Lilacs are fragrant deciduous shrubs or small trees, ranging in size from 6.5 to 32 feet tall, with stems 8 to 12 inches in diameter. They are noted for their flowers that bloom in mid-spring to early summer and grow in large loose clusters. Usually they are lilac coloured but can be white, pale yellow, pink or dark burgundy. Occasionally green lichen can be found growing on lilac trunks and branches that look like fungus.
Lichens are organisms consisting of a fungus and a green or blue-green alga growing together in a mutually beneficial relationship. The fungus obtains water and minerals from the air and the material it is growing on. The alga provides carbohydrates and vitamins. Although lichens grow on tree bark, they do not harm trees. Lichens can be flat, leafy, or branched and hairlike. All three forms live on tree bark, rocks and soil.
Most lichens will not grow in a smoky or polluted atmosphere and only grow where the air is clean. According to the University of Illinois, it was very difficult to find lichens growing in Illinois or the Midwest 20 years ago because of air pollution. Following a series of clean air acts, air has become a lot cleaner in the region and lichens are now very common.
Large Patches of Lichen
If a tree is growing, lichens will not be able to attach themselves to the stretching bark. Small patches of lichen that are 12 inches or less in diameter are not significant, but if large patches of lichen appear, it can be an indication that the tree is not very healthy. For example, it might have a disease or insect problem.
Lilacs grow best in full sun and well-drained soil and will not flower well in partial sun or shade. The shrubs may take three to four years to establish themselves in a new site, but once established they can live for centuries. Lilacs do well in a slightly acid to alkaline soil.
After flowering, prune flowering stems back to a set of leaves and remove dead, damaged, or diseased branches. Overgrown, leggy or poor flowering lilacs can be rejuvenated by a method called "renewal" pruning. Do this by removing one-third of the oldest stems at ground level each year for three years. This encourages growth of vigorous new stems from the base.