Dryback is how much a growing medium dries between one irrigation and the next. It is measured as the fall in volumetric water content (from a substrate sensor) or in pot weight (from a scale), and expressed as a percentage. It is the core control variable in crop steering.
In practice
A rockwool block or coco pot is brought to near container capacity in the morning, then allowed to dry during the day and overnight. If a sensor reads 65% water content after the last irrigation and 52% before the first one next morning, the overnight dryback is 13 percentage points, or 20% of the starting value. Smaller drybacks favour vegetative growth; larger drybacks favour generative growth and can concentrate root-zone EC. Too large a dryback stresses the plant and can damage roots on rewetting.
Always state which way you're calculating it: percentage points of water content, or percentage of the starting value.
Dryback (%) = (WC_max − WC_min) / WC_max × 100, or simply WC_max − WC_min in points.
Don't confuse with
- Runoff: water draining out; dryback is water lost by uptake and evaporation.
The amount of water in a material as a percentage of its weight; cured cannabis flower usually sits around 10-12% w/w, though measurement method matters.Rockwool is an inert growing medium spun from melted basalt rock into fibres, sold as plugs, cubes and slabs, that holds water and air in fixed proportions.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.Field capacity is the amount of water a medium holds after it has been saturated and allowed to drain freely; in pots it is called container capacity.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.Crop steering adjusts irrigation timing, volume and dryback, along with climate, to push plants towards vegetative growth or generative (flowering) growth.The wet-dry cycle is the alternation between watering a container and letting it dry back, which refreshes oxygen in the root zone between irrigations.
