IS TOWN WATER KILLING MY INDOOR PLANTS?

 

The chemical element chlorine, and mineral fluoride are the two additions that give rise to most concerns over the potential negative effects of town water on plant health.

Chlorine (which is used to kill microbes) and fluoride (used to prevent tooth decay) are both potentially harmful to plants.

I say potentially because the key is both the plant in question and the quantity of either the plant is getting.

So....first up, how much are they getting?

Using data from SEQ Water (the bulk supplier) here in South-East Queensland on levels in town water^:

• Average fluoride level - 0.76mg/L (mg/L is the same as ppm/parts per million)
• Average chlorine level - 2.10mg/L Total chlorine, 0.6mg/L free Chlorine

 

That doesn't sound like a lot, but what are your indoor plants likely to think?

Let's look at chlorine and fluoride individually:

FLUORIDE

Reputable horticultural science sources in the USA recommend a maximum 1ppm fluoride in irrigation water for both greenhouse and nursery use.

SEQ Water averages 0.76ppm (no individual region or sub-region recorded higher than 0.8ppm).

So that's safe right?

Maybe. Maybe not.

On face value it might seem ok but is it actually slightly marginal where plants which are known to be fluoride sensitive are concerned*?

You also have to consider the following (both positive and negative) factors:

• Fluoride can accumulate in plants over time. And it appears not enough trial work has been undertaken on this.
• Growing media pH can impact plant availability of fluoride - more acidity solubilises the fluoride, making it more plant available.
• Growing media clay and organic matter content can reduce plant fluoride availability - for instance, high clay content soils are said to be effective at binding the fluoride into plant unavailable forms. Potting mixes are not high clay content but do contain organic matter.
• There are other potential sources of fluoride other than tap water. Read more below in the section 'For the Technically Minded'

 

OUR CONCLUSIONS ON FLUORIDE

Having looked at quite a bit of the available information, we have personally come to the conclusion that fluoride toxicity from town water (in South-East Queensland) should not be an issue for most plants but with fluoride sensitive plants the answer is not clear cut.

For indoor plants (certainly fluoride sensitive):

• Adopt a conservative approach.
• Water your plants with rain water, tap water blended with rain water to dilute the fluoride or if you happen to have installed an RO (reverse osmosis) water purifier....well done!
• When you re-pot your plants the new potting mix will effectively dilute some of what fluoride may have accumulated in the existing mix.
• Don't bother trying to boil off the fluoride from tap water, it doesn't work.

 

CHLORINE

Chlorine is a micronutrient required by plants that is involved in several metabolic processes within the plant. 

The Australian Drinking Water Guidelines state that, for health considerations, chlorine concentrations in drinking water should be less than 5ppm.

SEQ water is nowhere near that.

Are plants a different matter though?

At the kind of levels in SEQ water you have to say no.

That is backed up by both the University of Nebraska and the CDC (Centers for Disease Control and Prevention) in the USA which states that chlorine treated water can be used to water plants "regularly" .

*CDC guidelines state the same for chloramine - refer to 'For the Technically Minded' below.

Despite all this, if you still have a concern over chlorine and want to get rid of it from your tap water you could look at activated carbon filtration as a pre-filter in an RO system but how well that will work for you and the specific system required is something best left to people with the particular expertise in those matters.

You could also look at boiling your tap water or leaving it to stand. Refer to 'Chlorine vs chloramine' below.  

 

TOXICITY SYMPTOMS

This is really interesting in that the symptoms of chlorine and fluoride toxicity in plants are similar as well as being similar to the toxicity symptoms for over-fertilising and even incorrect watering!

That makes it easy to diagnose problems right?

Symptoms of chlorine toxicity:

• Necrosis (death of plant tissue) along leaf margins and tips
• Occurs in older leaves first
• Also shows sometimes as chlorosis of the leaf, which is a lack of chlorophyll, so the leaf 'bleaches'
• Leaf drop

Symptoms of fluoride toxicity:

• As above!
• Note also, fluoride toxicity at lower levels can reduce plant growth without showing any other visible symptoms.

If your plants are exhibiting these kinds of symptoms we would suggest the order of diagnosis, based on general probability, should be:

• look at your watering regime (also include micro-climate with this)
• look at your fertilising regime, that you are not overdoing it
• fluoride toxicity 
• chlorine toxicity

 

FOR THE TECHNICALLY MINDED

Chlorine vs Chloramine

These are two different disinfection treatments and here in South-East Queensland both are used to some degree in the treatment of your tap water.

Chloramine is used as a secondary disinfection, is formed when both chlorine and ammonia are added, and provides a longer lasting effect than chlorine which makes it useful in large distribution systems ie; metro areas.

Does it matter which one where my plants are concerned?

Because the concentrations in the water are low for both, you would have to say no.

It might only be a consideration if you are currently (or are planning to) either boil your tap water or let it stand before watering your indoor plants with it.

This is because chlorine and chloramine have different properties:

• Leaving tap water to 'age' will get rid of the chlorine in the water over the course of a few days. But with chloramine it will take several weeks.
• Boiling tap water might get rid of the chlorine in a few minutes but it will take 15 times longer to get rid of the chloramine! 

So chloramine is tougher to deal with.

Do you have chloramine in your tap water?

