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Water and substrate

Rockwool Slab Irrigation

Rockwool blocks and slabs: volume of the substrate, flow rate of the drippers, drainage, root establishment, P0–P3 irrigation phases and root-zone EC.

Water and substrate21 sources~18 min to read
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Purpose and scope

Use the measured flow rate of the drippers to calculate the quantity of water to supply. Make sure that the water goes into the block and that the roots go into the slab. Make sure that the runoff flows out of the tray. After these checks, use the trends of the VWC and the EC to change the irrigation.

In vegetative steering, you fill the substrate again more frequently and the dryback is smaller. In generative steering, the irrigation window is shorter and the dryback is larger. Athena and CCI give information on how to change these conditions during the growth of the crop. The ranges of numbers below are examples from their documentation.[13][16]

Terms

Definitions

Field capacityThe water content that stays in a saturated substrate after the free water drains out, in specified conditions. In a slab, the measured value changes if the properties of the substrate are different. It also changes if the shape and size of the slab, the drainage openings or the position of the sensor are different.
Absolute drybackThe quantity by which the volumetric water content (VWC) decreases from the peak to the trough, in percentage points. When the VWC decreases from 70% to 50%, the dryback is 20 points.
Relative drybackThe absolute dryback divided by the start VWC, multiplied by 100. When the VWC decreases from 70% to 42%, the absolute dryback is 28 percentage points. The relative dryback is 40% of the start value.
P0 / P1 / P2 / P3P0 is the time from lights-on to the first irrigation. P1 is the phase that fills the substrate again. P2 is the phase with the maintenance irrigations. P3 is the time from the last irrigation to the next lights-on. The total dryback between the irrigation windows includes P3 and the P0 after it. The terms are not the same in all controllers.
Root-zone ECYou measure the feed EC and the runoff EC in the solution. You measure the bulk EC through the substrate. You can calculate an estimate of the pore-water EC from the data of the sensor. Compare readings of the same measurement method at almost the same VWC. The unit is mS/cm. The number is the same as the number in dS/m.[18]
RunoffThe drain volume divided by the applied volume, as a percentage. Catch the runoff from some typical plants or slabs in each irrigation zone. Do not use the pump time to calculate it.

Calculated example

Absolute dryback and relative dryback

Absolute dryback and relative drybackWhen the VWC decreases from 70% to 50%, the absolute dryback is 20 percentage points. The relative dryback is approximately 28.6% of the start water content. These values are only an example. To find the field capacity, make a different measurement of the drainage.Example VWC readingsPeak 70%Trough 50%70 − 50 = 20percentage points20 ÷ 70 × 100≈ 28.6% of startVWC = volume of water ÷ volume of substrate.The bars show measured VWC values.
When the VWC decreases from 70% to 50%, the absolute dryback is 20 percentage points. The relative dryback is approximately 28.6% of the start water content. These values are only an example. To find the field capacity, make a different measurement of the drainage. Move the diagram to the side to see the labels at full size.
Quality of the information

Evidence and limitations

The sources are journal papers, manufacturer instructions and grower reports. The product instructions are for the specified products. The irrigation procedures in the reports are for the facilities in the paper.

CORPUS / INFERRED: the diagrams, the calculated examples and the assembled irrigation ranges are only for information. OPERATIONAL: controller values must come from a specified local procedure or from a measured crop record. This paper gives no setpoints that are correct for your location.

Equipment

Slab layout and flow rate of the drippers

Shot time = necessary volume ÷ measured flow rate. Each table is 7.6 × 1.2 m and has three rows of seven slabs. Each slab has three plants. Thus each table has 63 plants and seven lights above the plants, with nine plants for each light. Three tables have 189 plants.

Substrate
Rockwool blocks of 15 cm (6 in), approximately 3.6 L (0.95 gal), on slabs of 1 m (39 in)
Volume of the root zone for each plant
Approximately 7.35 L (1.9 gal) for each plant. The root zone of each plant is a block of 3.6 L (0.95 gal) plus one third of a slab of 11.25 L (3.0 gal). The volume of the slab is a value that this paper uses to calculate the root zone.
Water supply to the block
Pressure-compensating drippers, short feed tubes, and a ring on the top of the block. You can use a different distributor that makes a wide wet area, if you do a catch test of it.

