Fluoride Toxicity in Plants – Is Tap Water Harmful?

Many municipalities add fluoride to tap water and fluoride can be toxic to plants, so is this a problem? Is tap water harming your plants and what can you do about it?

A quick search on the internet reveals lists of fluoride sensitive plants – how sensitive are they and should you stop growing these if your tap wat has added fluoride?

Let’s have a look at the facts regarding fluoride and plants.

What is Fluoride?

Fluorine is the elemental form and is usually found in its ionic form which is called fluoride. As fluoride it forms salts with things like calcium, magnesium and sodium. Combining it with sodium produces sodium fluoride, which is similar to table salt (sodium chloride). It is estimated to be the 13th-most abundant element in the earth’s crust and it’s widely dispersed in nature. It is found in soil, water, air and all living things.

Is Fluoride Toxic to Plants

As with all toxicities, this depends very much on the dose. If plants are exposed to too much fluoride it will harm them.

Fluoride can cause visible injury as well as reducing growth rate. These two are mutually exclusive so even if you don’t see damage, the growth rate can still be affected.

Fluoride toxicity affects germination, growth, photosynthesis and yield. It interferes with calcium which is essential in fertilization. Symptoms include chlorosis, marginal and tip necrosis (brown tips), decreased seed production, and the drop of leaves, flowers, or fruits.

Is Fluoride an Essential Nutrient for Plants?

Fluoride is found in all plants, fruits and vegetables, but it is not an essential nutrient. Plants grow just fine without it.

Fluoride in Soil and Plants

Fluoride is absorbed by plants through the stomata as they take in air and through roots when they absorb water, but it is not very mobile in plants. When absorbed through roots, much of it stays in the root. Fluoride that reaches a leaf usually stays there and migrates to the tip and margins of the leaf, where it accumulates. Brown tips can be caused by high levels of fluoride accumulating there.

Most of the fluoride in soil combines with other ions like calcium to become insoluble and then plants can’t absorb it. It is also held strongly by clay and organic matter, especially in more alkaline conditions. Fluoride is a bigger problem in soil with a low pH, or low amounts of clay and organic matter. Most houseplants are grown in soilless mix so they have no clay and little active organic matter, making fluoride toxicity higher in such soils. Peat moss is not as effective at holding nutrients than other forms of organic matter.

Samples of soil from Pennsylvania had values of 377 ppm for total fluoride and 0.4 ppm for soluble fluoride – only the soluble form (0.1%) is available to plants.

Some phosphate fertilizer contains fluoride as an impurity (1 to 3%) and there is some concern that this fluoride will build up in agricultural soil to a level that will be toxic to plants and livestock. Testing of fluoride levels shows this is not a warranted concern.

Fluoride Sensitive Plants

Some plants are reported to be more sensitive to fluoride than others. That should not be a surprise because each species of plant has a slightly different biochemistry. Some plants accumulate up to 4,000 ppm and do not show signs of toxicity while others show signs at 20 ppm. Note that these numbers are the accumulated fluoride levels in the plant, not the levels in the irrigation water.

This source has a list of fluoride sensitive plants. According to this reference a plant is considered to be sensitive to fluoride when “injury has been observed and when leaf analysis revealed a fluoride content less than 50 ppm”. There is a fundamental problem with this definition – the difference between cause and effect.

Just because a plant has symptoms of fluoride toxicity does not mean that fluoride is causing them. For example, high levels of fluoride in spider plants is reported to show up as brown tips on the leaves. But high salt levels will also cause brown tips even if there is very little fluoride in the plant.

What is the Toxic Dose for Fluoride?

This is really the key question we need to answer. How much fluoride is too much?

An experiment was carried out to determine the level of fluoride that causes brown tips in spider plants (Chlorophytum). Plants were treated with 0,  3.16, 10, 31.6, 100, 316 ppm fluoride. After 6 weeks there was no sign of fluoride toxicity at any dose. The same study also looked at Plectranthus, which is also reported to be sensitive to fluoride and doses of 100 and 316 produced “some marginal chlorosis on the older leaves of plants”.

The University of Florida reported that in spider plants, fluoride damage is much more severe when plants are grown at high light levels and high fertilizer levels. Leaves with fluoride toxicity symptoms were found to contain 70 ppm fluoride (dry weight) while healthy leaves contained only 25 ppm.

In 1977, the Soil and Plant Lab, tested spider plants in various growing media: Canadian sphagnum peat moss, clay, perlite, etc. Some of the plants were heavily fertilized with superphosphate. No measurable amount of tip burn was seen even with 3 ppm fluoride. “The studies concluded that many of the injuries previously attributed to fluoride probably were due to unfavorable cultural conditions“.

Tomatoes were tested with 0, 10, 25, 50, and 100 ppm fluoride in controlled conditions. Even at 10 ppm growth rate as well as other growth parameters were affected. No statistics was done on the data.

Cut gladiolus showed petal margin deterioration and necrosis at 1 ppm fluoride, and even 0.25 ppm was enough to cause damage to petals and leaves after 4 days.