In an email enquiry response, SEQ Water stated that Mt Crosby East, West Bank & North Pine water treatment plants use chloramine as a secondary disinfection treatment and that it is "useful for maintaining disinfection in a large distribution system such as the wider Brisbane-Ipswich-Logan-Moreton metropolitan area".

The easiest way to determine whether your particular water supply is from a chloraminated system or not is by the amount of free chlorine in the water.

In a chloraminated system the level of free chlorine will be what is termed 'less than the limit of reporting' ie; zero or near zero.

In South-east Queensland refer to Water quality report | Seqwater to determine levels.

 

Other sources of fluoride

The two most commonly cited are phosphate fertilisers (in particular superphosphate) and perlite.

Superphosphate, specifically single superphosphate (SSP), is credited with driving agricultural development in Australia and New Zealand.

Today, SSP is readily available for gardeners in Australia either online or at garden centres and hardware chains.

The sources of the phosphate rock used for it's manufacture, though, were for decades high in cadmium (Cd) and continue to be high in fluoride (can't change that, it's in the rock itself).

Cadmium is a heavy metal that works its way from the soil all the way up the food chain to us, accumulates in the kidneys and is a known carcinogen.

Nowadays the phosphate rock used in SSP manufacture comes from a range of sources to reduce the Cd content of the product.

Technical information from Incitec Pivot Fertilisers states the Cd content in their SSP (in 2015) was typically up to the maximum allowed (300mg Cd/kg P*).

*300mg Cd/kg P equates to 26ppm, down from the 38-48ppm CSIRO measured in SSP in 1973.

A cynical person might suggest though that the gradual reduction in Cd and maximum allowed levels (self-managed by the fertiliser companies) deliberately mirrors/mirrored the reducing availability of high Cd raw phosphate rock from the geographically close deposits of Nauru & Christmas Islands.

Anyway, I digress, sorry about that. 

Where were we? Ah yes, Fluoride. 

I found it hard to find a lot of data on the fluoride (F) content of phosphate fertilisers, but I did find a review in 2000 in the New Zealand Journal of agricultural Research^^ that showed the fluoride content of SSP to range from 1.08-1.84%  and in more concentrated phosphate fertilisers up to 3%.

The bottom line on fertiliser fluoride for indoor plants

The kind of fertiliser's being used for indoor plants aren't solely made up of phosphate fertiliser and the % is generally not high so the amount of fluoride in them is effectively diluted.

And I can't see any situation where a straight phosphorous fertiliser would be required anyway.

I looked at one indoor plant food (dry, granular, soluble) marketed in Australia.

Based on component analysis, the phosphate fertiliser (in this case a combined high-analysis phosphorous and nitrogen fertiliser) makes up only 20% of the product. 

And some data from a paper produced by the  North Carolina State University (NCSU) showed resin coated controlled release fertilisers (see also our blog 'feeding your succulents') had only 15% as much fluoride as SSP.

If this is still a concern personally for you though, you will need to look at fertilisers that do not use any form of phosphate rock for their phosphorous (P) source.

And if you happen across any marketing promoting 'soft' phosphate rock (also referred to as Reactive Phosphate Rock or RPR) as an organic source of P in fertiliser blends then our suggestion is to avoid the product for use in potted plants for these reasons:

• The availability of P to the plant from RPR is dependent on a few factors and acidity is the key one. Conversion of the P to plant available forms is proportional to the acidity of the soil and above pH6, release rate is very very slow. I couldn't find any data specifically on potting mixes (there is very extensive data from soil trials) but you would think the same applies to potting mixes as it does to soils? Your potting mix pH will probably be around 6.5
• In SSP manufacture some of the F in the rock is removed as a by-product and ultimately ends up as, guess what, F in town water. Some RPRs have just as much fluoride content as the raw phosphate rock used in SSP but that F remains in the product 100%. There is no process where some is removed.

 

Perlite

The lava and debris that spews out of volcanic eruptions contains fluoride (F), so it's not a surprise then that perlite, which is manufactured from volcanic rock, contains F.

Some recommendations suggest perlite should not be used in potting mixes for indoor plants because of the F content.

The same NCSU paper I refer to above identifies the following:

• The amount of F in perlite, although it does vary, is not high
• Although the F will readily move out of the perlite into growing media when first watered, it will also dissipate very quickly from subsequent watering
• Some commercial growers maintain a growing media pH of 6.5 to 6.8 because it has been reported that at that pH the F binds with calcium to form plant unavailable calcium fluoride
• Trials with a very acidic growing media containing 50% perlite failed to produce toxicity in three different types of fluoride sensitive plants

Conclusion

Any thoughts that perlite contributes to fluoride toxicity in plants are unfounded, in my humble opinion.

Perlite also contains aluminium but because the typical pH of potting mixes is well above the acid levels required to solubilise aluminium, this should not be a worry. 

 

 

 

^This data was averaged from the most recent monthly water analysis reports available online when researched at the end of February 2022 and included data from Brisbane, Sunshine Coast, Moreton, Redland, Logan and Gold Coast regions. No data was available online at the time for Somerset and Scenic Rim. 

*These include Peace Lily's (Spathiphyllum, Never Never Plant (Ctenanthe), Chamaedorea, Spider Plant (Chlorophytum), Calathea and some?Yucca.

^^Fluoride: A review of it's fate, bioavailability, and risks of fluorosis in grazed pasture systems in New Zealand, Sept 2000, S.Cronin, M.Hedley, P.Loganathan, V.Manoharan.