Example dimensions

Three-block slab cross-section

Three-block slab cross-sectionSide section of a slab with three plants. The substrate is continuous below each block, and the roots are only a symbol. The dimensions of the slab give 11.25 L, and a cube of 150 mm is 3.375 L. For the block, this paper uses a different value of 3.6 L for the volume of the product. The drain openings must agree with the procedure for the wrapper and the product that you select. [2]SECTION ALONG THE SLAB · example size1,000 mm slab length3 blocks go on openings in the wrapper.Roots go from the block into the slab, a solid substrate.Slab example: 1,000 × 150 × 75 mm = 11.25 LBlock example: 150 mm wide. Measure the volume.
Side section of a slab with three plants. The substrate is continuous below each block, and the roots are only a symbol. The dimensions of the slab give 11.25 L, and a cube of 150 mm is 3.375 L. For the block, this paper uses a different value of 3.6 L for the volume of the product. The drain openings must agree with the procedure for the wrapper and the product that you select. [2] Move the diagram to the side to see the labels at full size.

Specified layout · the heights are not accurate

Three-row slab layout and LED alternatives

Three-row slab layout and LED alternativesEach table has three rows of seven slabs, with 63 plants for seven top lights and nine plants for each light. Two air socks with holes go between the rows, and a 200 mm fan supplies each air sock. Alternative A has two LED lines along the table above the air socks, and alternative B has bars across the table at equal intervals. The seven bars show the spacing and are not a specified number of fixtures. In the two alternatives, the LEDs send light up below the canopy. Install the fixtures on supports that do not touch the air socks. Examine the clearance of the inflated air socks. After you put seven slabs of 1 m on the table of 7.6 m, a space of 300 mm stays at each end. The dimensions and the light areas show the specified layout. Measure the light at the canopy and the airflow.THREE ROWS · 7.6 m × 1.2 m table21 slabs × 3 plants = 63 plants for each table7 top lights × 9 plants for each lightA · LEDs along the table above the air socksB · LEDs across the table at equal intervalsA 200 mm inlet fan supplies each air sock.Dashed lines = top lights. Gold lines = under-canopy LEDs.CROSS-SECTION · layout AContinuous canopyairairSmall LED bars in the center above each air sock.Keep the fabric clear of the fixtures and their supports.
Each table has three rows of seven slabs, with 63 plants for seven top lights and nine plants for each light. Two air socks with holes go between the rows, and a 200 mm fan supplies each air sock. Alternative A has two LED lines along the table above the air socks, and alternative B has bars across the table at equal intervals. The seven bars show the spacing and are not a specified number of fixtures. In the two alternatives, the LEDs send light up below the canopy. Install the fixtures on supports that do not touch the air socks. Examine the clearance of the inflated air socks. After you put seven slabs of 1 m on the table of 7.6 m, a space of 300 mm stays at each end. The dimensions and the light areas show the specified layout. Measure the light at the canopy and the airflow. Move the diagram to the side to see the labels at full size.

Dripper layouts and shot time

A shot is one irrigation event. This paper uses a volume of 7.35 L for each plant to calculate the shot volume. With this volume, 3% is 220.5 mL and 5% is 367.5 mL. The table uses the specified flow rate of the drippers. Replace it with the volume that you collect, divided by the time in which you collect it.

Component diagram

Drip irrigation components

Drip irrigation componentsThe supply flows through the dripper and the microtube to a distributor on the block. The distributor in the drawing is a symbol. The number of outlets and the dimensions are different for different products. Examine the model of the ring or stake, the operating pressure and the fit on the block. Make sure that they agree with the documentation of the product. Measure the total output that each plant gets. [19]SUPPLY, DRIPPER AND DISTRIBUTORLateral with pressurePressure-compensating dripperMicrotube → distributorBlockThe distributor is only a symbol. Use a ring or stakes.Examine the fit and the connections. Catch the total flow.
The supply flows through the dripper and the microtube to a distributor on the block. The distributor in the drawing is a symbol. The number of outlets and the dimensions are different for different products. Examine the model of the ring or stake, the operating pressure and the fit on the block. Make sure that they agree with the documentation of the product. Measure the total output that each plant gets. [19] Move the diagram to the side to see the labels at full size.
Usual dripper layouts for each plant. The times are rounded to the nearest second.
LayoutTotal specified flow rate3% shot · 221 mL (7.5 fl oz)5% shot · 368 mL (12.4 fl oz)Use and effect
1 × 2 L/h2.00 L/h6:3711:02One outlet with a low flow rate. The shots are long. If this dripper does not operate, no other dripper supplies water.
2 × 2 L/h4.00 L/h3:185:31Two outlets. Use this layout if the hardware on the top of the block gives a different wet area for each outlet.
1 × 4 L/h4.00 L/h3:185:31An easy layout: one dripper supplies a ring. If this dripper does not operate, the ring has no water.
2 × 4 L/h8.00 L/h1:392:45The shots are short. Examine the ramp of the pump, the pressure regulation, and the minimum valve time at which the valve operates correctly.
2 × 0.3 GPH2.27 L/h5:509:43A usual pair of drippers in GPH, with a low flow rate. If one dripper does not operate, the other dripper continues to supply water. The time to make the block wet is long.
2 × 0.5 GPH3.79 L/h3:305:50A usual pair of drippers in GPH. The total flow rate is almost 4 L/h.
Shot time in seconds = shot volume in mL ÷ (total measured flow rate in L/h × 1000 ÷ 3600). At 4 L/h for each plant, the flow rate for the 63 plants of a table is 252 L/h. If all three tables irrigate at the same time, the flow rate is 756 L/h.

