Friday, July 9, 2010

The Lichen Problem

An acquaintance of my husband’s told him, not so long ago, about the problem he was having with lichen at his place. It was, he said, “growing on all the fauna”.

My husband, (after a moment’s astonishment), translated this to “growing on the flora”, and went on to hear the story about how this man had taken his problem to a local nursery, and had been given “some stuff” to kill it.
We wondered, together, whether this story was true. The idea of lichen on plants as a “problem” was a new one to us. We regard lichen as a beautiful ornament in any garden.

We also value the fact that, like a healthy frog population, it is a pollution indicator. Lichens won’t survive if the air isn’t clean. Lichen, in our book, is a Good Thing. It doesn’t harm healthy plants. That a nursery might sell someone a poison to kill it was a mind-broadening concept.

We delight in lichens in all shapes and forms, those “paint blobs” in delicate shades of white, soft green, yellow and orange, that you see on trees and rocks; the leafy-looking encrustations; the growths that cover the soils in the bush; and the preposterous hairy-looking growths that give rainforest trees that bearded look.

Obviously other people like them too, as there is a market for so-called “mossy rocks” - where most if not all of the “moss” is really lichen

Despite their plant-like appearance, lichens are not really plants at all. In fact they’re not even a single kind of organism. They are “associations” between fungi and algae. Some sorts of lichens have several species of each, a little community of friends all happily coexisting and presenting a united front as a lichen “species”.

The alga’s contribution to this team effort is photosynthesis, which provides the lichen’s food. The fungus half of the partnership contributes the structure of the lichen, defining its shape and protecting the algae. It also catches the water, and provides the minerals for the little community.

So is lichen a problem?

To quote the Yates Australia website, (which can be counted on to recommend a chemical for every garden problem), lichens “do not actually affect the health of the tree as they are non-parasitic organisms”.

The lichen which beautifies the dead tree at right was not responsible for its demise.
A few years ago, I noted that the Yates site said bluntly: “There are no approved chemicals for treating lichens and moss on trees and shrubs.”
Now, however, the site has been altered to recommend a cautious use of lime sulphur to destroy lichen on plants, which suggests to me that there has been a substantial body of customers who just can't be convinced that there's any beauty in a thing which finds its way naturally into their gardens.

Are there really people who dislike lichens so much that they would risk damaging their precious plants, to get rid of it?

What about lichen on other substrates?
It forms on rocks, concrete, timber, clay tiles, plastic shadecloth, and metal. Is it a problem?
Lichens do acidify their surrounds, and the acid certainly does, very slowly, erode rocks. The effect is greater where the substrate is damp, and almost non-existent in dry conditions - but in either case is minuscule in the timescale of a human lifespan.

Lichens have been described as “the termites of the plant world”, by those who see them as significant contributors to the long process of turning rocks into soil. However conservators of old British stone buildings and monuments have noticed that old stonework has actually broken down less where it been covered by a coat of lichen than on sections which are lichen-free and exposed to the weather. So it seems that despite their acid production they are not as effective at “termiting” as are frost, wind, and rain.
The same protective effect has been recorded on wood-shingle roofs, and there appears to be no actual evidence of it actually damaging even metal roofs.
Removing lichen does leave ugly marks behind, though, and these are well-nigh impossible to remove.
In all, those who are worried about lichen on their plants and buildings might simply do best to leave it in place and learn to enjoy its beauty.
Then bring on the lichen!How can we encourage it?
Aren’t we humans an interesting lot? While some of us are worrying about how to remove the lichens, others are going to a lot of trouble to acquire it. They want it on their rocks, terracotta and concrete pots, garden structures of various types, buildings, and even on their plants.
The simplest technique is simply to wait. Given an absence of pollution and the obvious lichen-destroyers (garden chemicals, flood and fire), and the presence of nearby lichens as a source of reproductive materials. The Toowoomba district is wonderful for lichen-growing. Long may these conditions last - mining and urban expansion are likely to have their impact in the future.
Many people paint their rocks etc., with various substances to hurry up lichen growth. Popular items are yoghurt, beer, skim milk, thin porridge, rice flour and manure slurry. Healthy lichen is sometimes broken up and added to the mix (or mixed in with a kitchen blender - perhaps not desirable for the manure option).
Results of these techniques have been mixed. Some people get lichen. Some get mosses, or ugly moulds (attracted by the sugars and starches in the slurry). Some don’t care what they get, so long as they’ve made new garden feature look older.
There are three processes involved, when the potions do work.
One is the addition of nutrients. The observation that some lichens grow best where there are bird-droppings has led to the manure slurry technique. (Urine is another favoured ingredient).
The second is acidification. Yoghurt, buttermilk and sour milk are all acid, as well as weak nutrients. They might be a help - or might be a hindrance. There is great variation in pH preference, between species of lichen. They grow on substrates with pH from 4 (quite acid) to 8 (alkaline). If you’re in the know, you can “read” the pH of rocks and tree bark by identifying the species of lichens which grow on them. For the most part, there doesn’t seem to be much justification in using up good yoghurt in the hope of encouraging lichen. The pH you alter might have been just what a local species of lichen really wanted.
An exception is new concrete, which is very alkaline, (pH about 11), and may need active modification before lichen will interest itself in the site. Left out in the weather, the surface of new concrete can drop to around pH8 in five or more years, but adding some sour dairy produce may speed up the process.

