Another Essential to Plant Health: Water Hardness

Do you have a film of white buildup on your pots or leaves? Pale, yellowish, and lackluster growth on the bonsai? If so, you may have very hard water, and that can be a limiting factor for plant health.


Above 150 ppm (parts per million) of hardness (Ca + Mg), we get beyond the zone where container plants may be maintained in good health. Over time the roots of containerized trees begin to be coated by the same minerals that coat the outsides of the pots, usually calcium and magnesium…which is essentially liquid limestone. Imagine that: Coating our roots with rock. (Which is less cool than it sounds, really.)

Hard water is a common problem in arid areas of low rainfall, but it can happen in many other places, too.

What’s the problem, then?

  • In extreme cases, very hard water may limit the root’s ability to draw in water (an osmotic issue)
  • Even moderately hard water can pose limits on the uptake of nutrients (If you fertilize with hard water, the combined total salts ppm is often very high, with attendant nutrient deficiencies)

These are serious issues that limit the health and growth of bonsai. And although there are some semi-effective ideas like flushing your soil out occasionally, there are really only two effective solutions this problem:

  • Collect and use rainwater for your bonsai
  • Set up a reverse osmosis (RO) system
  • Do not use a water softener, which introduces sodium into your water

The first is great if you get enough rainfall. In some areas even the rain might be suspect, but it’s often better than what comes out of our pipes or wells. Collection tanks may be set up, and then pumped or gravity fed into a hose for watering. Years ago I set up a gravity fed rainwater tank when I lived up in the mountains of Arizona, and I’ve fond memories of it as my most efficient and low-cost watering system.

Reverse osmosis systems will cost something to set up, and also to run (needing electricity and a loss of water in its functioning), but they do work very well. We get very high quality water out of these units which can almost magically change the health of bonsai in areas of very bad water quality, including a very acceptable pH. (Which is a double plus, as hard water is generally accompanied by high pH.)


Do pH and hardness tests on any reverse osmosis system to be sure it’s doing what you think it’s doing. Water coming out of them should have a pH of 7 or slightly lower, with almost no dissolved salts. Because of this nearly complete lack of minerals in both rainwater and RO systems, one does need to be consistent with fertilizing.

It’s worth investigating what’s coming out of our faucets. Proper pH and water hardness are game changers in growing bonsai. Try this post about pH:

Postscript: Definitely educate yourself on RO systems if you choose to use them, most need their filters changed periodically, with more frequent changing if the water is very hard. Definitely do not go for the low end units. They do not provide very good water at all.



  1. carterbeall says:

    This is very helpful. I will try to use mostly rainwater, but would it help to let the well water sit in a basin before use? Some people suggest that this allows contaminants and excess minerals to settle out. This issue is separate from pH, correct? That would mean that even if I can fix the pH the water will still be to high in mineral content.

  2. Todd says:

    Is there any way to get the “salts ” out of the soil/pots once they are in there? slightly more acidic water? flushing lots of water through when watering?

    • crataegus says:

      Yes, over time slightly acidic water, the 6-6.5 pH that we’ve been talking about, will break down the salts. And over time slightly higher volume of water while watering will flush them out.

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