Measure and calculate

Dripper flow rate and irrigation time

Dripper flow rate and irrigation timeAt typical outlets across the active zone, use vessels with labels and volume marks. Or use a container on a scale with the tare set. Compare the volumes that you collect in the same time at the operating pressure. The vessels in the drawing show where you collect the water. The shot time uses 220.5 mL and not a rounded value. It also uses a volume of 7.35 L for each plant. Replace the volume and the flow rate with values that you measure.COLLECT FOR THE SAME MEASURED TIMENearMiddleFarRecord the mL at each outlet. Write the time of the test.Example of shot time7.35 L × 0.03 × 1,000 = 220.5 mL220.5 ÷ (4,000 ÷ 3,600) ≈ 198 s = 3:184 L/h is an example total, measured for each plant.
At typical outlets across the active zone, use vessels with labels and volume marks. Or use a container on a scale with the tare set. Compare the volumes that you collect in the same time at the operating pressure. The vessels in the drawing show where you collect the water. The shot time uses 220.5 mL and not a rounded value. It also uses a volume of 7.35 L for each plant. Replace the volume and the flow rate with values that you measure. Move the diagram to the side to see the labels at full size.
Water movement

Movement of water between the block and the slab

The block above a wet slab can stay dry. Examine the block, and not only the slab, during root establishment.

Water moves through connected rockwool along hydraulic gradients. After drainage, the top areas can keep less water. The properties of the substrate, the irrigation and the uptake of water by the roots cause the water distribution.[5][6][7]

When the fibers of the block and the fibers of the slab touch fully, water can move between the block and the slab. The staged-drainage method of Grodan gives information on the drying of the block that the slab causes. It also shows that, if the slab keeps water at the start, this effect can be smaller. The drainage opening changes the quantity of water that the slab keeps and the drainage of the block.[3]

Cross-section without numbers

Block and slab water distribution

Block and slab water distributionWater moves between the block and the slab through their contact face. When the media drain, the top media can become drier and the slab can stay wet. The irrigation and the uptake also change the profile. The profile is different for different irrigation and drainage conditions. In its staged-drainage method, Grodan shows that water that the slab keeps changes the drying of the block. [3]SECTION · connected media that drainBlock feedWater movementPlant uptakeDrain boundaryColor = water content, without numbers.The colors show where there is more or less water.Water in the slab and outlet height change the profile.
Water moves between the block and the slab through their contact face. When the media drain, the top media can become drier and the slab can stay wet. The irrigation and the uptake also change the profile. The profile is different for different irrigation and drainage conditions. In its staged-drainage method, Grodan shows that water that the slab keeps changes the drying of the block. [3] Move the diagram to the side to see the labels at full size.

TDR-Sensor position drawing

MT22 sensor position and paper template

MT22 rockwool slab sensor positionViews from the top, the end and the side. Three rods go into the long side of the slab, near the middle block, in a horizontal direction. The long housing of the sensor goes along the length of the slab. Use the PDF to make a paper copy at 100%. MT22 in a Grodan Prestige slab Three plants for each 1 m slab | Recommended middle position, the same each time TOP VIEW Example position near the middle block. Not below the block. PLANT 1PLANT 2PLANT 3 Long housing along the slab length Rods are horizontal and go across the slab width. Keep clear of drain holes and the slab end. END CROSS-SECTION Only one pin is in view from the end. 53 mm Housing flat against rockwool No rods in the air. No air gaps. 75 mm slab: 37.5 mm above bottom 100 mm slab: 50 mm above bottom SIDE FACE / ALIGN THE TEMPLATE MT22 Zero height = bottom of the rockwool, not the tray edge. All three pin centers on one horizontal line Housing contact face: 88 mm long x 26 mm high Use the pin spacing of your sensor. Remove the paper. Then put the sensor in the rockwool. The rod spacing here is an example. It is not a dimension. Make a paper copy of the A4 template at 100% ("Actual size"). Measure the two 100 mm bars. Manufacturer data: housing dimensions and rod length. We recommend: middle height and middle position. The sizes in this drawing are not accurate. The PDF template has 1:1 marks to align the sensor.
MT22: put the sensor into the long side of the slab, near the middle block and not below the block. The housing is 88 mm long, and it goes along the slab. The three rods of 53 mm go into the slab across its width, all at one height. Put the housing flat against the slab and hold the cable. The template gives centerline heights of 37.5 mm for a slab of 75 mm and 50 mm for a slab of 100 mm. The heights are from the bottom of the substrate. The heights are positions to compare, and you must make sure that they are correct in your facility. Get the A4 template of two pages. Make a paper copy of the template at 100% (setting: "Actual size"). Measure the two scale bars. Make sure that each bar is 100 mm. Write the spacing of the pins of the sensor on the template. Open the drawing at full size · Notes on the position of the TDR-Sensor · AGPL-3.0 license. Move the diagram to the side to see the labels at full size.
Prepare the slabs

