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.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.
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.
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:
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.
- 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.
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
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.
, 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 flour
escence. 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.