Fluoride: What it is, and how it's spelled

I was inspired to write this post because of a recent Garden Web thread. It was one of those deals where I started writing a response about one specific thing, and kept adding in other stuff, and the response got longer and longer and longer until it reached the point where I thought I'd be kind of being a jackass if I posted it. Which is one way that having a blog is nice.

So here we go.

Fluorine is a chemical element. It's a greenish, very toxic and reactive gas, composed of diatomic (= two atoms) molecules, F₂.

Model of a diatomic fluorine molecule. Not terribly informative, I know: I looked for pictures of the actual gas but all such pictures I found were copyrighted. This image is by benjah-bmm27 and was taken from Wikimedia.
When a molecule of fluorine picks up a couple extra electrons from somewhere, you get two fluoride ions. Fluoride is toxic to humans in large amounts, but in small amounts added to drinking water, it can strengthen teeth. More on that later. Because fluoride is negatively charged (due to the extra electron it's carrying), it can't just float around without something positively charged to balance it, e.g. sodium fluoride or calcium fluoride. More about calcium fluoride later, too.

Most plants aren't particularly touchy about fluoride levels in water, but there are a few that are, including some of the more popular ones:

• Cordyline fruticosa is sort of famous for developing burnt tips and margins from fluoride; this, combined with its susceptibility to spider mites, probably accounts for why it's not a more commonly grown indoor plant.
• Chlorophytum comosum will get tip burn from fluoride, sodium, or boron.
• The entire Dracaena clan, except possibly for D. surculosa, gets terrible tip burn when fluoride levels are high; this is a difficult problem to deal with, as Dracaenas are often sold as large, floor-sized specimens which are hard to drag to a water source to flush out.
• Calathea and Maranta spp. (e.g. Calathea ornata, Maranta leuconeura erythroneura) also develop tip and margin burn, especially on the oldest leaves, often with a yellowish halo at the edge of the burn.
• Asparagus spp. don't seem to be that picky about fluoride in water, but fluoride in the air (which sometimes happens following volcanic eruptions, or near incinerators) can cause sudden, massive leaf drop.
• The kentia palm, Howea forsterana, is also sensitive to atmospheric fluoride, and is slightly sensitive to fluoride in the soil.
• Chamaedorea species (elegans, metallica, seifrizii, etc.) get tip burn, as will many other palms (Chrysalidocarpus lutescens, Rhapis excelsa).
• Yucca guatemalensis can experience fluoride toxicity, but it tends to be slow in developing and fairly easy to fix with a soil flush.
• Aspidistra spp. (elatior, lurida) and Tradescantia spp. (zebrina, pallida) are also somewhat, though not extremely, touchy about fluoride.

Probable fluoride burn on a Cordyline fruticosa at work. Or if it's not fluoride, it at least looks like fluoride damage would look. My own photo.
If your water supply contains a lot of fluoride, or if flushing your plants with large amounts of water is impractical for some reason, you should either steer clear of the above-listed plants or invest in a water purifier of some kind: reverse-osmosis filtration is popular, if expensive. Distilled water may also be an option. One can also just avoid the plants on the above list and grow plants which are less susceptible. (Most of the sensitive commonly grown species are in a pretty small number of families: avoid the Marantaceae, Arecaceae, Ruscaceae, and Liliaceae, and you should be relatively safe)

Some people also advise letting water sit out overnight before using it, to give the fluoride a chance to evaporate. This isn't quite wrong, but it doesn't work as well as you'd expect. Fluoride in water can escape, particularly in acidic conditions, by finding a stray hydrogen ion and combining to form the gas hydrogen fluoride, which will then drift away with the air. However, as best as I can determine, this happens too slowly to be useful for horticultural purposes. If the air is dry enough, you may find that letting water sit out to remove fluoride actually removes enough water to concentrate the fluoride.¹

Changing your water supply may not be enough, depending on what other things you do. Certain fertilizers contain superphosphate, the name for phosphates derived by dissolving phosphate rocks with sulfuric acid.² This is, you know, fine, except that the resulting superphosphate can have any number of other things in it, depending on what rocks were dissolved, and fluoride is a common impurity. Time-release fertilizers like Osmocote are typically safe; other fertilizers may specifically state that they do or do not contain superphosphate, or will indicate what amount of fluoride ion the product contains, or etc. Read the labels.

Another possible accidental source of fluoride is perlite, a common -- and very useful -- soil additive which keeps potting mix light and well-aerated, enabling soil to dry quickly and getting air to plants' roots. Alas, perlite is also made from rock,³ and so depending on the source, it may or may not contain harmful amounts of fluoride.⁴ Perlite can still be used for fluoride-sensitive plants if it's rinsed well with water first, though obviously if you're rinsing with fluoridated water then you're not necessarily helping. Also I imagine it feels kind of silly to wash perlite.

Growers add calcium to soil, and raise the soil pH, to keep fluoride levels from affecting plants. Calcium and fluoride form calcium fluoride (CaF₂), a relatively insoluble mineral which is named fluorite when found naturally. The fluoride ions are still there, but the calcium prevents them from dissolving in water, so they're not available for the plant's roots to absorb and consequently don't cause problems.

