Author: Linda Chalker-Scott

You all remember the story of Chicken Little, right? Chicken Little thought she’d been hit on the head by a piece of the sky and ran around alarming the rest of the barnyard animals, who assumed Chicken Little knew what she was talking about. Had they not all been eaten by Foxy Loxy, I’m sure they would have felt foolish discovering that an acorn, not a piece of the sky, had bopped Chicken Little on the noggin.

On to today’s science rant.

Ann McCormick, one of my GWA (Garden Writers Association) colleagues, alerted me to an online story from Live Science entitled “Water Drops Magnify Sunlight and Burn Leaves.” A quick look at the internet shows that this report has gone viral, with similar headlines from other websites including the venerable Scientific American (“A study in the journal New Phytologist confirms the gardener’s belief that droplets of water resting on some types of leaves can focus sunlight until the plant’s surface actually burns”). It gets more and more ridiculous by the day (“Sun shining?  Then don’t water your plants” courtesy of the Daily Mail in London; “Water droplets can form forest fires” from Calcutta). And so on.

I tracked down the original article in New Phytologist, entitled “Optics of sunlit water drops on leaves: conditions under which sunburn is possible.”  You should take a look at it, if only to become completely intimidated by the physics and computer modeling it contains. I’ll be honest – I didn’t even try to understand this portion but focused on the plant science.

The authors had three actual experiments in addition to the optical modeling.  The first experiment involved placing glass spheres on detached leaves (Acer platanoides – Norway maple) and exposing them to sunlight. Yup, glass spheres caused leaf burn on sunny days – no big surprise there. The second experiment substituted water droplets for the glass spheres and tested Ginkgo biloba as well as maple leaves. Not surprisingly (to me anyway) there was no damage to leaves of either species. The third experiment repeated the second, but tested the leaves of the aquatic fern Salvinia natans and voila! Leaf damage!

I have a lot of issues with this paper and maybe we’ll have to extend blog coverage for a day or two to keep today’s discussion as short as possible. Let me point out just two of the experimental problems

The leaves for experiments 1 and 2 were detached from the plant prior to treatment. How a detached leaf resembles anything in a natural situation is beyond me. Furthermore, these leaves were laid out, covered with glass spheres or water droplets, and left in the full sun for as long as nine hours. (Even so, the leaves covered with water droplets didn’t burn! You go, detached leaves!)

The Salvinia experiments were conducted on leaves scooped out of a pond, placed in two containers, sprinkled with water, and left in the sun for two hours. Then, in the authors’ words, “the experiment was concluded by cutting and scanning several Salvinia leaves – still holding water drops – in the laboratory in order to document their sunburn.” We are not told (a) how many leaves were scanned, (b) how the leaves were chosen, since they didn’t scan them all, and (c) how the reported damage was proven to be from water droplets. >Worse, there are no statistical data. We are simply asked to believe their report in the absence of any evidence except a handful of photographs. (As an aside, I really would like to know how they were able to cut and transport leaves without the water droplets moving!)

The lack of scientific rigor in this article is disappointing, especially in a peer-reviewed journal. Is there any plant scientist would consider detached leaves to be a model for those on an intact plant? Is there any gardener who would consider an aquatic fern comparable to trees and shrubs? Would any species – including aquatic ferns – easily burned by the combination of water and sunlight survive in the real world for long? I don’t think so – hence my myth posting on this very topic several years ago.

Let’s review: leaves of one species of aquatic fern was damaged by something – possibly sunlight – but without enough data presented to really evaluate the claim. None of the tree leaves tested were affected, even though they were detached from the plant and could not benefit from transpirational cooling. Yet the alarm has gone out! Don’t water your garden plants when it’s sunny, or the leaves will burn!!!

Be sure to take everything you read with a grain of salt – or an acorn.

(I will continue discussion of this article further if there is enough interest – that means you need to post a comment!)