Building & Running a Drip System with Rockwool Slabs

If anyone is focused on yields it’s commercial growers.  And rockwool slabs and drippers are the mainstays of many professional greenhouses.  But what about growing using a “commercial” style, high-yielding hydroponic set-up at home?  Gareth Hopcroft takes us through how to build your own rockwool slab dripper system and then shows us how his plants develop from germination right through to harvest! Gareth also shares details on how to gauge and respond to the changing needs of quick growing, light-loving annuals.

Whether or not you’re a tomato lover, there are plenty of important hydroponic lessons to be learnt here.  Enjoy!

THE BASICS

What is a Dripper System?

A dripper system is a method of automating the irrigation of your plants.  At the heart of the system is a pump.  This is placed inside a nutrient reservoir attached to some piping.  This piping comes out of the reservoir and runs near to the plants. It’s terminated with a stopper or looped back into itself to form a “ring” or “circuit” so that when the pump is switched on it creates pressure throughout the pipe.  Narrower tubes are inserted into the piping. Pressure from the pump pushes the nutrient solution through the narrower tubes and into “dripper stakes” which are inserted into the growth media and deliver the nutrient solution to the root zone.

Why Drippers?

Drip irrigation is a flexible way of watering your plants. You can easily switch between draining to waste or re-circulating your nutrient solution and you can choose from a wide variety of growing media.  But perhaps best of all, you have total control over your nutrient solution and irrigation and therefore, the growth of your plants.

Why Rockwool Slabs?

Rockwool slabs might seem a bit alien to some of you dirt-loving gardeners but there’s a reason they are such a firm favorite among commercial growers.  Rockwool slabs don’t take up much space and they yield like crazy!  They are also easy to use and boast amazing moisture and air retention.

Rockwool Basics

Rockwool is used extensively in commercial agriculture worldwide to grow a variety of vegetable crops – especially heavy fruiting plants such as tomatoes, peppers and.cucumbers. Its amazing ability to hold nutrient solution as well as a minimum of 20% air even when saturated creates a fantastic environment in which roots can thrive.

Rockwool is an inert growing medium (meaning that it doesn’t contain any nutrients in itself) made up of thin, strand-like fibers made from volcanic rock.  It is available in different forms to cater directly to the different stages of plant development.  Propagation of cuttings and seedlings usually takes place in 1.5” cubes. Once the plant starts to develop the whole cube is placed into a larger 3” or 4” block.  The fact that smaller cubes are designed to fit into larger blocks allows for quick, easy, and stress-free transplanting.  Once the plant is established within a larger block it can then be placed on top of a rockwool slab in order to reach full maturity.

Why Drain to Waste?

Many growers find drain-to-waste systems easier to manage as the pH of the input nutrient solution does not fluctuate as with re-circulating systems.  While this method certainly uses more water and nutrients it can often to lead to higher yields as the plants are receiving fresh, full spectrum nutrients on each irrigation.  The run-off can be diluted and used elsewhere in the garden too.

Shopping List

30 x rockwool cubes for starting seedlings
20 x rockwool blocks for transplanting seedlings
6 x rockwool slabs
6 x rockwool slab support trays
Hydroponic nutrients
1 x run-off tank (70 gallon, 85” x 20” x 10”)
1 x 60 gallon reservoir
10’ of metal extrusion with 6 x tee pieces
13’ of ¾” supply tubing
1 x ¾” compression stop end
4 x ¾” compression elbows
1 x 1000 gallons per hour submersible pump
1 x ¾” high-pressure filter set
1 x digital timer (must have 20 programs or more)
6½ ’ of flexible ¾” hose (Tricoflex)
14 x pressure compensated drippers (2.3 quarts per hour)
1/10” hole punch
10 x cable ties
1 x black plastic corriboard sheet
1 x permanent marker
1 x scalpel
1 x kettle
Somebody on hand to make coffee as required.

Let’s Get Growing!

January 1 - Germination

New sprouts!

I’m growing a tomato variety called ‘Cedrico’ in my heated greenhouse.  It’s an old favorite of mine and very popular among exhibition growers due to its uniform round shape, disease resistance and excellent flavor.

