Several of the posts here about guttation said it's an indication of over-watering. My guess is no, but of course that's coming from no experience, so I'm asking.

First of all, is there really even such a thing as over-watering? I sort of thought there was only inadequate drainage, sometimes compensated for by scant watering. And for some plants in some situations I think there can be soil ecology issues, where the plant's favorite mycorrhizae will be displaced by other soil microbes if the soil doesn't dry between waterings as much as the mycorrhizae are used to.

I haven't re-potted my pothos since it was given to me, and I didn't ask what the potting medium is. But it seems adequately coarse, the pot doesn't seem excessively heavy after watering, and the plant is growing at what seems to me like a normal rate.

Another thing my search turned up is that guttation fluid isn't unmodified xylem sap.
http://www.plantphysiol.org/content/41/10/1743.long
The leaves are getting mineral nutrients from that sap. Salt, in the terminology of the article, but the authors are talking about osmotic potential, not sodium chloride. So I think the plant is doing it intentionally, sending up the minerals to the leaves, and disposing of the excess water (along with anything else in the sap that the leaves can't usefully remove).

Another thing that makes me think so is that only the last couple leaves at the end of the shoot were guttating. Those leaves are presumably still growing, and need the minerals.

Any opinions?

I have always considered guttation a normal function of plant metabolism. Many of my plants do it, both the ones grown in soil or hydro and I've seen this fluid squirting out of the tips of leaves our raspberries in our garden, particularly after a good rain so water quantity certainly has some effect on it.

Personally, I just clean it up where my girlfriend will yell at me about it and ignore it elsewhere.

Chemically speaking, when referring to "salt" in a solution, it can mean any metal or mineral dissolved in that fluid.

It doesn't seem to be related to anything but soil moisture that I can tell although it does happen faster in higher heat/humidity, from unscientific, sporadic observation.

That's just supposition, glad you brought this up. I'm curious too.

I don't put much stock in the sources that say it's excess moisture overwhelming the stoma when transpiration isn't sufficient to release as much moisture as the plant has taken in. None of them are science-y, just speculating I think, since these sources seem to indicate that guttation is a sign of a problem. I completely disagree.

This is a sentence I found that seems wrong to me, "As a result, root pressure builds up - water rushes into the plant but with low temperatures, a traffic jam occurs at the stomata exit." And, " Certain uncommon conditions have to be met for guttation to occur.... Guttation happens when there is excessive water in the soil and there is an absence of water loss through transpiration." My plants that guttate are not in unusual conditions. Just because it's written on the internet, doesn't make it true.

I've seen guttation mostly during the daytime... 'cuz I'm asleep at night. ...or did it happen during the night and it's just still dangling there when I notice it later?

I found many "bad" sources that seemed incompetent or incorrect, but not a "good" one good enough to share. Nothing that seems conclusive one way or another. What links have people found that seem reputable, competent, based on science?

Josh

"Experimental and control plants were exposed to mineral solution during the night (MN-plants) and day (MD-plants), respectively. ... Finally, the water movement caused by root pressure is taken as 150 ml. This is based on the fact that guttation drops appeared only on MN-plants during the night, and the difference in the water lost during the night between MN- and MD-plants was 150 ml ... ."
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC55440/

Last night I was thinking that guttation helps with mineral transport. But that article also points out that mineral transport is done much, much more by the flow of water for transpiration, growth water, and circulation of phloem sap back to the roots. Transpiration exceeds guttation by something like a hundred-fold for the plants in that article.

Now I'm thinking that the main function of guttation is to get rid of elements the plant doesn't want. For example, "Salt stress aggravates boron toxicity symptoms in banana leaves by impairing guttation."
http://www.ncbi.nlm.nih.gov/pubmed/22765264

So I conjecture that heavy guttation, in a type of plant that doesn't always have it, means your micros are out of balance. If someone has the materials to try it, here's an experiment. Grow a bunch of some plant that commonly guttates, but not universally. Use a potting medium, a main fertilizer, and a water supply that can be expected not to supply an excess of any micronutrient. Add micros in various ratios and amounts. I predict that the plants will guttate more with increasing amounts of micros beyond the level where they have enough, up to a maximum that the plant is capable of, which will be the same level where toxicity from the micros first starts to appear. As you vary the ratio of the micros, I predict that plants will guttate enough to dump the micro they have the greatest excess of. Increasing that one will make them guttate more, whereas increasing others will have no effect. Unless, that is, the plant is deficient in one or more micros. In that case, remedying the deficiency will make the plant grow more, therefore take up more water, therefore take up more of the greatest-excess micro, and therefore guttate more.

However, I would expect that some plants have evolved under circumstances where one or more micros are almost always present in excess. Those plants will guttate all the time (unless prevented by dry/salty soil or drowned roots), because there's no point regulating something that you always want to have turned on. And other plants shed leaves fast enough (or lose them to herbivores fast enough, or die back for the winter soon enough) that they can just let their excess micros accumulate in the leaves, and not worry about it. Hence the specification of a plant that guttates readily but not consistently.

And another thought on the purpose of guttation: the sugar in guttation fluid may not just be lost because retaining it would cost more than it's worth. Maybe it's there to help support commensalists.

Suppression of Bacterial Blight by a Bacterial Community Isolated from the Guttation Fluids of Anthuriums
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC91139/

For several years, I never once fertilized or re-potted my Pachira,
and yet it guttated then as well. Would that count for testing the null-hypothesis?

Here's my plant...grown from a single broken leaf.

Josh

Xylem tissues carry liquids UP from the root hairs all the way to the leaves where it evaporates via the numerous stomata located on the upper and lower surfaces. Evapo-transpiration is a mighty force.....a mature tree can transpire several tens of thousands of gallons......a day!

It is entirely driven by evaporation. At night, evapo-transpiration ceases. If water continues to load up at the roots, it has to get out somehow. That's where those little hydathodes along the margins come into play.

Some kinds of plants guttate practically every night; it's simply part of the natural physiology. Haven't you ever noticed how your lawn always seems to have dew (condensation) on it every morning, even when nothing else does? It's not dew.

If your houseplants are guttating excessively and often, you might be watering too frequently....or the potting medium stays too moist for long periods of time.

Poetically: It's not dew... on your shoe.

Regarding the handful of plants I have that do it, I would have to see signs of some kind of unhealthiness before I would consider anything was being done too much to a plant, water, fert, humidity, any of the maladies proposed here or elsewhere out there that are supposed to be the cause of guttation.

I can't find any time-lapse vids of guttation, btw... but if they didn't call it time lapse guttation, how would they expect my search for such to find it?

(P.S. Who's got a little part of them who thinks it's plant pee?)

If plants have specialized organs to do it, that means they're doing it for a reason, rather than having it be something that just happens as a passive result of root pressure. Plant pee is my current favorite.

Here's a page that mentions guttation in zinc-deficient plants. I wonder whether that's because of antagonism from iron or phosphorus. (Sticking with the plant-pee hypothesis.)
http://www.bananas.org/f356/deficiency-essential-minerals-9038.html

Here's someone saying they notice it especially on new leaves of their banana plant:
http://www.bananas.org/f311/dew-leaves-10339.html#post109681

I've never given it a second thought, really...other than to mention to others that it happens
on this particular plant. I just went through all my Pachira pics and found only one with
a drop on it. Pic is from March 9, 2011:

Josh

Planto

Josh

The above article contains some commentary in its introduction about the state of ignorance on the subject of guttation. Plant pee still sounds likely to me, but no one really knew as of 2004.

Alternatively, what evidence is there that eliminating excess water is the purpose?