The difference between rainwater, tap water, and groundwater

There really isn’t a competition between water sources for your beloved cannabis plants. Most domestic water available to the average indoor grower should be trouble-free.

It is second nature to check pH, and the use of chlorine varies all over the world. Just ask your local infrastructure authority for their water assessment. It is an easily obtainable document that will have a broken down analysis of your water. A simple rule of thumb is that if you wouldn’t drink the water yourself, don’t use it to mix nutrients for your plants.

A confronting reality is that human pollution has penetrated deeply into the ecology. To talk of rain as a pristine water source has been a fairytale for decades. The poisonous trace of human consumerism has insinuated itself into mountain springs and aquifers. Molecular plastic, novel chemistries not seen in nature, and raw pollution are difficult to avoid anywhere at all.


Ostensibly, all water is rainwater—being a general term to cover water in all its forms coming from the sky. A majority of the world’s major rivers and aquifers swell with water as rain comes down and snow melts. Many go on to fill dams that feed cities with water and hydroelectric power. Except for glacial water sources and prehistoric water trapped in rock strata, all water was rain at some stage.

Rainwater does have some magical properties, but they are short-lived. Once it is contained, the momentary benefits of extra molecules fizzle away as the water reaches equilibrium. Every gardener has noticed that plants react with great vigor after being drenched by rain. Much more so than after a simple watering from above. These peculiar qualities of rainwater are fleeting; once captured in dams and cisterns, barrels or tanks, it becomes mere fresh water.

Rainwater is usually mildly acidic, with dissolved carbon dioxide making very weak carbonic acid. After rain, the alchemy of plants absorbs the carbon dioxide out of the rainwater acid solution, and the plants get a growth spurt. Similarly, nitrogen can be dissolved in rain water, giving plants a second boost of an essential molecule.

A generous rain seeps deep into the ground, wetting the complete root systems of the plants, carrying its payload of extra nitrogen. Saturating rain penetrates much more efficiently than artificial watering. Most people will not water their outdoor plants enough.


Groundwater is generally okay. Potable water is always sought for humans, plants, and livestock. Wells or aquifers that have unbalanced pH because of dissolved solids, such as calcium from limestone, are tolerated as a necessary evil when there are no better sources of water available. It is advisable to get an assay of your water if it is particularly hard. If your soap doesn’t lather and the water is not thirst-quenching, there are minerals present making the water more alkaline.

TDS meters only measure total dissolved solids, they cannot tell you which particular solids are dissolved. It is usually calcium and sodium chloride, but if you really want to know, water assays are available. If it’s a private well or bore, you are going to have to pay for an analysis.

Wells and aquifers have played an integral part in the evolution of many human civilizations not near the coast or navigable water. When far from rivers and tributaries, wells and groundwater are of great importance. There are countless towns and cities in the mountains of the world where the town centre is a water source. The Ancient Silk Road was strung between water sources.


Tap water for the urban grower could be from a number of sources. Anything listed above are all used for tap water. The majority of urban water supplies are filtered before reaching the consumer. Ultraviolet sterilization and reverse osmosis are often used on industrial scales to clean billions of litres of water for human consumption. Always a topic that can raise controversy.

Fluoride and chlorine are added to many domestic water supplies. It is a personal choice. These chemicals do not really exist in nature in a form that can be absorbed by living things. Yet they do perform a primary public sanitation service. In-home, domestic-scale water purifying units are common and inexpensive if you want to be sure.

Older cities may find their water hardening as minerals build up in old infrastructure. Similar things happen with older houses that may get scale deposits building up in old pipes, which can harden water. Simple devices are available for the contemporary grower that can give you information that will warn you of such water troubles. An EC/pH/TDS meter is one of the fundamental tools in the grower’s arsenal. Use it often. It is your friend.


Toxin-free, well-oxygenated, clear, pH-balanced water is one of the cornerstones of a good cannabis grow. Water is quite safe in most of the developed world. Quality may fluctuate from source to source, but the swing in quality usually stays within a human-friendly range.

If it is okay for you to drink, it will usually be okay for your plants.In fact, you are a much more sturdy organism than a plant. If your plants get ill, it is an early warning system for you to check the water you are consuming yourself.

Here are some basic guidelines to keep in mind. You want your water to be in pH range 5,8-6,2 depending on the plant life stage to remain in full safety zone. If your native water has below 150ppm TDS (Total dissolved solids), you can use it without further filtering. If your water has above 250ppm at the start line, consider doing an analysis to break down the ingredients before you mix in fertilizers.

As much as short term you might not see a point of that, in longer perspective it will help you fine tune your feeding schedules and nutrition plans to meet exact needs of your plants.