I planted my seeds in small rockwool cubes pre-soaked in nutrient solution (1.2 E.C.) at pH 5.5.  (EC stands for Electrical Conductivity and it is a measure of the strength of a nutrient solution.)

January 11 – Transplant into 4” Rockwool Blocks
Germination was quick in my heated propagator and the plants are developing well.  I’m using one 250W metal halide grow lamp raised three feet above the plants to supplement light levels.

I propagated my seedlings in an automated flood and drain air-pruning table. An air-pruning table is simply a grid that raises the blocks slightly off the surface of the tray.  When roots grow out of the bottom of the blocks they immediately come into contact with air and ‘prune’ themselves.  This is a great way to create super vigorous seedlings.  (See “Power Propagation” – UGM issue 5, page 27) They grew quickly and needed irrigating once a day with a nutrient strength of 1.8 E.C.

February 4 – Preparing the Slabs and the Drippers
All the tomato plants are up to size and have a good amount of air pruned root tips showing on the bottom of the blocks.

Now it’s time to prepare the rockwool slabs.

I made up a pre-soak solution.  My personal recipe is a standard one part grow nutrient at 2.2 EC and pH 5.5.  I also added a concentrated liquid beneficial biology supplement crammed full of endo and ectomycorrhizae, all sorts of beneficial bacteria, trichoderma and biostimulants.  This will help the plants to establish a good root system.

I placed the slabs upside-down in their plastic trays and placed the trays on a level surface on top of the run-off tank. Next I cut slits in the plastic wrappers on the slabs and poured the nutrient solution through until the slabs were fully saturated.

TIP: Be sure to turn the drainage tubes at the end of the tray to face upwards so the nutrient solution stays in the tray. Always pre-soak rockwool slabs for 24 hours in pH 5.5 nutrient solution.  This balances the alkalinity and loads the slabs with nutrients and microbes.

While the slabs were soaking I moved on to setting up the dripper system itself.

Setting up the Dripper System

I placed the pump into the reservoir and attached ¾” Tricoflex hose to an inline filter. The filter will help reduce clogging in the drippers and can be used to regulate supply pressure and mix the solution in the reservoir.

I used white PTFE tape on the threaded fittings.  This helps to keep the fittings water-tight as they can form small leaks when the supply tube pressure builds up. Cable ties came in handy when I attached the pipe work to the support bar in order to keep the pipe work off the floor.  The less stuff to trip over, the better!

I used drippers with compensators – these are small brown plugs that are inserted into the ¾” supply tube and held in with a barb. Compensators only allow water through when pressure is generated in the supply tube (i.e. when the pump is on). When the pump switches off the pressure drops in the supply tube and the compensators close. The green tool is a 1/10” hole punch which I used to make small holes in the supply tube (into which I inserted the compensators.)

TIP: Put the kettle on.  No, it’s not time for a coffee-break just yet.  You need to use hot water to soften the rigid plastic pipe before attempting to insert the barbed fittings. When the pipe cools it shrinks around the barbs making good water-tight fittings.

I cable tied the piping to the support bar all the way along. From the support bar the supply tubes go through three elbows and up to the filter set in the 60 gallon reservoir.

The supply tube runs along the back of the system and is plugged with a ‘terminator’ fitting.  Next I installed the drippers. I used the hole punch to cut a hole into the supply tube then I insert the barbed compensator onto the hole.

TIP: After the compensator has been inserted gently pull it up while squeezing the supply tube. This helps form a nice tight seal between the barb and the internal wall of the supply tube.

Draining the Slabs

I turned the drainage tubes down to allow the solution to drain out. You can see in the picture that the run-off tank is weighted with bricks, This stops it from flipping up under the weight of the saturated slabs. I position the plants on the slabs.  (Yes, I knew the slabs were still upside down!) My plan was to plant two tomato plants per tray in the middle and three plants per tray on each end.

TIP: It’s important to mark where your plants are going to be positioned before you cut the drainage slits in the plastic cover.  Be careful not to cut into the rockwool slab itself. The drainage slits should be diagonal and be positioned between the plants, not directly under where they will be planted. One horizontal slit should also be cut at the drainage end of the slab.

Once the drainage slits were cut, I put my plants to one side and flipped the slabs over. I then gave them some time to drain a little more.

