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Lichen, moss or algae -- what's that green stuff on my tree?

Moss thrives in shady spots but is generally

Moss thrives in shady spots but is generally harmless to trees. Credit: USDA Agricultural Research Servi / Charles T. Bryson

DEAR JESSICA: I have a pear tree that's about 55 years old and have noticed what looks like grass or moss on its limbs. Due to a neighbor's very tall house, this tree does not get much sun, especially in the winter. I love this tree, which was planted by our parents. Will the tree be OK? What should or should not be done for this tree?pear

DEAR EMAILER: Green masses growing on tree trunks or branches could be lichens, moss or algae.

Lichens are unsightly organisms that look menacing but in reality cause no harm. They actually indicate good environmental air quality and will not grow in polluted areas. You can leave them be.

Moss thrives in shady conditions and prefers low soil fertility, a low or acidic pH and compacted soil. Prune branches in the center of the tree to allow more light in and more air to circulate. You can try scraping moss off the remaining branches and trunk, or apply a copper fungicide (read labels carefully and follow instructions precisely).

There's nothing you can do about your neighbor's house, but you can correct the pH, improve drainage and apply a nitrogen fertilizer to the soil. Get a pH test of the soil under the tree. You can do this yourself with a home-test kit, which you can find at most garden centers, or bring a few tablespoons of soil collected from a 4- to 6-inch depth to the Cornell Cooperative Extension office in Riverhead (423 Griffing Ave.) or East Meadow (832 Merrick Ave.). Pear trees can tolerate a wide range of pH levels, which works in your favor, so if the test indicates the soil is acidic, lime can be applied without compromising the tree's health.

Also, keep the tree as dry as possible. If you use an irrigation system, be sure to water deeply and infrequently and point sprinkler heads away from the tree. You likely shouldn't need to water the lawn very often anyway, since it's not getting much sunlight either. Core aeration in spring will help with drainage.

Algae, usually considered an aquatic plant, thrives in moist and/or shady areas and can be present on trees, houses, walkways and fences. It can easily be scrubbed off inorganic surfaces, and should be removed when safety is a concern, such as when walkways become slippery. Algae doesn't pose a threat to your tree's health. If you are intent on removing it, however, you can spray the affected area with 1 teaspoon of copper sulfate in 8 gallons of water. Keep in mind that if shading, low soil fertility and inadequate draining aren't corrected, the algae will likely return.

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