How Cannabis Plants Use Water and What To Know

Everything you need to know on how cannabis plants use water, how and when to water, and what to avoid.
09 March 2020
6 min read
How Cannabis Plants Use Water and What To Know

  • 1. Understanding transpiration
  • 1. a. Transpiration through the foliage
  • 1. b. Transpiration through the roots
  • 1. c. What can go wrong?
  • 2. The ideal growing medium for water retention
  • 3. The different types of water to consider
  • 3. a. Hard water
  • 3. b. Soft water
  • 4. Reverse osmosis
  • 5. Rain water
  • 6. Top tips on watering cannabis plants
  • 7. In conclusion

Water is an essential building block for all carbon based life forms that exist on the planet. Especially when it comes to Cannabis plants that require large amounts of nutrient solution on a regular basis. Having the discipline to not over water or underwater your plants is one thing, however, understanding how and why Cannabis plants use water in the first place is the key factor. Below Fast Buds better explain all you need to know about water, transpiration as well as our top tips. 

1. Understanding Transpiration

The best way to think about how transpiration works with plants, is to relate it to our own human bodies. We rely on water to keep our brain hydrated, allow salts and osmosis to occur, keep our kidneys and other organs functioning and is our body’s natural defense to cooling us down when we sweat.

Now if you imagine how a Cannabis plant depends on water to function in the same way we do, then you can consider how they use water to maintain salt levels and nutrient uptake. 

Transpiration Through The Foliage

One way in which Cannabis plants will utilize water is through their leaves. The amount of relative humidity present can play a role in how much water the plants release through their leaves, or in fact how much water is absorbed by the root zone. 

Transpiration Through The Roots

The main network of the Cannabis plant that is responsible for sourcing nutrients. The root hairs only need to tap into a very thin film of water to access available nutrients. Now on the basis that the humidity levels meet the requirements of the plant’s life cycle, the ratio of nutrient uptake to transpiration through the foliage should be perfectly balanced.

The amount of moisture a growing medium is capable of holding can be up to 18 times its own weight, however, the rate of transpiration via the foliage rapidly fluctuates. In order to overcompensate for a dry and harsh environment such as the Mediterranean in the peak of Summer, or a badly ventilated indoor garden, plants must cool themselves down very much the same we do after demanding physical exercise.


What Can Go Wrong?

When an environment becomes too hot for a Cannabis plant, and all of the water from the root zone has been used up, it is just a matter of time before the plants will lose their essence and diminish in vigor. This will also cause a list of deficiencies to occur and for the plants to ultimately suffer in growth and performance. 

•  E.C Levels can fluctuate

•  Plants can become stunted

•  Deficiencies will occur

•  Visible leaf damage

2. The Ideal Growing Medium For Water Retention

It is very important to have a fine balance within your growing medium of water retention, and drainage. The ratio and substrate a grower uses can very much determine how fast or slow transpiration will take place.

•  Rockwool can hold up to 18 times its own weight in water, meaning it is an excellent substrate for holding onto water during hot spots indoors or outdoors. More commonly used in hydroponics, it is a great additive to use at the very base of pots, or within a mixture of coco, perlite and soil.

•  Coco is made up of coconut husks and packaged as an inert growing medium. An extremely user-friendly and plant-friendly substrate that has incredible air capacity, as well as water retention. Growing Cannabis in coco is often performed in hydroponics, or as a blend into organic living soils.

•  Vermiculite is a hydrous phyllosilicate mineral that expands under intense heat. Another inert medium that is lightweight and can keep a growing medium well saturated.



3. The Different Types of Water To Consider

Depending on which part of the country you live in, it can play a massive role in the water source that will be used for your indoor or outdoor plants. As well as how you access the water by tap, local town deposits, or by collecting rainwater.  

Hard Water

This described the mineral-rich content of the water that has been leached minerals from rocks. Hard water has a far higher mineral content due to percolation through deposits of chalk, gypsum, and limestone. These leech off into the water, meaning the water becomes rich in: 

•  Calcium 

•  Magnesium

•  Magnesium carbonates 

•  Bio carbonates

•  Sulfates

You can boil a saucepan of hard water and will notice a white, chalk-like substance left behind in the saucepan. This is the easiest way to test the hardness of your water.

Soft Water

Soft water has a very low mineral value and is low in ions, in comparison to hard water. As the name suggests, the water itself when drinking is very soft with very little after taste. It may be wise to supplement your garden with a Calcium and Magnesium nutrient, to maintain the balance between primary nutrients and trace elements

•  Low ions of Magnesium 

•  Low ions of Calcium

•  Requires Cal + Mg supplement


4. Reverse Osmosis

A method that purifies water by removing ions and larger-sized particles from drinking water is reverse osmosis (R/O). This is an excellent way to reduce the electric conductivity (E.C) to the lowest starting point and then add nutrients accordingly. An excellent solution for growers who suffer from poor water quality and rely on a filtering process that is adequate for safe drinking, external use and most importantly feeding Cannabis plants and maintaining the optimum living soil conditions.

5. Rain Water

Unfortunately, the days of collecting clean and free rainwater are over, with parts of the world now restricting the collection of rainwater. If you are currently relying on Mother Nature for a free water source, or are considering starting doing so, then it is important to consider the following:

•  As rain water falls to the ground, it pulls ions from the air and traps them into the water drops. If you live close to a major town, then be careful the air pollution will not affect the rain and cause it to become toxic or unusable for plants or drinking. 

•  Make sure that the materials being used to collect the rain are clean and sterile. An old drainpipe or rusty corrugated steel roof may cause contaminants to leach into the water over time. Using plastic corrugated sheet works very well, and make sure you thoroughly clean out drain pipes that will collect the water.

•  It is important to store the collected rainwater in darkness with high amounts of dissolved oxygen and well out of direct sunlight. By doing so, this will prevent algae and anaerobic bacteria from beginning to form.


6. Top Tips On Watering Cannabis Plants 

•  Watering your plants close to the grow lights switching off can cause excessive humidity levels and for the growing medium to become oversaturated during the darkness period. It is much more beneficial and practical for the plants to be fed once an hour after the lights turn on, or twice a day in smaller intervals.



•  Use an airstone or pump to keep your water aerated and oxygen-rich. It is impossible for bad bacteria to live in a highly oxygenated environment. Not only will adding oxygen to keep your reservoir fresh, but it will also increase the pH at the same time. 

•  In the event, your plants have been treated with stagnant water, or have formed pathogens, then an excellent remedy is to use hydrogen peroxide. The additional oxygen molecule attached to the H2O will kill off any disease or infection. 

•  Reverse osmosis is well recommended for growers who have access to a hard water source. It may take time to filter and purify the water, however, the end result will be safe to drink and contaminant-free water source. 

7. In conclusion 

When it comes to watering Cannabis plants, there are many variables to consider. Not only does the maturity of the plant, pot size, growing medium, light intensity, air intake, temperature, and humidity play a role, but also the cultivar and the experience level and understanding of the grower. Learning how much water and nutrient solutions your plants need is a big difference-maker in the final harvest and quality of your crop. 

Good luck finding the perfect balance and growing some top-shelf flowers!

09 March 2020