The right watering water plays an important role in plant care. However, the hardness of the tap water is often too high, so it should be decalcified in advance. We explain how this is done.
For plants to thrive, they need water. But the water from the tap is not always suitable as water for watering.
If the hardness level is too high, you may have to descale the water for your plants. Tap water contains, among other things, various dissolved minerals such as calcium and magnesium. Depending on the concentration, this results in a different degree of water hardness. And many plants react very sensitively to water with a high degree of hardness.
Rhododendrons and azaleas, heathers, camellias, ferns and orchids in particular should be watered with low-calcium water if possible. In the long run, watering water that is too hard leads to a lime build-up in the potting soil and increases the pH value, i.e. the acidity of the soil. As a result, the plants can no longer absorb nutrients through the substrate – and eventually die. Here you can find out how to decalcify water and exactly what the hardness of water is.
Watering water is one of the most important elements for plants, whether they are kept in the garden, in a tub on the balcony or as a houseplant in a decorative pot. Depending on the species, the plants require a different pH value in order to thrive.
It becomes problematic for many, especially tropical, plants in Central Europe due to the hardness of the water and the high lime content, which can lead to deficiency symptoms and diseases such as chlorosis.
Your cabbage feels comfortable in the lime soils? You have no problems watering numerous vegetables with calcareous water? Not all plants prefer alkaline and calcareous soils, let alone hard watering. Plants such as rhododendrons, hydrangeas, azaleas, various types of bamboo, and primroses want neutral to slightly acidic soil so they can easily absorb nutrient additions. Hard tap water has some disadvantages that can negatively affect plant growth and even lead to disease:
- Nutrients are washed out
- pH value of the substrate is increased in the long run
- white coatings are formed on substrate and plants
- numerous deficiency symptoms such as chlorosis occur
The more often you administer calcareous water, the worse off the plant will be. The nutrient deficiency is simply too high, which the plants cannot withstand in the long run. Therefore, you should definitely check how hard the irrigation water is in your region and decalcify accordingly or lower the pH value.
What does water hardness mean?
Whether water is suitable for watering or needs to be decalcified depends on the water hardness. In Europe, this so-called total hardness is expressed in “degrees of European hardness” (°dH or °d). According to the European Institute for Standardization (DIN), the unit millimoles per liter (mmol/L) should actually have been used for a few years now – but the old unit has persisted, especially in the gardening sector, and is still ubiquitous in technical literature.
The total hardness of water is calculated from the carbonate hardness, i.e. the compounds of carbonic acid with calcium and magnesium, and the non-carbonate hardness. This refers to salts such as sulfates, chlorides, nitrates and the like, which are not due to carbonic acid. The carbonate hardness is not problematic – it can be lowered quite simply by boiling the water – by heating the carbonate compounds decompose and the calcium and magnesium are deposited on the wall of the boiling vessel.
This phenomenon will have been observed by anyone who owns a kettle. The dissolved carbonic acid compounds therefore only cause the so-called “temporary hardness”. This is in stark contrast to the permanent hardness or non-carbonate hardness: this usually accounts for a good two-thirds of the total hardness of the water and is difficult to lower.
Determining the water hardness
You can find out the water hardness from your local water supply company – or simply determine it yourself. You can obtain the required indicator liquids from pet shops with a range of aquarium supplies.
Or you can visit a chemical store or pharmacy and purchase a so-called “total hardness test”. This contains test sticks which you only have to dip briefly into the water in order to be able to read off the water hardness on the basis of a coloration. The test sticks usually cover a range of 3 to 23 °dH.
However, experienced amateur gardeners can also rely on their eye. If lime rings form on the leaves of plants in summer after watering, this is a sign that the water is too hard. The water hardness is then usually around 10 °dH. The same applies to white, mineral deposits on top of the potting soil. If, on the other hand, the entire leaf is covered with a whitish layer, the hardness level is over 15 °dH. Then it is time to act and decalcify the water.
