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One of the many choices for hydronponic growing are systems using rockwool as the growing medium. Rockwool offers many advantages in producing top-notch chilies as well as it is demanding control from the gardener.
This article was written to explain some of the most important concepts when gardening with rockwool and to help aspiring chili rockwoolers to have a good start.

Chilies in Rockwool – Starting indoors

Like expanded clay, vermiculite, perlite and other inorganic material rockwool is a soilless medium that contains no nutrients whatsoever but has an incredible potential for holding water.
Rockwool consists like the name implies of molten rock that is heated until fluid and then pushed in spinning cylinders where the fluid rock hardens into thin fibers that are pressed into rockwool.
This is then cut into slabs or cubes which already is the form of rockwool you can buy in horticulturist stores.

It has to be stressed that only horticulturist rockwool can be used to grow anything since the rockwool you can buy in hardware stores, for example as insulation material, is treated with special chemicals and oils that will kill plants.
One of the best things about rockwool is that due to the fibrous texture it will hold plenty amounts of water and still more than enough air to prevent roots from rotting.
Rockwool will need no special care to sterilize it since it comes that way, will not attract
many fungi that cause root rot and cause leaf fungus as well, it is light unwatered and will produce no mess indoors.

A dutch greenhouse producing tomatoes operating on rockwool and a drip system. Copyright by Wikipedia.

If handled right rockwool will enable the grower to achieve breathtaking results – one of the reasons it is the number one choice for commercial greenhouse vegetable production worldwide.
Of course to all the good sides there is a bad side to rockwool. It will demand the grower to constantly pay attention to certain parameters like pH.
Most basic parameters will be explained how.


Safety first – as it should be. It is widely assumed that rockwool in dry form can cause lung cancer by the same way asbestos does. This is not true. Rockwool fibers on the one hand can be inhaled but will not break down lengthwise into much thinner fibers as asbestos does.
It is advisable to wear some form of breath protection when handling larger amounts of dry rockwool though to prevent irritations.
What dry rockwool can cause though is irritation of the skin. I always wet the surface of dry rockwool with a watersprayer a little before handling it.
Here is an excerpt from WikiPedia regarding health issues:

The EU risk and safety phrases associated with this material in general are:
R38 – Irritating to the skin
R39 – Danger of very serious irreversible effects
R40 – Possible risk of irreversible effects
S36/37 – Wear suitable protective clothing and gloves.

All European produced rock (stone)wool and glass wool is bio soluble and R39 and R40 do not apply. For these products only the risk phrase R38 remains. This irritation to the skin however is not a chemical irritation but only a temporal mechanical irritation, comparable with exposure of the skin to straw, grass or hay.

By all means if you happen to have some doubts still, go ahead and google some more!


I often hear anti-hydroponic arguments mostly regarding the taste of the product. Many associate the inferior products from their supermarkets with hydro and rockwool. It is true that it used to be that especially dutch greenhouse products had a poor taste and seemed to consist of pure water.
But the primary ingredient of flavor and aroma of a plant product still is and always was the genetics. More often than not poor taste stems from the selected strain that was bred for looks, speed of ripening, resistance to diseases and pests and resistance to damage done by transportation.
Plants breed for those properties and not for taste will always taste dull – when grown hydroponically or on the best earth there is. Furthermore most commercial vegetables are harvested ahead of their ripening time and have to ripen in dark containers without any nutrient beside certain gases at all.
A plant grown hydroponically by a skillfull gardener with the potential of great aroma and flavor will be just that in a short time – with no drawbacks to growing pure organic. I have tasted fantastic products grown "bio" as well as grown hydropocic and as a hobby gardener I just prefer the absolute control and fast results hydroponics offer.