Slab saturation and drainage openings

Saturate the slabs equally in all parts. Make the drainage openings. Put each block directly on the fiber of the slab, with no plastic between the block and the fiber.

  1. Examine the surface below the slab

    Before you put the slabs on the tray, examine the tray and the direction in which the runoff flows. Make sure that the surface below each slab is the same for all slabs. Let the runoff flow to the runoff container.

  2. Saturate the slab with the nutrient solution

    Saturate the slab with the nutrient solution through the specified openings. Record the EC and the pH. The staged-drainage method of Grodan gives an instruction: keep the slab saturated for a minimum of 24 hours.[2][3]

  3. Select the drainage procedure

    In the staged-drainage method of Grodan, you first open the wrapper above the seal. Then only a part of the water drains. After this, you make the openings below the first opening. Obey the procedure of the product that you select for the position and the time of each opening.[3]

  4. Open the wrapper at the position of each block

    Obey the instructions on the wrapper. Then each block goes on the fiber of the slab, with no plastic below the contact face of the block.[2]

  5. Put the blocks on the slab and examine them

    Put each block, with the roots of its plant in the block, on the slab. Make sure that the fibers of the block and the slab touch on a flat face. Examine the water content of the blocks and the roots. Do this before you use the procedure for root establishment that you select.

Root establishment

Irrigation after transplanting

At the start, the roots are in the block of the transplant. The irrigation must keep this block wet while the roots become longer and go into the slab.

The volume of 7.35 L for each plant, which this paper uses to calculate the shot time, includes slab volume. It is possible that the new roots are not in this volume at this time. Set the irrigation at the start from the water content of the block and the growth of the roots.

  1. Record the condition at the transplant

    At the transplant, record the water content of the block and the VWC of the slab. Also record the feed EC, how the block touches the slab, and the roots that you can see.

  2. Examine the water content of the block

    Make sure that the water goes into the block and makes it wet equally in all parts. Examine the block between the shots and after the irrigation.

  3. Examine the growth of the roots

    Examine the roots at the interface of the block and the slab. Record if the water uptake increases when the roots go into the slab.

  4. Do a check of root establishment

    Use the cycle of each day for plants after root establishment. Use it when the roots are in the slab and the measured uptake is stable. Before you increase the irrigation, make sure that the water supply and the drainage are correct.

Drawing of root growth

Root growth from block into slab

Root growth from block into slabThe drawing shows the same assembly in three stages of root establishment. The roots are in continuous substrate. The drawing shows many roots because the example is a plant from a clone. The roots and the water uptake show when the crop can go to the cycle of each day for plants after root establishment.Roots in blockRoots at interfaceRoots in slabThe block–slab assembly is the same in all stages.Examine roots, block water content and stable uptake.The growth rate changes with the crop and conditions.
The drawing shows the same assembly in three stages of root establishment. The roots are in continuous substrate. The drawing shows many roots because the example is a plant from a clone. The roots and the water uptake show when the crop can go to the cycle of each day for plants after root establishment. Move the diagram to the side to see the labels at full size.
Irrigation each day

P0–P3 irrigation phases

Phases of the irrigation each day after root establishment.
PhaseTaskInformation
P0Transpiration from lights-on to the first irrigationUse the measured dryback and the water content of the block to start the first irrigation. Keep the VWC more than the minimum VWC that you set for the crop.
P1Fill the substrate again without channelingThe example procedure uses shots of 2 to 6% of the substrate volume. Shots of 3 to 5% are frequent. The interval between shots is 15 to 30 minutes. These ranges are start values. Make sure that they are correct in your facility.
P2Keep the VWC stable and control the root-zone ECReplace the water that leaves the substrate during the light period. Drainage removes dissolved salts. Measure the EC and the runoff to see the effect.
P3From the last irrigation to the next lights-onThe dryback starts after the last shot, frequently before lights-off. In this paper, P3 stops at the next lights-on.

