What is dryback?

Dryback refers to the reduction in moisture content within your substrate between irrigation events. It measures how much the volumetric water content (VWC) decreases from its peak level after watering to a lower level before the next irrigation cycle.

Understanding Volumetric Water Content (VWC)

• VWC is the amount of water held in your substrate (like coco coir, Rockwool, or soil), expressed as a percentage.
• It represents the volume of water per volume of substrate.
• Monitoring VWC is crucial for assessing how much water is available to your plants at any given time.

The importance of drybacks

Properly managing drybacks is vital for several reasons:

1. Root Health

• Oxygenation: Allowing the substrate to dry back ensures roots receive adequate oxygen, promoting healthy development.
• Prevents root suffocation: Overly wet conditions can lead to root suffocation and hinder growth.

2. Nutrient Uptake

• Optimal absorption: Correct moisture levels facilitate efficient nutrient uptake by the roots.
• Enhanced growth: Adequate nutrients support overall plant vitality and development.

3. Disease Prevention

• Reduces risk of root rot: Monitoring drybacks prevents overly wet conditions that can lead to diseases.
• Healthy environment: Maintains a balanced microbial ecosystem in the root zone.

4. Efficient Water Use

• Conservation: Helps in using water more intentionally, reducing waste.
• Plant stress mitigation: Prevents water-related stress by ensuring plants receive the right amount at the right time.

5. Crop Steering

• Controlled plant stress: Manipulating drybacks allows you to apply controlled stress, steering the plant towards vegetative or generative growth.
• Desired outcomes: Achieve specific goals like more bud sites, tighter nodes, or accelerated flowering times.

How to calculate drybacks

Accurately calculating drybacks is essential for effective irrigation management. Here's how you can do it:

Step-by-Step Guide

1. Measure pre-watering moisture content:
   • Use a substrate sensor to record the VWC before irrigation.
   • Alternatively, weigh the plant and pot if sensors are unavailable.
2. Irrigate the plant:
   • Water thoroughly until runoff is achieved to ensure uniform moisture distribution.
3. Measure post-watering moisture content:
   • After allowing excess water to drain, measure the VWC again using the same method.
4. Calculate the dryback:
   • Subtract the pre-watering VWC from the post-watering VWC.
   • Dryback (%) = Post-Watering VWC (%) - Pre-Watering VWC (%)

Example Calculation

• Pre-Watering VWC: 60%
• Post-Watering VWC: 80%
• Dryback: 80% - 60% = 20% Dryback

Factoring drybacks into crop steering

Crop steering involves manipulating environmental factors to control plant growth stages. Drybacks play a pivotal role in this process.

Vegetative Steering

• Goal: Promote rapid plant growth and foliage development.
• Strategy:
  • Implement smaller drybacks (5-10%).
  • Increase irrigation frequency to minimize stress.
  • Encourages upward growth of stalks and leaves.

Generative Steering

• Goal: Focus on reproductive growth to enhance yield quality.
• Strategy:
  • Allow larger drybacks (15-30%).
  • Reduce irrigation frequency to introduce controlled stress.
  • Stimulates the plant to produce more bud sites and tighter nodes.

Adjusting Drybacks According to Growth Phases

• Early Growth:
  • Maintain consistent moisture to support root establishment.
• Flowering Stage:
  • Gradually increase dryback percentages to steer generative growth.
• Late Flowering:
  • Monitor plant response closely and adjust drybacks to optimize ripening.

Determining the right dryback dercentage

There's no universal dryback percentage suitable for all situations. Factors to consider include:

Cultivar Sensitivity

• Genetic variation: Different strains respond uniquely to moisture levels.
• Observation: Monitor how your specific cultivar reacts to changes in dryback percentages.

Environmental Conditions

• Temperature and humidity: These affect transpiration rates and substrate drying.
• Light intensity: Higher light can increase water uptake and evaporation.

Substrate Type

• Field capacity: The total amount of water a substrate can hold affects dryback calculations.
• Drainage properties: Substrates like coco coir mixed with perlite may require different management.

Understanding Dryback Measurements

• Absolute Percentage Method:
  • Dryback is measured as the actual percentage decrease from the peak moisture level.
  • Example: From 50% to 20% VWC is a 30% dryback.
• Field Capacity Percentage Method:
  • Dryback is calculated as a percentage of the substrate's field capacity.

  • Example: 30% of a 50% field capacity equals a 15% dryback (50% - 15% = 35% VWC).

     

Tips for managing drybacks

• Monitor plant responses:
  • Look for signs of overwatering or drought stress.
  • Adjust irrigation schedules based on plant feedback.
• Use precise sensors:
  • Invest in high-precision substrate sensors (e.g. TEROS 1) for accurate VWC and EC readings.
• Record and analyze data:
  • Keep detailed logs of moisture levels, irrigation times, and plant responses in software platform.
  • Use this data to refine your dryback strategies over time.
• Consider environmental factors:
  • Adjust for changes in weather conditions in greenhouse settings.
  • Control indoor environments to maintain consistent drybacks.

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.

Implementing strategic dryback practices allows for:

• Enhanced Root Development
• Improved Nutrient Uptake
• Efficient Water Use
• Disease Prevention
• Targeted Crop Steering

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