pH and EC

If you should decide to start growing chilies hydroponically those two parameters determine if you are feeding the babes right or not.
pH – short for potentia Hydrogenii – describes the acidic properties of the solution. A value of 7.0 describes a neutral solution, 0.0 is pure acid and 14 is pure base. While with growing in earth the pH value can be well at and around 7, for rockwool the optimum is between 5.5 and 6.5. In this range all of the needed nutrients are available to the roots.
Even if plants can take variations the pH must never get below a pH of 5 since under this value the manufacturers claim the rockwool will take damage. Below 4.5 the roots will begin to be damaged.
The EC – short for electrical conductivity – measures how well a certain electric current is
conducted in a given solution. This indicates how many salts are dissolved in the solution. Pure distilled water should have an EC of 0.0.
From my own experience and what I have read from various other sources mature chilies thrive best in values between 1.5 and 1.8. Young seedlings and just rooted clones do best in 1.0-1.2.
Well above 2 the plants will begin to show signs of overfertilization – the leaves wilt and roll and will start to show necrosis, dead and brown leaf tips. Rockwool is chemically inert which means it will not contribute to any chemical reactions and therefore it will not alter pH or EC. It has to be conditioned the first time used though – more on that later.

Needed equipment

When growing with rockwool there are two measurent devices that are mandatory: an EC-meter and a pH-meter. With these two a high degree of control over the nutrients feeding the plants can be achieved. Without them failure is a sure thing because the nutrient solution will almost always not fit the requirements for rockwool (or any other hydroponic system). Especially the pH is prone to cause troubles.

Advanced equipment: pH meter and EC meter by Milwaukee – my cat guards them cause she knows they will help to produce a bigger garden she can enjoy more to destroy later

Of course the basic ingredient, the nutrients, are needed. Brands of fertilizer will have to used that are specially designed for use in hydroponic systems. That exludes most of the brands used when growing in earth and of course those "secret" ingredients used by organic growers like egg-shells or fish emulsion.
In earth there are many strains of bacteria and fungi that are necessary to break down those organics into forms usable by plants. Those are missing in rockwool and all you get is a pungent stench and starving plants if not worse.
For conditioning the nutrient solution also pH conditioner is needed. It comes in the flavor of pH up and pH down. pH down should be the only one you need since tap water everywhere is up in the 7 and above region due to its content of dissolved minerals. To mix water and ingredients in defined proportions a measurement cup of sufficient size and some syringes in the milliliter range are highly recommended.

Measurement devices (cup and syringes),large stirring spoon and pH down with buffering agents by GHE

Water Quality

You should first determine your water quality. Many regions will have hard water meaning a lot of dissolved magnesium and calcium dissolved in the water. This is done at the water plants to bind up unwanted metals that otherwise could be dangerous to the health. So while this water is good for the consumers it is not to household appliances like washing machines or water heaters where it produces clog ups and stones and it is also high in pH and already in EC.
For example my tap water has an EC of 0.8 and a pH of 7.5. It also has a high level of carbonate/bi-carbonate which makes it somewhat resistant to acidification. It takes me some pH downer to achieve the wanted pH of 5.8.
After some time the pH will rise again and I will have to correct it once more. This can go on for a couple of times until I have a stable pH.
But since I use a reservoir to water my plants holding 40litres I am done then for about 1.5 to 2 weeks.
So if you also should happen to encounter problems with keeping the pH stable it could well be the water quality. In severe cases devices called reverse osmosis filters can be used to filter out the high contents of carbonate/bi-carbonate.

The nutrient solution

Rockwool contains no nutrients whatsoever. It is the task of the chili farmer to provide the plants with the needed nutrients in the solution in the needed pH range. Fertilizers have a set formula of main and secondary nutrients.
Most important is the main nutrients formula which is noted in the NPK form which stands for nitrate, phosphorus and potassium (kalium).
Good fertilizers also have a declaration of all needed secondary nutrients on them. From my past experience and the current grow in progress I swear by three part hydroponic fertilizers. They come in three different bottles containing different formulas each. Mostly two bottles refer to different grow stages and one contains micro or secondary nutrients.
With those you will have control over the nutrients for the different phases the plants are in. Those are: Rooting, vegetative growth, flower production and fruit production. With chilies flower and fruit production overlap somewhat since even while fruits are already growing most plants will continue to produce new flowers during the season. In the rooting and flowering phase the needs of the plants for phosphorus rise, during vegetative growth the plants need more nitrate and potassium and during fruit production the focus lies on potassium with somewhat less nitrate to keep the leaves healthy.