Example VWC data

Irrigation phases each day

Irrigation phases each dayAn example of 24 hours, with a light period of 12 hours, shows the phases. P0 is the time from lights-on to the first irrigation. P1 fills the substrate again, and P2 keeps the VWC stable. P3 is the time from the last shot to the next lights-on. The total dryback between the irrigation windows includes P3 and the P0 after it. The values and the times are only an example.EXAMPLE DAY · hours after lights-onP0P1P2P3455565VWC(%)04812162024LightNightP3 starts after the last irrigation, before lights-off.In this paper, P3 stops at lights-on. Then P0 starts.Blue lines = shots. The values are an example.
An example of 24 hours, with a light period of 12 hours, shows the phases. P0 is the time from lights-on to the first irrigation. P1 fills the substrate again, and P2 keeps the VWC stable. P3 is the time from the last shot to the next lights-on. The total dryback between the irrigation windows includes P3 and the P0 after it. The values and the times are only an example. Move the diagram to the side to see the labels at full size.

Make a graph of the VWC, the shots and the EC on the same time axis. Write the EC method on the graph. Record the position of the sensor, the calibration and the growth stage of the crop.[18]

Growth of the crop

Irrigation for each growth stage

These example ranges are from the information of Athena and CCI together. Before you compare EC values, make sure that the measurement methods are the same. Before you use dryback targets, set a minimum VWC for the crop and the substrate.[13][16]

Example ranges for plants after root establishment in rockwool. The dryback is in percentage points of VWC. To compare EC values, use the same measurement method.
StageSteeringPeak and runoffDryback in the controllerRoot-zone EC (mS/cm, for one measurement method)Signal to change
Vegetative stage after root establishmentVegetativeThe peak is equal to or more than the measured field capacity. The runoff is 8 to 16%.10 to 15 points3-5The roots are in the slab, the water uptake is stable, and you can change the light cycle for the plant.
Flower set, approximately days 1 to 21GenerativeThe peak is equal to or less than the field capacity. The runoff is 1 to 7%.Start at approximately 15 points. During the first three weeks, increase the dryback to 20 to 25 points. Do not let the VWC become less than the minimum VWC that you set in your facility.5-10The vertical stretch becomes clearly slower or stops.
Flower bulking, approximately days 22 to 42VegetativeThe peak is equal to or more than the field capacity. The runoff is 8 to 16%.10 to 15 points3.5-6The flowers expand more slowly, and the signals of ripening are the primary signals.
Last stage, usually the last 10 to 14 daysLower EC and generative drybackThe peak is equal to or less than the field capacity. The runoff is 1 to 7%, but not when you correct an EC that is too high.At the start, use 20 to 25 points. Use a larger dryback only if you have data for the cultivar. Keep the VWC more than the minimum VWC.3-4Signals of maturity for the cultivar
The days are approximate. Record the stretch, how the flowers expand and the maturity. Use these records to find the changes of stage.
From the podcast

Measurements of a slab with three plants

In episode 31 of We The Growers, Sipkoi tells how to collect the irrigation water and the runoff from one slab with its three plants. When you divide the quantities by three, the result is the average for each plant. The average cannot show which dripper has a blockage.[15]

Setup diagram

Collect slab runoff

Collect slab runoffThree blocks are on one slab above a runoff tray. Gold shows how all the drainage flows to a container with volume marks, and blue shows the feed tubing. The positions of the slits are different for different products and drainage procedures. Catch all the drainage, and do not let the slab stay in the runoff in the tray. When you divide the slab totals by three, the result is an average and not a measurement for one plant. [15]SIDE VIEW · all runoff from one slabSupplySlab above the runoff trayAll slab drainage → tray → outlet → containerRunoff % = collected drain mL / applied mL × 100Use the same slab and interval for the two values.
Three blocks are on one slab above a runoff tray. Gold shows how all the drainage flows to a container with volume marks, and blue shows the feed tubing. The positions of the slits are different for different products and drainage procedures. Catch all the drainage, and do not let the slab stay in the runoff in the tray. When you divide the slab totals by three, the result is an average and not a measurement for one plant. [15] Move the diagram to the side to see the labels at full size.
Procedure from the podcast, for that facility only

After approximately 16 days in the vegetative stage, the plants go from blocks of 10 × 10 × 6.4 cm (4 × 4 × 2.5 in) to slabs with saturated substrate. One slab is above a runoff tray. Personnel divide the applied volume and the drained volume by the three plants of the slab. They record the volume in and the volume out each day. They measure the pH and the EC of the runoff one or two times each week.

