In this article we will look at crop steering with rock wool using precision irrigation methods for saturation and dry back. Crop steering is the maximizing of growth rates, minimizing nutrient waste, pushing a crops potential for aromas, yields, essential oils, etc. Rock wool over all other mediums offers this much precision at the growers fingertips.

Rockwool is unique in design and function. Volcanic rock is melted down into thin quasi dense fibers making it easy to wick water and hold a lot of water. So unlike other grow mediums such as coco, or hydroton, rockwool requires short burst of water to allow a gradual saturation followed by dry back during the night cycle. Timer cycles can even be less then a minute in some instances. So precision timers and higher PSI pumps are highly recommended.

The following examines crop steering methodology during the flowering regenerative cycle. The key is getting the dry back right during the night cycle.

The science of crop steering

Ideal irrigation cycle for rock wool. Notice how staggered on/off watering reaches peak saturation (drain/run off cycle at P2 cycle. Notice of the EC gradually increases in the stone wool, then drops when saturation (field capacity) is reached.

Crop steering (generative i.e flowering cycle)

The first watering cycle starts about 2-4 hours after the plants first receive light to begin just saturating the block slowly as the plants wake up (transpiration before saturation). The goal in the next 2-3 hours is to slowly reach saturation, then maintain that level to achieve minute runoff. The graph provided here shows in blue the irrigation slowly building up to full saturation until P2. The red line is the EC value of the rock wool medium. Now the rock wool slowly pushes out yesterdays salts achieving a trickle flush of yesterdays nutes. Up to 0-2 hours before the night cycle watering is stopped to allow the rock wool to almost completely dry out until the next morning. When this is done correctly water run off from the rock wool is less than 10% and the roots in the block will fill the rock wool evenly.

Ideal crop steering irrigation for rock wool in regenerative cycle.

The irrigation system

Watering rock wool requires some monitoring and measuring the rock wool saturation and dry back times as the plants mature. This is a moving target and is also greatly effected by the growing environment. 

Top feed drip is much more accurate way to monitor instead of ebb and flow watering methods however either way can be done. It is recommended to use a short interval timer for very short burst of water anywhere from half a minute up depending on environment and crop, and desired crop steering.

Manifolds best achieve equal pressure to each site when built as a loop instead of capping off a manifold line at one end. Use a good quality pump that can handle  back pressure of flow restrictions to feed emitters (.5 gph is recommended). Cube caps available today allow water to evenly pass through the top reducing salt channels.

Higher grade pumps rated by PSI instead of gallons per hour (GPH), will give quick accurate delivery of nutrient solution to the block.

So how do you determine the cycle?

Easiest Grodan has a real time meter called the GroSens but without this meter there are short cut methods to  measure out how to time your water cycles. It is easier to tune a top feed drip irrigation as opposed to ebb/flow so please keep that in mind.

Take one drip emitter from the table and put it in beaker to collect water that will collect water for that days cycle and compare this to the entire volume of water that runs off for that day for the remaining drip sites. Divide the total volume of run off by the number of sites, thus giving the volume of run off per cube. The dripper with no cube collecting straight water should equal something around 10 times the volume of water passing through each cube on a tray.

For instance I have 48 sites and I collected 1440 milliliters of water. 1440 divided by 48 = 30 militers per plant. If the sizing is right here, the dripper collecting water with no cube should be roughly 300 ml. Or about 10% of 300ml is 30 ml. If you are getting much more run off adjust duration and frequency to be lower. If your cubes still are not getting enough dryback the next day, use a smaller cube rock wool medium. Sounds counter intuitive sure, but even a 4” cube can hold a 7 foot plant!

In conclusion rock wool is about ultimate control in crop steering and is not suitable in instances where someone cannot put in the irrigation infrastructure for precision. This means timer cycles that go down to increments of rock wool will work even if not applying proper irrigation methods, but with proper saturation and dry back methods rockwool is worth taking on the learning curve.

Have a commercial operation using or wanting to use rockwool? Grodan has a team of technichians that can help dial in an operation. Call now or email for more information.