The third requirement is some reproductive material from existing lichens. If not included in the slurry, lichens may still be “seeded” from the air, provided there are lichens growing locally.

My own experience, in this district, is that lichen happens.
It needs no help from me, to get established. I am thankful for it, and hope we can all manage to protect the good, clean air that keeps our lichens, our frogs, and us, healthy.

Are Lichens Really Killing Your Trees?

Before blaming the lichen for a tree’s ill-health or death, it is worth considering whether something else is the real culprit.
Dying trees become unable to fight off an overload of lichens, so it can appear that the lichen is killing the trees. (Mistletoes are blamed for killing trees under the same circumstances.) But it almost certainly wasn’t the lichen’s (or the mistletoe’s) fault in the first place.
We humans hate to admit that something we’ve done might be causing our own problems, but with trees, we sometimes kill them despite ourselves.
Common causes of “tree-slaughter” are:
ROOT DISTURBANCE (digging; building; road-making; ploughing)
SOIL COMPACTION (piling dirt higher over roots;  driving and parking vehicles over their root zones: introducing hoofed animals. Life is very tough for any poor tree that finds itself turned into a cattle camp.)
OVER-FERTILISATION (fertilising trees that were previously happy without it; allowing manure and urine from crowded livestock to over-enrich the soil)
DRYING OF THE SOIL (covering soil with asphalt, concrete, or a building; clearing hillside trees thereby causing the water table to drop; installing a bore which can do the same, altering drainage flows with building works)
MAKING THE SOIL WETTER (installing a dam or pond; mulching previously bare soil; altering drainage flows with building works)
ALTERING SOIL SALINITY OR pH. (or adding inappropriate quantities of trace elements)
USING “GREY WATER” (It may be the "grey" that's the problem - or it may just be the suddenly soggy soil)
In our district, the faster our burgeoning population “develops” the land, the more we can expect to have “mysterious” tree deaths from the above causes.
Factors beyond our control may also cause tree deaths. Once-healthy plants can die suddenly from unusually wet or dry seasons, or from new diseases or pests which move into a district.

Some people seem to forget that rather obvious fact. "The tree's been here since my grandfather's day", they say. "I can't understand why it should fail now, after so many years."

But could lichens be causing a problem, all the same?
It is true that lichens can accelerate the decline of aged or unhealthy trees.
This is how they do it:
All bark is dotted with lenticels. These little “bumps” are really openings filled with corky tissue. Oxygen is absorbed through them, and carbon dioxide released, just as it is through our lung tissues. On some plant species these are large and conspicuous. On others you might have difficulty finding them.
Plants need their lenticels for breathing. They absorb oxygen through the pores on their leaves, and through their roots, but this is not enough. Bark (like our skin) is not permeable to gases, but unlike us, plants have no bloodstream to carry oxygen around their bodies. Bark-covered tissues would die of oxygen starvation, without their lenticels.
But then, why don’t lichens always smother trees?
A growing tree is continually stretching and cracking its bark, and lenticels form in the cracks. The same cracks also disturb lichens. It may not look like much, but it’s enough to allow the trees to go on growing, in excellent health, sometimes for more than 1000 years! Some of these trees seem to be blanketed in lichens for the whole of their long lifespans.
Trees continue to grow bigger all their lives, but when they go into decline, or if they’re just not thriving for some other reason, they stop. The bark finishes expanding and disturbing the lichens. At last, they can cover those pesky little lenticels. Lichens that are saturated from prolonged wet weather, or over-enthusiastic use of sprinklers, have the edge when it comes to blanketing ability. It is questionable, however, whether they cause the tree to die any faster than it would otherwise have done.
A beloved, ancient tree might need protection from lichen if it is to go on living, just as it may need other kinds of mechanical and chemical help to prolong its life beyond what nature would have normally allowed.
So what about lichens on younger plants?
Lichen problems on young plants are rare and may even be mythical. If all other possible causes of death have been examined and rejected, and the problem does really seem to be lichen attack, then it might be time to examine whether the species of plant chosen is something that just can’t cope with the native lichens of a particular place.
Some natural environments don’t support lichens. They don’t for instance, occur in places subject to annual flooding, like the Amazon valley, or the flood plains of northern Australia. It’s not unreasonable to theorise that plants native to those places might have problems when we try to grow them in our lichen-friendly part of the world.
Those gardeners who are sure that they do have lichen problems must decide whether the cost, the work, and possible environmental unfriendliness of using chemicals to make war on them is a game that’s worth the candle.
I’d leave them alone. If the problem is imaginary, they’ll live. If the lichen, or some other cause, is killing them, they’ll die.
Perhaps it’s time to let it happen, and move on to planting something that is happy to live where you do.


Patricia Gardner said...
This comment has been removed by the author.
Fran Blank said...

This is, by far the BEST explanation of lichens on trees that I have read. I have taught pruning and gardening classes for over 30 years and always get the question "How do I get rid of the lichen - it is killing my trees". I even had one person rent a pressure washer and "wash" his trees. I will use your wonderful descriptions - Thank you for your work!
Fran Blank

Patricia Gardner said...

So glad it,s been helpful, Fran.
Lichens are lovely things, and part of the ecology, needing to be valued. There are some pretty moths that rely on them for survival, and no doubt they have other virtues that we mere humans don't know about yet.
All the Best