In the first 48 hours on the slabs, the start procedure in the podcast is approximately 12 shots during the lights-on period. Each shot is two minutes through an outlet of 1.9 L/h (0.5 GPH). Then the grower stops the irrigation for a period of three to four days. After this, the grower increases the irrigation to the full quantity. At approximately day 14, a typical procedure in the podcast is 10 to 12 shots of four minutes. The grower changes the frequency for the quantity of water that the cultivar uses and for the drainage that the grower sees.

The podcast gives information on how to put the slabs in position at 27:33-28:12. It gives information on how to collect the runoff from a slab at 31:43-32:33. It gives information on the first irrigation of flowering at 39:44-41:12. It gives information on the change of the irrigation in the first three weeks at 48:55-53:37.[15]

Control of nutrients

Root-zone EC and nutrition before harvest

Before you correct a high EC, examine the measurement method and the water supply. Do the product procedures before harvest and the diagnosis of salt that collects in the substrate as two different tasks.

  1. Examine the EC measurement

    Find the type of the EC. It can be the feed EC, the runoff EC, the bulk EC, or an estimate of the pore-water EC. Examine the calibration. Make sure that the probe touches the substrate correctly. Compare the readings of the substrate at almost the same VWC.[18]

  2. Salt that collects in the substrate

    If readings that you can compare continue to increase, measure the feed EC. Also measure the volume that the system supplies and the drainage that you collect. Drainage removes dissolved salts. After you change the feed or the irrigation, examine the next readings of the runoff and the substrate.

  3. Athena Fade procedure

    In the nine-week example of Athena, Fade replaces Pro Core in weeks 8 and 9, and Pro Bloom continues. The example gives these instructions: keep the feed EC, and measure the pH after Fade replaces Pro Core. This example is a procedure for these products. The example does not show that this procedure is necessary for all cannabis in rockwool.[17]

  4. Last irrigation step of Athena

    On the last day in rockwool, the same example uses Cleanse in water from reverse osmosis. The example also uses measurements of the EC to see the result. This procedure does not show if the quality of the flower becomes better when you flush the substrate before harvest.[17]

Environment

Climate and water uptake

Light, temperature, humidity and air movement have an effect on the transpiration. After a change of the climate or the lighting, compare three values with the values before the change. The three values are the rate at which the VWC decreases, the volume of irrigation and the runoff. Measure at the near end, the middle and the far end of the table. If one area dries more quickly, examine the local climate and the supply from the drippers. Do this before you make a change for all of the zone.

Plant clearance and lollipopping at the start

In this layout, the plants must be sufficiently high to keep the foliage and branches clear of the bars and their supports. At the start, use lollipopping to remove the bottom growth that can touch the fixtures. Also remove the bottom growth that touches the air socks or fills the space below the canopy. Keep the foliage that makes the crop above the lights. When the branches become wider and the flowers become heavier, examine the clearance again. Use the minimum crop clearance of the fixture manufacturer.

Foliage that touches a fixture, or that is very near to a fixture, can have local damage from heat or light. Bottom growth with a high density also causes a blockage of the airflow, and it is not easy to examine the plants. If the plants are too short for the bars, do not use the bars until the plants are higher. You can also put the bars in a different position.

How to move light from the top to the bottom of the canopy

You can use under-canopy lighting to move part of the lighting power from the top of the canopy to the bottom. Decrease the setting of the top lights in steps. At the same time, measure the light through the crop. The same electrical watts give the same total lighting power. But the efficacy, the spectrum and the light interception are not the same for all fixtures. As a result, the same watts do not always give the same photon output or the same effect on the crop.

You measure PPFD at one surface and in one direction. It is not correct to subtract a reading below the canopy from a reading above it. Measure the PPFD of the top, middle and bottom foliage, with the same sensor positions and directions each time. Also include the photoperiod in the results. The tests in cannabis add under-canopy light to top light. These tests do not show that the same quantity of bottom light always replaces top light.[20]

Heat and root-zone temperature

When you add lighting power, the heat that the room must remove increases. If you decrease the watts of the top lights by the same quantity, the total lighting power stays almost the same. But the heat is then near the slabs. The air socks move the heat to other areas, but the cooling system or the exhaust must remove it. Keep the fixtures away from the wrappers of the slabs and let air flow around the bars. Make sure that the cooling surfaces of the bars are clear.

Record the temperature in typical slabs with roots. Include positions near the bars and positions with a lower temperature to compare. Also record the air temperature and the leaf temperature in the bottom canopy. A room sensor above the crop cannot always find an area with a high temperature below it. Compare the temperature trend for the full lights-on period before and after each change of the lighting. Also compare the VWC, the EC and the runoff.