5 February – Planting Out

I cut an X in the slab wrapper for each plant, making sure that the X’s were not directly above the drainage slits.

TIP: Before planting onto the slabs pour through a gallon of fresh nutrient solution through each slab, this solution should be at the same EC and pH you used to pre soak the slabs. As the slabs drain collect some of the drainage solution and check the EC and pH. You may find that initial pH readings are slightly high (6.3 or more) but as the drainage slows to drips the pH should start reading around 5.8.

After preparing the planting sites I placed a plant on top of each ‘X’, so that the triangular flaps are around the outside of the block.  I then poured a cup of nutrient solution through each block. That was the last irrigation they would get for three-four days as the slabs and blocks now contained all the solution they needed.

Over the following three-four days the roots anchored into the slabs.

I increased light levels with 2 x 400w metal halide lamps to extend the day length to 14 hours and to supplement the low levels of natural light provided by the winter sun.  The lights switched on at 7am and off at 9pm.

TIP: Tomatoes are insensitive to photoperiod when it comes to flower initiation and fruit growth, although they do grow better and with more vigor if the day length is longer than 12 hours.

8 February – Checking on Rooting Progress

Three days after planting onto the slab the tomatoes looked like they were doing very well. To tell how well rooted the plants were I gave the rockwool blocks a very gentle twisting movement.  They still felt a bit loose.  I decided that they could do with a little more time before starting regular irrigations. At this point I gave them each a cup of nutrient solution at EC 2.2 and pH 5.8. A small amount of run-off drained out the slab after doing this.

At this point I placed support canes in the block to prevent any plants from falling over.

My goal was to maintain the relative humidity at 60-75% with temperatures reaching a maximum of 77-79ºF (25-26ºC) in the day and 64-68ºF (18-20ºC) at  night. This environment would enable the plants to set off well without being under any stress through excessive transpiration.

TIP: Keep the day lengths at 14 hours. On bright days switch the lamps off from any time between 10am and 3pm as the sun provides more than enough light.

12 February – Starting Irrigation

The tomato plants were now firmly rooted in – a light twist of the 4″ block now showed they were strongly held to the slab. I removed the canes and supported the vines with string and vine clips. The string was on a spool attached to a ‘roller hook.’ The bottom of the string was tied to the slab tray. This went up above the plants where the wheel and hook were attached to a support bar. This allowed the vine to be supported by the string and the vines could be lowered when they became too tall. I also covered the run-off tank with some black plastic corriboard to prevent algae growing in the run-off water.

I inserted the drippers into each block. These type of drippers need to be inserted at 45º. I set the digital timer for the irrigation pump to come on for three minutes every two hours during the day. It’s not necessary to irrigate during the night. I set the first irrigation to start two hours after the lights came on and the last two hours before the lights went off. On these first and last irrigations the plants received a longer irrigation of six minutes.

TIP: It’s a good idea to wait two hours before the first irrigation to give your plants a chance to transpire.

TIP: To help promote vegetative growth, tomato plants like to be fed little and often. This creates a consistently moist but not overly wet root zone. To steer the plants into a more generative (fruit producing) growth habit the root zone is allowed to dry slightly, but not excessively, between irrigations. This will mean changing at a later date to longer but less frequent irrigations. This influence of controlling water content in the root zone to promote a more vegetative or generative growth habit is a very useful controlling factor and can be applied to many plant types, not just tomatoes.

14 February – Noting the Importance of Run-Off
No Valentine’s cards.

My nutrient solution had an EC of 2.4 and pH of 5.8. To minimize salt build-up inside the slab I made sure that during all irrigations some solution dripped out – this helped to flush the slab of any residual salts.

TIP:  Your run off should be around 20%-30% of the input volume so if you give a slab a quart of nutrient solution you should aim for 6-7 fluid ounces of run-off. I find the easiest way to check I’m getting enough run-off is by using the clock: if your irrigation length is three minutes then you should be seeing the slab producing run-off around two and a half minutes after the irrigation starts.

