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A few months ago I was trimming back the trees which continually encroach on my deck (left), when I noted that on the handrail, lichen I had completely removed only four years ago had spread in parts to the full 150mm width of the handrail.
Then I made an interesting observation: there were clear areas surrounding the nails used in the construction (left). It appeared there was something discouraging the lichen from growing close to the nails.
This gave me an idea. I wondered if the iron of the nails was responsible, and if I could use this phenomenon to discourage the lichen from growing on the rest of the rail.
I decided to conduct an experiment. At my age I find I am prepared to take a long view of things, and decided to just leave a piece of iron attached to the rail over the lichen and see what happens. I had a broken old-style butterfly can opener (left) that I had used as a sacrificial anode while experimenting with electrolytic rust removal (which works a treat) and just tied it to the rail to see what happens.
Then the other day while talking about this at work, I speculated that maybe as the nails are galvanised, it is not the iron of the nails, but the zinc of the nails' galvanising that is inhibiting the lichen.
The video at left shows someone making the same observation I did.
This opened a very fertile (or should that be infertile? Ha-ha) area of research. I found that in the United States and Canada, where roofing of wooden shingles is common, strips of zinc sheet or ribbon are fixed to the roof so that leached zinc discourages growth of both lichen and moss. Someone has figured it out well before me.
I decided I should know more about lichen.
Click "Read More" for exciting research results!
Crustose: Flat, close to the surface
Fruticose: Branching, 3D
Being living things, many of them do not fit neatly into these boxes we have made, but a lot of effort has gone into classifying them.
NZ has approaching 2,000 species with more to be discovered — 10% of the world’s lichens in 0.18 % of global land area!
I found two excellent references:
Lichens of New Zealand: An Introductory Illustrated Guide (pdf 104MB),
and a key to identification,
New Zealand's Foliose Lichens: An Illustrated Key by Nancy and Bill Malcolm and Allison Knigh Micro Optics Press. (pdf 20.5MB)
It is not an easy task to identify them as many look similar, and you soon run into a maze of technical terms for the structure and parts. This is not to say one cannot tell differences, but rather it is just difficult dealing with so many unfamiliar terms at once. A typical identifying choice: -
Does it have: - Fruiting bodies a sessile or stalked mazaedium
or: - Fruiting bodies not a mazaedium
so.... off you must go to look up "sessile" and "mazaedium" to proceed any further.
My deck lichen seems to be "foliose" to me. Just by illustration eyeball scan in "Lichens of New Zealand: An Introductory Illustrated Guide" mentioned above, a probable ID is: -
OMG! a Google search for "Xanthoparmelia scabrosa" zinc brings up 26 health remedy hits, chiefly it seems concerning erectile dysfunction, before something useful!
The 27th hit, coincidentally, although from the US Geological Survey, is a New Zealand paper entitled "Element content of Xanthoparmelia scabrosa growing on asphalt in urban and rural New Zealand", and deals with the lichens ability "to accumulate high levels of Cu, Pb and Zn [which] may make it useful as a remediation tool."
Here's a problem.... I want to discourage it with zinc, yet find it is able to absorb high levels!
I found only one other useful hit amongst many, many more male remedies, but it was a good one.
From The New Zealand Botanical Society newsletter, December 2009, in an article entitled
"Lichen notes 2: Lichens on the move - are they telling us something?" by David Galloway,
This lichen is characterised by its pale yellow-green colour and the presence of superficial pustular isidia, that are effective agents of its dispersal. It is probably the most aggressive coloniser of man-made substrata in New Zealand (asphalt, concrete, glass (notably greenhouses), metal (of abandoned cars), plastic sheeting, gravestones, wood, roof tiles etc), and certainly in the last 30 years it has noticeably increased in cover on asphalt paths and roads and on margins of airport runways in many parts of New Zealand, and is probably the most common urban lichen that we have in terms of area colonised (see Green & Snelgar 1977; Galloway1980,1985, 2007; Green1997; Bennett & Wright 2004;). It grows rapidly once it has begun to colonise a surface, with Green(1997) recording annual radial growth of 16mm per year. Indeed, 30-odd years ago the abundant growth of X. scabrosa on roads in New Zealand had people complaining that the lichen when wet was hazardous, causing cars to skid (Green & Snelgar 1977). Although it has a well-known chemistry (see Foo & Galloway1979; Ernst-Russell et al.1999), X. scabrosa is notoriously and inaccurately touted as a “male enhancement product”, with a plethora of lurid websites attesting to this lichen as being... “an ancient Chinese secret” and... “in use for thousands of years by traditional practitioners”. Whatever the Viagra-like properties claimed for this lichen, its active spread on asphalt in New Zealand towns and cities over recent years is an established fact (Bennett & Wright 2004). It is the Southern Hemisphere counterpart of Xanthoparmelia conspersa which is the dominant macrolichen of disused tarmac runways in Britain (Gilbert 2000a)
Interesting to see the remarks on the health benefits!
Searching for a remedy
The questions remaining are method, and cost.
So far my searches have thrown up: -
- Zinc oxide as a mixture with about 0.5% iron(III) oxide (Fe2O3) is called calamine and is used in calamine lotion.
- Info on the medical use of zinc oxide as a remedy for lichens that grow on people! Who knew!
- Info on monitoring lichen zinc levels to track environmental pollutants
- Info on use of lichens to absorb pollutant zinc (as above)
- Info on use of zinc to discourage lichen (as above)
- Info on lichen's internal chemistry
I kept encountering references to copper being used in the same way to discourage lichens and mosses; some Americans and English have simply tossed pennies on the roof! I will keep this under consideration.
I wondered if any other forms of zinc could be used, e.g. zinc oxide appears to be easily available in powder form. I found an MSDS (Material safety data sheet) for ZnO powder, it appears pretty non-hazardous as long as you don't snort it or mix with linseed oil! It is used as a food additive as a source of zinc, a necessary nutrient. (Zinc sulfate is also used for the same purpose). It is insoluble in water. thus not dissolved by rain.
I have not been able to find anything specific to the mechanism of the toxicity of zinc to lichens, which might help on the usefulness of other forms.
This is a possibility, but I don't see how it could be used on a handrail without aesthetic, tactile and safety problems. That is, it would look crap, feel sharp, and could cut your hands.
CONCLUSION, err interim decision..
I will try just scraping off the lichen in some area and applying zinc oxide powder to penetrate the myriad cracks and crevices of the wood. It is insoluble in water so rain should not reduce the effectiveness.