Mastering dry backs: The key to optimizing yields and plant quality

What is dry back?

Dry back 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 dry backs

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: Maintaining correct moisture levels facilitates efficient nutrient uptake by the roots.
• Enhanced growth: Adequate nutrients support overall plant vitality and development.

3. Disease prevention

• Reduced risk of root rot: Monitoring dry backs 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: Monitoring water use helps growers use 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 dry backs 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 dry backs

Accurately calculating dry backs 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.
   • If sensors are unavailable, weigh the plant and pot.
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 as in step 1.
4. Calculate the dry back:
   • Subtract the pre-watering VWC from the post-watering VWC.
   • Dry back (%) = Post-Watering VWC (%) - Pre-Watering VWC (%)

Example calculation

• Pre-Watering VWC: 60%
• Post-Watering VWC: 80%
• Dry back: 80% - 60% = 20% dry back

Factoring dry backs into crop steering

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

Vegetative steering

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

Generative steering

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

Adjusting dry backs according to growth phases

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

Determining the right dry back percentage

There's no universal dry back 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 dry back percentages.

Environmental conditions

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

Substrate type

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

Understanding dry back measurements

• Absolute Percentage Method:
  • Dry back is measured as the actual percentage decrease from the peak moisture level.
  • Example: From 50% to 20% VWC equals a 30% dry back.
• Field Capacity Percentage Method:
  • Dry back is calculated as a percentage of the substrate's field capacity.
  • Example: 30% of a 50% field capacity equals a 15% dry back (50% - 15% = 35% VWC).

Tips for managing dry backs

• 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 ONE) for accurate VWC and EC readings.
• Record and analyze data:
  • Keep detailed logs of moisture levels, irrigation times, and plant responses in a software platform.
  • Use this data to refine your dry back strategies over time.
• Consider environmental factors:
  • Adjust for changes in weather conditions in greenhouse settings.
  • Control indoor environments to maintain consistent dry backs.

Improving the bottom line

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

Implementing strategic dry back practices allows for:

• Enhanced root development
• Improved nutrient uptake
• Efficient water use
• Disease prevention
• Targeted crop steering

To learn more about dry backs, check out Office Hours Episode 3 and Seth’s breakdown of dry back math from Episode 101.