When the root zone is too hot, the roots and the microbes use more oxygen, and warm water holds less dissolved oxygen. In rockwool, the pores that contain air and the drainage also supply the oxygen. Thus a slab that is warm and stays wet for a long time can give the roots oxygen that is not sufficient. The uptake of water and nutrients can decrease. As a result, the plant can show wilt although the substrate is wet, and the growth can be unsatisfactory and the yield can be lower. Warm conditions can help the growth of some species of Pythium, but only when the pathogen is in the root zone.[21]

If the slab temperature is more than the limit for the crop, stop the under-canopy bars or decrease their light. Do the same if the plants show damage after the change of the lighting. Examine the cooling, the clearance, the airflow and the drainage. Examine the roots and make sure that the condition of the plants becomes good again. More irrigation without a check of the cause can decrease the oxygen in a slab that is wet.

Diagnosis

Irrigation faults

Irrigation symptoms, possible causes and checks.
SymptomPossible causesFirst step
The runoff is high and the VWC almost does not increaseChanneling, a block that does not touch the slab correctly, or an incorrect position of the dripperExamine the block and the slab by hand. Use a slower shot, or make the block wet again by hand with balanced feed. Do not increase the volume before you know the cause.
The root-zone EC increases each day and is not in the band for the stageThe runoff does not remove sufficient salt, the dryback is too large, the feed is not correct, or the plants use too much waterMake sure that the VWC is the same for the readings that you compare. Examine the feed EC and the climate. Then add P2 shots or runoff, with one change that you record.
The root-zone EC is less than the targetThe peak and the runoff are too high for the type of steeringDecrease P2 or the peak by a small quantity. Monitor the result for one light cycle.
The VWC does not increase to the P1 targetThe target is more than the peak VWC that this slab can make. A dripper has a blockage. The flow rate that you use to calculate the shot is incorrect. The ramp has a number of shots that is too small.Do a catch test. Measure the peak VWC that this slab can make and the substrate volume. Then change the model.
New transplants do not become larger while the slab stays wetThe water content of the block is not sufficient. The block does not touch the slab correctly. The water supply does not operate. The roots have damage.Examine the block, the roots, how the block touches the slab, and the flow of the outlet that you catch. Do not use the slab sensor as the only trigger.
The dryback suddenly becomes largerA shot did not occur, or the plants use more or less water because of a change of PPFD, VPD, CO2 or airflowExamine the records and the climate first. Add P2 shots only after you find the cause.
One plant wilts and the plants near it have the usual valuesOne dripper does not operate, or the block does not touch the slab correctly at one positionMake the flow correct again. If it is necessary, make that block wet by hand with balanced feed.
Records

Record of the irrigation each day

Record the values for each irrigation zone and each sampling position.
RecordInclude
Crop and substrateCultivar, growth stage, product and dimensions of the block and the slab, number of plants for each slab
Water supplyFlow rate for each plant that you catch, time of each shot, number of shots, first and last irrigation
Water contentPeak VWC and VWC before the irrigation, dryback in percentage points, position of the sensor
DrainageApplied volume and drained volume from the same slab in the same interval, and the runoff percentage
EC and pHResults for the feed and the runoff. EC method, VWC and time of sampling for the substrate.
Under-canopy lightingSettings and watts of the top fixtures and the bottom fixtures. Position and direction of the PPFD sampling. Slab temperature near the bars and at positions to compare. Leaf temperature and clearance.
Changes and faultsShots that did not occur, outlets with a blockage, water that you apply by hand, and changes of the climate. The correction and the effect that follows.