A level of 1 ppm fluoride is considered safe in the horticulture industry, but they do caution some plants may be more sensitive.

What does all this mean? There is no single dose that is toxic. It depends on the plant type and growing conditions. Several of the reports I looked at suggested that fluoride gets blamed for a lot of damage that is probably due to other factors.

Most studies are short term and it is known that fluoride accumulates over time. Low doses may not affect plants very much short term, but over the long term the affect may be more pronounced. A lot more research is needed.

Fluoride in Tap Water

How much fluoride is there in tap water? That depends on where the water comes from. Natural drinking water in the US has an average fluoride level of about 0.2 ppm, although in some places it can be much higher. Most natural water in the US has less than 2 ppm, but areas in Colorado can have levels as high as 14 ppm.

Water sources with high natural fluoride affect up to 60 percent of populations in Pakistan, the African continent, Thailand, China, and Sri Lanka. Values as high as 18 ppm are found.  Levels in bottled water in Australia tend to be below 1.6 ppm.

The United States Public Health Service recommends an upper level of 0.7 ppm. The Canadian Drinking Water Guideline maximum recommended level for fluoride is 1.5 ppm and in the Alberta Health Drinking Water Guideline it is 2.4 ppm.

Removing Fluoride from Water

Some people suggest boiling water to remove fluoride. This is a myth and may be spread because people confuse fluoride with chloride. Chloride can be boiled off, fluoride can not.

Most home tap filtration systems, like Brita and Pur, do not remove fluoride. “The types of filters that do remove fluoride are activated alumina filters, reverse osmosis units, and distillation setups”.

Fluoride in Tea and Coffee

A lot of people take their cold tea and coffee and pour it on plants. I discussed all the human food that people put on houseplants in Feeding Plants From the Kitchen – Which Products Actually Work?

Tea plants are known fluoride accumulators. Brewed green tea contains fluoride in the range of 1.9 to 6.8 ppm (made with deionized water), depending on the type of tea. When brewed black tea was tested (54 brands) the range of fluoride was 1.6 to 6.1 ppm.

Brewed coffee has less than 1 ppm fluoride. A second study reported values between  0.013 and 0.502 ppm.

Pouring cold tea on plants is not a good idea.

Will Fluoride in Tap Water Harm Plants?

When I started writing this article, I was quite sure that the fluoride in tap water would not harm plants, but this may not be true. The answer is certainly much more complex than gardening reports suggest. Here are some key points to understand.

1. Plants can be harmed by fluoride without showing any visual symptoms. Gardeners frequently comment with, ” my water is fluorinated and my plants are fine”. The problem with this statement is that gardeners rarely do controls and the absence of symptoms does not mean plants are not harmed. They could be growing slower than normal due to fluoride.
2. The presence of fluoride toxicity symptoms does not mean they are caused by fluoride. These symptoms can be caused by a number of other factors, include other salts, low humidity or even plant shine products.
3. Some plants are more sensitive than others.
4. The soil or potting media has a significant effect on the amount of fluoride that is absorbed by plants. Clay, organic matter, high levels of calcium and higher pH all reduce the impact of fluoride on plants.
5. Fluoride is accumulated in plants which means the amount in plants goes up over time. Most testing is short term and plants held by gardeners is long term.

Most of the research I found looked at fluoride levels of 10 ppm or even 100 ppm, while tap water levels are in the order of 1 ppm. I found very little data on the effect of fluoride at 1 ppm and no long term studies. To be fair a lot of research on this was done prior to the 1950’s and may not be readily available. It is quite possible that low levels of fluoride do not harm plants during the relatively short production/nursery phase, while harming them once grown for several years by gardeners.

The level of fluoride in most tap water, even if it is fluorinated, has a level of about 1 ppm. That level does not seem to cause visible symptoms on most plants. We don’t know if it stunts growth.

Garden plants are less likely to be affected because they tend to grow in soil containing clay and organic matter. This is especially true if the pH is above 6.0. Sandy soil or peat based soils with very low pH can be a problem.

Houseplants are more likely to suffer damage from fluoride since they don’t grow in soil. The common media used is peat based which tends to be acidic. Indoor plants are also watered more often since homes are drier, which adds more fluoride to the plant. If your tap water has a higher pH or is more alkaline, you will have less of an issue with fluoride.

Reducing Damage from Fluoride

What can a gardener do to reduce possible fluoride damage?

• Use fertilizer that has zero fluoride, nitrogen in the form of nitrate instead of ammonium, and contains calcium. Calcium acts like a sponge absorbing fluoride.
• Repot more frequently because fluoride accumulates in the soil and is not washed out easily.
• Use rain water more often.
• Keep the pH above 6.0 and preferably closer to 7.0, provided plants grow at a higher pH. You can add one teaspoon of gypsum or two teaspoons of limestone to a six-inch pot of soil.
• Don’t pour your cold tea on plants.
• Don’t blame the symptoms you see on fluoride.
• Check with the local authority and find out the fluoride level in your tap water. If it is below 2 ppm it should be safe for most plants. Mine is 0.33 ppm.