of stalks and leaves.

During the generative phase, when the focus is on reproduction, allowing intentionally targeted stressors promotes more compact growth. Larger, longer drybacks with fewer irrigations initiate ripening, leading to traits like tighter nodes in veg and early flower and more bud sites per length of stem.

Simply put: having insight into drybacks means more control for growers.

Conclusion

Understanding and effectively managing drybacks when crop steering is essential for achieving optimal plant health and maximizing yield potential. Growers who carefully monitor and calculate drybacks can ensure that their plants receive the right balance of water and oxygen, promoting robust growth and resilience against environmental stressors – leading to target outcomes that ultimately better the bottom line.

To dive deeper into drybacks, check out Office Hours Episode 3 and Seth’s breakdown of dryback math from Episode 101.