Decalcifying and softening water
The first step to decalcify water is – as already mentioned – boiling. In this process, the carbonate hardness decreases while the pH value of the water increases. This is a quick way to reduce water hardness, especially if it is slightly too high. If you dilute hard water with desalinated water, you also lower the concentration of lime. The mixture depends on the degree of hardness. You can get the desalinated water for dilution in supermarkets, for example, in the form of distilled water, as it is also used for ironing.
However, you can also use water softeners from garden supply stores. Keep in mind that these often contain potash, nitrogen or phosphorus. Therefore, if you additionally fertilize your plants, it is essential to apply the fertilizer in diluted form. It is also possible to treat water with the help of sulfuric or oxalic acid from a chemical store.
However, both are not entirely safe for the inexperienced and are more difficult to handle. As a home remedy, the addition of vinegar is often recommended, but also of, for example, bark mulch or peat. Since they are also acidic, they compensate for the hardness of the water and thus lower the pH value to a level that plants can tolerate – provided it is not too high.
If the water hardness is above 25 °dh, the water must be desalinated before it can be used as watering water for plants. To do this, you can use ion exchangers or bring about desalination by reverse osmosis. In normal households, ion exchange can be achieved with commercially available BRITA filters.
Devices for water treatment by reverse osmosis are also available in specialized shops. These were mostly developed for aquariums and are offered in pet stores. Osmosis is a type of concentration equalization in which two different liquids are separated by a semipermeable membrane. Through this wall, the more concentrated liquid draws in the solvent – in this case, pure water – from the other side, but not the substances it contains.
In reverse osmosis, pressure creates a reversal of the process, that is, tap water is forced through a membrane that filters out the substances it contains, creating “compatible” water on the other side.
Guideline values for the water for watering
For hobby gardeners, some guideline values for watering water are particularly relevant. Soft water has a hardness level of up to 8.4 °dH (equivalent to < 1.5 mmol/L), hard water above 14 °dH (> 2.5 mmol/L). Watering water with a total hardness of up to 10 °dH is safe for all plants and can be used. Harder water must be decalcified or desalinated, especially for lime-sensitive plants such as orchids. From a degree of 15 °dH this is essential for all plants.
Important: Completely desalinated water is unsuitable for both watering and human consumption. It can cause damage to health in the long term, such as heart disease!
The alternative: rainwater
Many hobby gardeners turn to rainwater for watering if the tap water in their region contains too much lime. However, especially in large cities or densely populated areas, there is a high level of air pollution, which is naturally also found in rainwater in the form of pollutants. Nevertheless, you can collect it and use it for watering plants. It is important not to open the inlet to the rain barrel or cistern immediately when the rain starts, but to wait until the first “dirt” has rained off and also rinsed the deposits from the roof.
Measure water hardness
Before using any of the descaling solutions below, you should measure the hardness of your water. This will allow you to check exactly what the hardness level is and act accordingly. The typical water hardness levels are:
- soft (1): under 1.5 mmol, 0 to 8.4 °dH
- medium (2): 1.5 to 2.5 mmol, 8.4 to 14 °dH
- hard (3): 2.5 to 3.8 mmol, 14 to 21 °dH
- very hard (4): from 3,8 mmol, from 21 °dH
Watering water must be medium to soft. Values above 3.8 mmol should be avoided, but are frequently found in many regions of Europe. The units of measurement refer to carbonate hardness and either mmol (millimoles per liter) or °dH (European degrees of hardness) are used, depending on the test medium. However, the use of mmol is convenient for test kits from abroad. Carbonate hardness refers to the solution of substances with carbonic acid, which make the irrigation water harder:
- Other salts
Except for magnesium and calcium, the other salts cannot be dissolved by descaling, but only by lowering the pH. You can find out the water hardness and pH values from the following sources:
Tip: a quick check for hard tap water is to use a clear glass that you fill with fresh, cold tap water. If the tap water is cloudy white, it is lime, which makes for high water hardness.