Some of my varieties fed by tubes – drip system

Many of the available hydroponic fertilizers have been formulated to feed medical herbs that can be for example smoked. But the nutrient intake of chilies is significantly lower than that of cannabis. Also remember that the focus is not on flower production like many of the manufacturer's label imply.
So it is a good choice to start with a nutrient solution half the strength of the manufacturer's instructions. Measure the EC and see if it fits the stage of growth your chilies are currently in. After the desired EC has been established go on and adjust the pH.
It is highly recommended to keep a log of your activities. This way you do not have to experiment each time but apply the amounts you have found work well. Keep on monitoring your solution of the pH drifts over time. It may be necessary to adjust the pH several times but always go in small steps since a too great dose of pH down can ruin your solution.

Conditioning rockwool

Before starting to put plants into the rockwool, be it slabs or cubes, the rockwool has to be conditioned right so it will help maintain proper conditions inside. Though chemically inert fresh rockwool has a tendency to have a pH of 7 and it will alter the solutions pH upwards.
I have experienced this myself and it took some days of measuring and correction to overcome it.
Prepare a solution of fresh tap water and a pH of about 5.2. See that the pH is stable and soak the rockwool with the solution for at least 12, best 24 hours with it. This not only will make sure the pH will be in the safe range but also will wet any pockets in the rockwool that may stay dry otherwise. Let the solution drain from the rockwool and water with nutrient solution.


Rockwool can be used in ebb/flow and drip systems.
It is not advisable to put rockwool into airated solution reservoir systems. The design with rockwool is that draining solution pull fresh air into the rockwool from above while rockwool sitting in a reservoir even with the solution being airated will cause root suffocation and rot.
The weapon of choice for pure rockwool systems is the drip system. IN this system the nutrient solution is appllied by emitters placed above the surface. The solution is administered from one to several times a day.
The amount of solution with each watering usually is calculated that about 10 to 20% of the solution drains from the rockwool causing a leeching effect flushing away unwanted nutrients from previous waterings and the plant's toxic digestive by-products.
The rockwool should be additionally leeched each one to two weeks with pure water that has a conditioned pH.

Seedlings grown in starter cubes

Of course it is also possible to water by hand. I have found it to give still very good results if in earlier phases the plants are watered on a schedule like if based on earth. Still enough solution has to be applied to achieve the leeching effect. Letting the rockwool dry out a little in the rooting phase seems to help with rooting because the roots grow more to search for water and nutrients.

Starting out

Starting seedlings in starter cubes

In the beginning a rockwool garden will require some labor.
A lot of measurements will have to be taken so in the later phases you know what the values are and should be. Measurement samples from the rockwool can easily be taken with syringes. Expect the values directly taken from the rockwool to differ from those of the solution.
The pH will be a little higher and the EC should be a little lower caused by nutrient uptake.
A significant higher EC will mean salt buildup and should be remedied immediately by leeching the rockwool with pH conditioned pure water.
A high pH can indicate that you have not conditioned the solution long enough and the carbonates/bi-carbonates from the tap water have caused a rise in pH.


Rockwool has the great advantage that it can stack and root will grow right down into the new rockwool. Starter cubes can directly be stuck into larger cubes which in turn can just be placed on top of slabs. So there is no mess and no plant shock associated with transplanting here. Right after transplanting I go for a higher phosphorus value in my solution and at the same time I lower the nitrate to encourage the plants to grow roots. But keep in mind to have at least some nitrate in the solution so the plant will not start to build up a nitrate deficiency.

Starter cubes transplanted into precut holes of growcubes