Published 20 November, 2018

With autumn comes the time of indoor growing, and many cannabis growers take advantage to review and update your equipment, or start a new crop, always with views to improve the quality of their crops. In recent years and at present, we are witnessing an increase in both the supply and the quality of the means for cultivation. Technologies are evolving, fertilizers and additives are being perfected, and techniques are refined.

But it also improves our knowledge of all the processes that participate in the growth of a plant, which allows us to make the most of our resources to obtain the best possible result: light, humidity and temperature control, nutrition, water quality irrigation … There is practically no aspect that the good grower of cannabis does not care with zeal, but perhaps it is the water issue, of which more determining factors for an optimal harvest depend.

How should we treat water in our crop?

The water with which we irrigate our plants and apply our nutrients will determine a huge percentage of the final quality of the buds, and not only in terms of size, aroma and properties, but, and this is very important in any product for human consumption, of health. That is, if we want to achieve a healthy and quality end product, we must pay close attention to the quality of irrigation water.

Watering or not watering: that is not the issue

No grower who addresses his next indoor crop will consider whether or not to light his plants, but how: will he use more versatile equipment that he can use throughout the development of the crop, or will he use a more refined result? different equipment adapted to each phase? Will you seek maximum energy savings, or will you risk an extra cost by betting on a higher result?

Why not apply the same principle to water? Since the question is not whether we should irrigate or not, the demanding grower must ask himself: how am I going to treat the irrigation water?

Of course, to answer this question, first we must respond to the following: What is in my irrigation water?, Why should I treat the water with which I irrigate my plants? The answer depends fundamentally on the origin of that water.

The ideal situation would be that, knowing precisely the water composition of our irrigation source, we would have verified that it has an acceptable quality in terms of electroconductivity (EC), pH, oxygenation, amount of dissolved salts, etc., to apply directly to our plants, without the need to treat it. But this situation is extremely rare.

Water comes from three main sources: rain, groundwater and the water network.

The rain

The rainwater is, generally, a soft water and, freshly fallen, very oxygenated, and therefore ideal for irrigation. However, you should keep in mind that their best properties are ephemeral and disappear as soon as it is stored.

It contains more CO2 and nitrogen than common water, which makes it slightly acidic. These, along with the oxygen, pass to the plants through the newly fallen rain, which gives them a boost of growth.

But be careful: depending on the place where you collect the rainwater, it may contain different traces of elements. If you live in a big city, very likely the water you pick up directly from the rain contains pollutants washed out of the atmosphere. In certain regions, too, there are often very hot weather conditions that contaminate rainwater and load it with undesirable elements. One option to sediment these elements is to let them rest, but in this way you will also eliminate the properties that make it particularly special.

When we talk about indoor crops, direct rain, whose saturation makes it penetrate much better in the soil than artificial irrigation, is not an option. Storage as an alternative has its counterparts: water loses its properties and, if we are not careful, it can stagnate or reach unfavorable temperatures for irrigation.


Using water directly from a well without knowing the salts it carries is reckless. Groundwater can have very different compositions, present solids that alter its pH, as well as contaminants. Yes, because of the location of your crop you have no alternative, you must order an analysis. Depending on the characteristics of the aquifer, it will be advisable to repeat the analysis every so often.

Running water from the tap

Although the tap water that flows out of the tap is previously filtered, it may contain, in hard water regions, a large amount of dissolved salts that make its EC and pH values not appropriate for the plants. Old plumbing installations can also add accumulated lime and other unwanted elements to running water. In addition, tap water is commonly treated with chlorine, as a sanitary measure necessary to prevent the proliferation of viruses and bacteria.

As we already told you in this entry, although chlorine is essential in the running water system to ensure that it reaches your tap completely sterile, from that point on it can become a great enemy of the grower, especially the organic grower. Chlorine will not distinguish between beneficial and harmful organisms and will eliminate beneficial fungi from organic fertilizers and crop stimulators. In some areas of the world fluorine is also added to water. Fluorine can inhibit photosynthesis and phosphorus absorption, producing damage to the leaves.

Heavy metals

As we can see, the three main sources of water have a common disadvantage: the possibility of presenting unwanted elements. Although tap water is safe in terms of sanitation, can, like groundwater and rainwater, runs the risk of being contaminated with heavy metals.

Cannabis is a biological accumulator: everything you give it will incorporate it into its biomass and, if it can not transform it through its common biological processes, it will accumulate it. Bioaccumulation It is a characteristic of heavy metals. These can reach the plant through its cultivation in contaminated soils, the use of poor quality fertilizers … or the water!

The term “heavy metals” refers to a set of metals and some semimetal that, without being essential, they have a toxic effect on living matter. Cadmium (Cd), mercury (Hg), arsenic (As), copper (Cu), cobalt (Co) and lead (Pb) among others, can accumulate in the plant and pass to the human being through its consumption.

Heavy metals can inhibit plant growth, structural damage and oxidative stress, in addition to poor functions in their physiological and biochemical activities, affecting the photosynthesis and the water potential of the leaves among others.


Although the plant has mechanisms of resistance against heavy metals, its exposure to them is always a risk, especially considering the possibility of its presence in products intended for human consumption. The effects of heavy metals in the human beings cover a wide spectrum, from headaches to nausea, passing through metabolic insufficiency and in cases of prolonged or extreme exposure, cancer.


The best treatment is, without doubt, prevention. Watering our plants with water whose composition we do not know is not very different from using cheap fertilizers that have no guarantees.

So, it’s not about whether you need to treat the irrigation water or not. It is about what water treatment system you need: Filtration or Reverse Osmosis.


“Professional growers say that quality water should be used”

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