Most people never make a point of flushing their plants' soil out, so minerals of all kinds accumulate, fluoride among them. For this reason, even if you're using fluoridated tap water, rinsing the soil heavily with water can reduce fluoride toxicity problems just by bringing the level down to whatever it is in the water. Also, flushing soil out at every watering (which is what I do) can prevent toxicity issues from ever developing in the first place.

Picture of fluorite crystals. Picture by Ryan Salsbury, taken from Wikipedia.
Finally, a note on spelling. I know that this is dickish of me, to rant about spelling, but it bothers me to see this done wrong, so I feel like I have to say something. It's not aimed at anybody in particular, if this reduces the dickishness at all. And as far as it goes, I know uor is a lot less common sequence of letters in English than our; it's easy to make typos, and I do it too. But:

The mineral fluorite (calcium fluoride, CaF₂: I told you we'd get back to it eventually) is what started this whole stupid-spelling business. Fluorite glows under ultraviolet light; this action was named fluorescence after the mineral, which led to fluorescent lights and highly-fluorescent chemicals like fluorescein. The element fluorine was prepared from molten fluorite, and consequently was named after it. Then the fluoride ion was named for the element fluorine, as were things like chlorofluorocarbons and water fluoridation (which changes the mineral in tooth enamel from apatite to the stronger fluoroapatite).

Fluorite the mineral was named fluorite via the Latin fluo, meaning "to flow," which referred to its low melting point, and the stupid Latin vowel order has been passed along through all the above links, plus several hundred others.

More fluorite, obviously crystals again instead of molten. Molten fluorite is apparently difficult to come by, or not very photogenic, or something. Picture by photolitherland, taken from Wikipedia.
Flour, by contrast, is the generic word for grain which has been ground into a powder; there's no meaning as far as I know for the words flourescent, flouride, flourite, or flourine.⁵

(Though I suppose if flour glows when you shine UV light at it, the glow could reasonably be called flourescence. I don't know whether flour glows under UV or not. Probably not. But if it did, that'd be a name for it.)

So, to sum up: 1) some plants don't like fluoride in their water. 2) Those plants should get water without fluoride. 3) Sometimes you can rinse fluoride away or bind it with calcium. 4) The low melting point of calcium fluoride has made us all bad spellers.

-

¹ I think a lot of the confusion here stems from the fact that one can remove chlorine, a very similar element, from chlorinated water by letting it sit out. The trick is that the chlorine in chlorinated water is present as a complex mix of ions and molecules, including chlorine (Cl₂), hypochlorous acid (HOCl), hydrogen chloride (HCl), hypochlorite (OCl^-), chlorite (ClO2^-), chlorate (ClO3^-), and perchlorate (ClO4^-), with chloride ion (Cl^-) forming a very small proportion of the total. Some of these species are more easily evaporated than others.
Fluoridated water, by contrast, contains most of its fluoride as hexafluorosilicate ion (SiF₆^2-) or fluoride ion, depending on the city adding it, and so the chemistry is very different: there are no fluorine analogues for many of the chlorine compounds listed above. Fluoride ion pretty much only leaves under acidic conditions; hexafluorosilicate can decompose in water to form hydrogen fluoride (HF) and silicon tetrafluoride (SiF₄), both gases. I couldn't find anything that addressed the question of whether the hexafluorosilicate was stabilized to prevent this from happening, but it seems like something really ought to be, because if it's not going to stay in the water then it's not going to serve any purpose to add it.
Iowa City's drinking water fluoride concentration ranges from 0.8 to 1.5 ppm, according to the 2006 Water Quality Report for the University of Iowa (pdf). I could not determine whether it was added as fluoride ion or as hexafluorosilicate. Chlorine levels (including all forms of chlorine, not just chloride ion) were similar (1.3 ppm) in the most recent report I could find.
² Yeah, sulfuric acid is pretty badass stuff. For fuck's sakes, be careful with it.
³ Perlite is essentially rock popcorn: pieces of rock, heated very rapidly, without giving the water in the rock time to evaporate. When the water can't take it anymore and evaporates anyway, it breaks and expands the rock as it goes, turning it into glass foam, basically. The bright white color is due to reflection from all the little bubbles so formed (like with the head on a beer, or thick soap suds). Actual popcorn is essentially the same thing: quickly heated water vapor, in a starchy seed, expands all at once and produces a light, fluffy white foam of starch.
⁴ According to Wikipedia, perlite is usually produced from obsidian, which does not contain enough fluoride to be worth mentioning, but you know how reliable Wikipedia can be. And anyway, even if it's not a lot -- not a lot relative to what? We're talking about parts per million here; it doesn't have to be a lot in order to hurt sensitive plants.
⁵ Also I believe someone should write a children's book about a young female baker named Flourine. Possibly she could have a partner named Sugarette and a dog named Butter. I'm not sure what the plot of the book would be, but c'mon: I shouldn't have to come up with everything.