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This article is an extract from "Bonsai Inspirations 2" by Harry Harrington.
Having spent many years growing bonsai in the North West of the UK using local slightly acidic, soft tap water, I was a little apprehensive of maintaining my bonsai with the notoriously hard water of the South-East when I moved to the London area in 2008.
Having now maintained my bonsai for 4 years with hard tap water that has a high lime content, I feel it is worth sharing the experience that I have acquired. Most importantly it should be understood by all enthusiasts that if their tap water is hard, it does have an effect on their plants and most importantly, their bonsai.
The hardness of water roughly describes the amount of 'lime' or 'limestone' dissolved in the water as calcium carbonate and magnesium carbonate. A 'hard water' having a higher amount of lime (as calcium and magnesium) and a 'soft water' having a lower amount of lime.
The occasional watering of a bonsai with hard water, high in lime content, causes no ill-effect on a bonsai and should not be regarded in itself as bad practice. Nonetheless, over a prolonged period of watering with hard water, the lime content of the water increasingly accumulates in the soil of the bonsai, and in turn, this heightens the pH levels that the bonsai is exposed to. As the majority of tree species prefer a neutral to acidic soil, the alkaline conditions caused by the build up of lime can have serious consequences.
Plants and pH
There are a few notable examples that many of us will be familiar with already as being lime-haters. These are trees that have a strong preference for an acidic soil and include species such as Azalea, Bougainvillea, Camelia, Ilex, many juniper species as well as coniferous species such as Spruce, Larch and Cedar. Prolonged use of hard-water will cause lime-induced chlorosis, often identified in lime-haters by the appearance of pale and yellow leaves caused by a deficiency in iron.
Though the majority of shrub and tree species we use for bonsai are tolerant of a reasonably wide pH range, there are a large number of these that still dislike an alkaline soil (one with a pH higher than neutral or 7.0) caused by the accumulative effects of the lime in hard water. Examples of note that fall into this category include Acer palmatum and Beech, both of which will begin to lose vigour over a couple of years before their health begins to deteriorate.
The Practicalities of Watering Your Bonsai and high pH in Hard Water Areas
At the end of this appendix is a guide to the pH tolerance of many tree species used for bonsai as well as a number of familiar garden plants. The practicalities of regularly measuring the pH balance of the soil of ones bonsai and trying to reach optimum pH levels for the wide range of tree species that many of us own, make the science of soil pH an unwelcome proposition to many of us, myself included.
However the implications of the continued use of hard water on our trees must be addressed or there is a very real possibility of a lack of vigour and deterioration in health in our bonsai.
Watering your bonsai with collected rainwater (which will tend to be very slightly acidic) is an excellent idea and obviously reduces the risk of lime building up in the soil. However, during dry periods particularly during the Summer or when maintaining a large collection of bonsai, watering with rainwater is unfortunately not a practical solution.
In real terms, the regular, fortnightly application of something to reverse the alkalinity of the lime in your bonsai soil is enough to keep your trees healthy. Fortnightly I use a liquid fertiliser marketed specifically for use with ericaceous or lime-hating plants during the growing season and this is more than sufficient to counteract the effect of hard water on lime-hating species such as Azalea.
For the rest of my bonsai that prefer a neutral to acidic soil, irrespective of the bonsai soil-mix, I use an ericaceous fertiliser OR a mixture of ONE teaspoon of vinegar (the type of which seems irrelevant) with 7 litres of water once a month. Again, this is sufficient to counteract the effects of lime in hard water areas.
While 'soft' water which is low in mineral and lime content is excellent for bonsai and can be used without any problem, 'softened' hard water contains a high salt-content and its use will cause more damage than lime and lime-scale.