I tested the EC and pH of my run-off every one to two days to get an idea of what was happening inside the slab. I collected a sample of run-off in the middle of the day. My aim was for the EC of the run off to be within 0.2 points of the input solution and the pH to be slightly higher but still within the 5.2 – 6.5 range. My run-off EC was 2.6 with a pH of 6.2 – I was happy with that.

Commercial growers use specialized EC probes that are inserted into the slab to give the actual EC around the root zone. Unfortunately for me these are very expensive and although I’ve had one on my Christmas list for many years I make do with the slightly less accurate method of testing the run-off solution. Some growers insert a syringe into the slab to withdraw a small sample of nutrient solution; I do not like doing this as it permanently damages the rockwool fibers.

This picture shows the high-pressure filter set in the nutrient reservoir. I kept the overflow valve open slightly so, during irrigation periods, the solution was able to mix and aerate.

Irrigation Tips

• If you test the run-off and find the EC is much higher than the reservoir it’s an indication that you’re not irrigating enough.
• If the EC is lower in the run-off than the reservoir you should increase the EC in the reservoir.
• If the pH is low in the run-off coupled with the same EC as the reservoir you are watering too much.

Plant Maintenance
Once the plants start to establish themselves into vegetative growth they start to grow side shoots.

Cedrico, like most greenhouse tomato crops, is an ‘indeterminate’ tomato variety which means it prefers to grow with one main stem and will produce fruits throughout the season. Conversely, a ‘determinate’ tomato variety (often called a bush variety) will grow to a fixed height and produce fruit over a short period. It’s important when growing indeterminate tomato varieties that you remove the side shoots, this turns into a weekly job during the peak of the growing season. Rather than using scissors or a knife, that can spread potential infections from plant to plant, simply bend the shoot with your fingers towards you, then away from you and they will snap off nice and cleanly.

You can use the unwanted side shoots for cutting material to create new plants for friends.

21 February – Diagnosing Run-Off Problems
I checked the run-off and the EC was 2.9!  There was also suspiciously little water in the run-off tank. Yesterday was really bright and sunny which must have increased the plants’ water uptake.  This caused the plants to produce very little run-off and an increase in EC. Luckily today has been particularly overcast and the slabs were producing plenty of run-off, so I decided to leave things be.

22 February – Verifying Run-Off EC Correction
I checked the run off EC again and it had lowered down to 2.7 so things were getting back on track. I made a mental note to keep a close eye on the approaching spring weather as sunnier days will cause the plants to demand larger irrigation volumes.

TIP:  As plants grow, their need more water and nutrients.  You need to adjust the length of your irrigation times accordingly.

25 February – Observing First Flowers
Towards the end of February the plants were growing vigorously. So far I had observed no nutritional problems and the plants were looking nice and healthy. I had been maintaining the reservoir EC at 2.4 and trying to keep the root zone EC around 2.6.  The plants were receiving four minutes of irrigation every two hours with seven minutes on the first and last irrigation.

The plants were also producing their first flowering truss. I was tempted to cut these off to keep the plants firmly in vegetative growth but I felt that they were well established and I was excited about the prospect of harvesting tomatoes by the end of April!

However, I made sure I kept the irrigation regime in such a state as to influence a more vegetative growth habit until the plants were bigger with a larger fruit load.

3 March – Developing Small Fruits
Early March saw incredible rates of growth.  Small fruits were also starting to form. At this stage I irrigated for six minutes every 1.5 hours with eight minute irrigations on the first and last.

TIP: As you observe fruits starting to develop it’s a good idea to increase the EC to accommodate the increased nutrient demand.

TIP: Remember to increase irrigation times to match the increased water requirements of larger plants. A sure sign is increased EC of your run-off.

The 400W lights were still being used but I anticipated not for much longer. The plants were getting very close to the lights and the natural day lengths were approaching 13 hours so I decided to turn the lights off and take them down.

March 19 – Training the Plants
It was time to start training the plants before they hit the roof! With day length, sunlight levels and warmth increasing as the season moved into Spring, the plants in the greenhouse were rocking along nicely.

Once the tomatoes had grown too tall for their supports they were lowered by releasing the spool of string on the roller hook.