Decalcifying irrigation water can be easily accomplished using two methods. These ensure that the salts in the water are dissolved, making it softer and therefore ideal for watering all kinds of plants such as orchids. These methods do not lower the pH of the water, which is something you should definitely keep in mind. If you only need water that is low in calcium, but not directly soft, these methods are recommended:
In desalination, you mix H2O from the tap with demineralized water, which causes it to become decalcified. This is offered under various terms, for example, battery water, where it is H2O, which is free of dissolved salts. For a water hardness of 3, mix 1 part demineralized with 2 parts tap water, for a water hardness of 4, mix 2 parts demineralized with 1 part tap water. You can now use this for watering.
If you heat the water, the lime will be dissolved and it can be used. Either a kettle or a pot is suitable for this purpose. After heating, you must leave the tap water for a day until it is decalcified.
You can easily use water filters to decalcify the water. To do this, simply run the tap water through a water filter and you can water, as the water is deionized in the process.
Tip: You can use all forms of water that are completely pure for desalinating, except for ironing water. This has the same properties, but it is often mixed with perfume, which has a negative effect on the plants.
Lower pH value
Not only the measures for descaling can be used to soften your water for watering. Once you know what the hardness level of your tap water is, you can easily use some means to bring it down to a pH of 4.0 to 6.0. 6.0 is the ideal value for the majority of ornamental plants and many trees, however, a value of 4.0 is fine, as lowering the value can sometimes be a bit inaccurate. Use one of the following methods:
- bark mulch.
With bark mulch, you do not run the risk of lowering the value of the water to the point where it is too acidic. This must come from conifers, as they have acidic properties. Alternatively, you can simply use coniferous wood if you do not have bark mulch available. You need 500 g of bark mulch for a quantity of 10 l of liquid, so that the value decreases. Fill the bark mulch into a cotton bag and let it work in the watering can for 24 to 48 hours. Over this period, the value of the water will drop enough to soften it.
- pine needles
Just as effective as bark mulch are pine needles. Firs and spruces are especially suitable for this purpose, but you can also choose conifer-based compost, as it has the same properties. For 10 liters you need 300 g of pine needles, which must act in the same way as the bark mulch for 24 hours. However, be sure to close the bag with a rubber band and weigh it down with a stone, as the needles like to float to the top.
Vinegar is a method that can go wrong quickly. Since it is an acid, the pH can drop as low as 4.0 in a short period of time. On average, one teaspoon of white wine vinegar lowers the value of six liters of irrigation water by 0.5. However, you should always take a measurement of the pH with this method to be on the safe side. Do not be surprised if even after small amounts of vinegar the value drops significantly, this is normal.
Peat is a classic when it comes to lowering the ph in the substrate. However, you can also use the sediment for the irrigation water. It is especially good if you are afraid of acidifying the water. This with the material is not possible. If you want to use the material, proceed as follows:
- for 10 l of watering water use between 100 and 200 g of peat
- works with hard water
- put it in a small bag or a stocking
- leave to act for at least 24 h
You need so much of the sediment, as one gram lowers the value by 1 °dH. However, keep in mind that this method is not really environmentally friendly and inexpensive. Therefore, the other methods are more recommended.
One advantage to lowering the pH is the preservation of the important nutrients that are completely lost during decalcification. In particular, the content of calcium and magnesium is not lowered, which has a positive effect on the growth of plants. The only substance that is flushed out by these agents is carbonate. In addition, lowering the pH value has a much better effect on the plants than decalcifying, since lime is no longer released to the plants from a pH value of less than 7.0.
Tip: An environmentally conscious and at the same time inexpensive method to enable the water to be neutral is coffee grounds. Brew a coffee and use the dried coffee grounds in the same amount as the peat portion.