Trees and shrubs are showing signs of life as they swell in preparation of budding out. Let’s hope that they have survived the extreme cold that followed some unseasonably warm weather in February when they started to appear. Although we are still weeks away from seeing canopies of leaves and flowering shrubs the weather is becoming nice enough to enjoy a walk through the landscape. And without leaves and flowers to attract our attention our sight is drawn to other details that might normally go unnoticed.
As I was walking around the yard looking at the pussywillows and the lilac buds I noticed lichen growing along the side of the lilac trunk. We get many calls at the Home & Garden Education Center regarding grey-green growths along trunks and limbs of woody ornamentals. Most lichen are so unworldly-looking that the common misconception is that they must be causing harm to the host plant, especially since they are commonly first noticed when a tree is in distress. But a sparse canopy simply lets in more sunlight which is beneficial to the lichen.
The truth is so different. In fact, lichen may be a benefit to the host plant by bringing extra moisture and environmental protection as the lichen take root. Further, removal of lichen may damage the underlying bark may create open wounds that would allow pathogens to enter. It is best left alone.
What are lichen, then? They not only live symbiotically with host plants, they can be found on soil and on rocks. Lichen are composite organisms and although they sometimes appear plant-like, they are not plants. They are algae (or cyanobacteria, a name that reflects their blue-green hues) that live among the filaments of fungi. They do not have roots to absorb water and nutrients but they can produce food through photosynthesis by the algae component. Lichen are sometimes called moss and may grow amongst them but they are not related. This image shows them on the same tree:
Lichen can be correctly called an epiphyte though. Epiphytes grow harmlessly on other plants, only relying on the physical support for its structure and getting moisture and nutrients from the air and rain. Orchids are a beautiful example of an epiphytic plant and more can be read about them in the Ladybug Blog: A Visit to the Bahamas.
As lichen grow the forms that the thallus take determine the grouping that they fall within. The thallus are the obvious vegetative body parts and they can grow in a variety of ways and colors. On the left is the Parmotrema sp. in a foliose growth form. On the right is the Caloplaca sp. in a crustose growth form.
Lichen are long-lived but can have slow growth rate, as little as 2/100” in a year although there are varieties that can measured at 1 ½’ per anum. Lichen can be the first species to colonize freshly exposed rocks and can survive under the harshest conditions, such as arctic tundra and desert. It can survive a complete loss of water and then rehydrate when it becomes available. This moss has been growing on this rock for years. The cup-like structures are the apothecia, the fungal reproductive structure that produces the spores. While these spores will produce new fungi it won’t lead to new lichen. New lichen are formed when soredia are dispersed. Soredia are clusters of algal cells wrapped in fungal filaments.
So there is no need to panic when you see lichen. If the host plant does seem to be in decline, look for another cause. It could be due to an insect infestation (have Gypsy Moth caterpillars defoliated the canopy?), a vascular disease that has caused a general decline in vigor, or uneven watering practices. Check with the UConn Home & Garden Education for verification of any of these possibilities.