References

  1. Grodan (ROCKWOOL Group). The right block–slab interaction ensures healthy plants. Grodan crop guidance. (source not from a journal, for example from a manufacturer). https://www.grodan.com/global/crops/sweet-pepper/the-right-block-slab-interaction-ensures-healthy-plants/
  2. Grodan (ROCKWOOL Group). Handling and placing of the slabs. Technical sheet TS 3.3. (source not from a journal, for example from a manufacturer). https://www.grodan.com/syssiteassets/downloads/tools--services/english/ts-3-3-handling-the-slabs-en.pdf
  3. Grodan (ROCKWOOL Group). Cutting drainage holes in stages is good for plants and saves water. Grodan knowledge base. (source not from a journal, for example from a manufacturer). https://www.grodan.com/global/knowledge/root-zone-management/irrigation-and-nutrients/Cutting-drainage-holes-in-stages-is-good-for-plants-and-saves-water/
  4. Grodan (ROCKWOOL Group), with B. Nikaj; trials with Wageningen University & Research (2020–2022). Grodan research reveals new insights into optimal irrigation strategy for large-scale production of medicinal crops. Whitepaper. (source not from a journal, for example from a manufacturer). https://www.grodan.com/
  5. Bougoul S, Boulard T (2006). Water dynamics in two rockwool slab growing substrates of contrasting densities. Scientia Horticulturae 107(4):399–404. https://doi.org/10.1016/j.scienta.2005.11.007
  6. Bougoul S, Ruy S, de Groot F, Boulard T (2005). Hydraulic and physical properties of stonewool substrates in horticulture. Scientia Horticulturae 104(4):391–405. https://doi.org/10.1016/j.scienta.2005.01.018
  7. da Silva FF, Wallach R, Polak A, Chen Y (1998). Distribution of nutrients and water in rockwool slabs. Scientia Horticulturae 72(3–4):277–285. https://www.sciencedirect.com/science/article/abs/pii/S0304423897001441
  8. International Society for Horticultural Science (ISHS). Utilizing the HYDRUS model as a tool for understanding soilless substrate water dynamics. Acta Horticulturae 1168. https://www.ishs.org/ishs-article/1168_41
  9. Caplan D, Dixon M, Zheng Y (2019). Increasing inflorescence dry weight and cannabinoid content in medical cannabis using controlled drought stress. HortScience 54(5):964–969. https://doi.org/10.21273/HORTSCI13510-18
  10. Malik M, Tlustoš P (2025). Soilless growing media for cannabis cultivation. Agriculture 15(18):1955. https://www.mdpi.com/2077-0472/15/18/1955
  11. Nemali KS, van Iersel MW (2006). An automated system for controlling drought stress and irrigation in potted plants. Scientia Horticulturae 110(3):292–297. https://doi.org/10.1016/j.scienta.2006.07.009
  12. Owen J, Norden D (Profile Products). Understanding drainage in horticultural growing media. Greenhouse Management. (source not from a journal, for example from a manufacturer). https://www.greenhousemag.com/article/growing-media-defining-drainage-improve-substrate/
  13. Athena Agriculture. Precision Irrigation Strategy, metric edition, document A01.002. (information from a manufacturer) Official Athena procedure. https://support.athenaag.com/hc/en-us/articles/25975395644315-Precision-Irrigation-Strategy
  14. We The Growers podcast, E.37 — Bones Grows (ZBRA / Wow Town). Practitioner discussion of block-on-slab irrigation and root-zone management. (source from a grower, without peer review) Official episode. https://www.youtube.com/watch?v=l8WU-sRxNnI
  15. We The Growers podcast, E.31 — Sipkoi, published 13 August 2024. Slab placement 27:33-28:12; runoff collection 31:43-32:33; initial flower irrigation 39:44-41:12; first-three-week adjustment 48:55-53:37. (source from a grower, without peer review. We compared the times with the captions of the episode.) Official episode. https://www.youtube.com/watch?v=Fv2jqkHsaBM
  16. Whipple J. The CCI Black Book, first edition, 2023, Garden Management chapter, pp. 56-63. (information for commercial cultivation) Publisher. https://ccibook.com/
  17. Athena Agriculture. Fade Procedure. Replace Core with Fade for the final two weeks, feed Fade + Bloom at full EC, then use Cleanse in RO water for the final 1-3 days, one day in rockwool. (procedure for a product from a manufacturer) Official Athena Fade procedure. https://support.athenaag.com/hc/en-us/article_attachments/14949498083739
  18. METER Group. TEROS 11/12 manual. Sensor installation, measurement volume and bulk-to-pore-water EC conversion. (information from a manufacturer) Manufacturer documentation. https://publications.metergroup.com/Manuals/20587_TEROS11-12_Manual_Web.pdf
  19. Netafim. NetBow product information. Consult the selected model and assembly instructions. (information from a manufacturer) Manufacturer documentation. https://www.netafim.com/en/products-and-solutions/product-offering/drip-irrigation-products/netbow/
  20. Subcanopy and Inter-Canopy Supplemental Light Enhances and Standardizes Yields in Medicinal Cannabis (Cannabis sativa L.). Plants 14(10):1469 (2025). Research paper. https://doi.org/10.3390/plants14101469
  21. Sutton JC et al. (2006). Etiology and epidemiology of Pythium root rot in hydroponic crops: current knowledge and perspectives. Summa Phytopathologica 32(4). Evidence from hydroponic crops; not a cannabis-specific temperature threshold. Research review. https://doi.org/10.1590/S0100-54052006000400001

The Cannabis White Papers · CC BY-NC 4.0 · version 1.2 · updated 2026-07-18