As the individual plants were lowered, the roller hook was moved onto the next hanging site. I move the plants around the drip system in a clockwise direction.  Over a whole season they can end up moving past their original location four or five times and be well over 30 feet in length! With the lowering of the stem, the foliage at the bottom was removed by cutting leaf stems with a pair of sterilized scissors as close to the main stem as possible. I tried to remove a maximum of two leaves off a plant in one go to avoid undue plant stress.

I noticed the run-off EC was climbing so I stepped up irrigation to seven minutes every 1.5 hours with nine minutes on the first and last irrigation.  An extra irrigation was added and the first and last were reprogrammed to accommodate for the increase in day length. I also changed the nutrients to a bloom formulation that has lower levels of nitrogen and higher levels or phosphorus and potassium to help with fruit growth.

March 23 – Pruning Trusses
I trimmed all the fruiting trusses down to five or six fruits – about optimum for this variety. Leaving anymore than six fruits on the truss results could end up with them taking too long to ripen and producing uneven sized tomatoes.

31 March – Achieving Plant Balance
The tomato plants were maturing well and had a moderate fruit load. To help the fruits along and to steer the plant into a slightly more generative growth habit I changed the irrigation strategy to allow the root zone to dry slightly more between irrigations.

TIP: A dryer root zone influences the plant to put more effort into its generative growth, help with fruit production, and put less energy into vegetative growth and leaf production.

When the tomato plants were young, my aim was to influence vegetative growth by keeping the water content high. By this time I was aiming to have the plants in a state where they were still putting some effort into vegetative growth while at the same time producing fruit. Understanding and adjusting this plant balance is a handy tool when growing tomatoes.

TIP: To tell if a tomato plant is generative (flowering, fruiting) the top of the plant stem becomes thin, inter-nodal distance increases, leaf area becomes reduced and fruit load will often be high. If the plant is more vegetative the top of the plant becomes very dominant with a thick stem, the whole plant will produce large leaves and fruit production will be slow. Striking a balance by observing the plant growth and adjusting influencing factors like irrigation regime, root zone EC, day and night temperature and relative humidity (RH) is important in achieving good production and high yields.

I changed the irrigations to 12 minutes every 2.5 hours.  This maintained the total daily volume of nutrient solution at similar levels but the longer gaps between irrigations influenced the plants to put more effort into producing fruit. The first and last irrigations were 14 minutes long. I also increased the EC in the reservoir to 2.6 and tried to keep the root zone EC around 2.8.

8 April – Ripening Fruit
The tomatoes were beginning to ripen and, in order to do this effectively, they needed plenty of potassium. I noticed a small amount of uneven ripening on some fruits so I decided to increase the potassium in the nutrient solution to encourage quicker, more even ripening. When the reservoir ran out I topped it up with fresh water, added a flowering additive (potassium and phosphorus booster) at 1 teaspoon per gallon, and then added bloom base nutrients until the EC reached 2.8. My target was to keep the root zone EC around 3.0.

The new irrigation regime was running fine and hadn’t needed any changes over the past week. The temperature and RH in the greenhouse had been good up to this stage – rarely leaving the ranges 64-79ºF (18-26ºC) and 50-70% RH.

16 April – Nearly There!
Adding the flowering additive seemed to help and the fruits were now ripening well. I wished the greenhouse was taller so I could have let the plants grow bigger before lowering them, that way the fruits wouldn’t had been so close to the ground.

22 April – First Harvest
I had stolen the odd tomato here and there in the last few weeks, but this was the first full truss. As you can see, this truss of tomatoes were fully and evenly ripe, they were big, colorful, and very sweet and juicy. I was really glad I chose to use rockwool slabs on the drip system this year as they had been a hassle free growing media and really easy to control.

Looking Ahead
As we started to get higher outside temperatures in May and June the greenhouse temperature unavoidably increased and the RH dropped. This caused a harsher environment that increased the plants’ water uptake. To account for this I lowered the reservoir EC down to 2.5 to allow the plants easier access to the large amounts of water they required.

TIP: High root zone EC can limit plants’ water uptake and should be avoided in summer. Running a lower reservoir EC gives you a little more of a buffer for the EC to increase during hot spells.

Whoooa!  That was a beast! Tell us what you thought of our pro grow diary! Did you learn new stuff or was it just another dose of tomato 101?