The easiest way to tell the age of a tree is to cut it down and count the interior rings. But what do you do when you don't want to cut down the tree but want to obtain a general estimate of its age? One way is to have a professional obtain a core boring of the tree and count the annual rings. This method, however, is invasive and may damage the tree. Another method, developed by the International Society of Arboriculture, requires only some simple measurements and calculations to obtain a good estimate of a tree's age.
Wrap the tape measure around the tree at about 1.35 m (4 1/2 feet) above the ground. This measurement is the tree's circumference. Write down this measurement.
Use the circumference to find the diameter of the tree. The formula for finding diameter is: diameter = circumference divided by 3.14 (pi).
Determine the age of the tree by multiplying the diameter by the growth factor. Here are the growth factor rates for common trees:
0.8: Aspen, Cottonwood
1.2: Silver Maple, Pin Oak, Linden
1.4: River Birch
1.6: American Elm, Green Ash, Red Oak
1.8: Black Walnut, Red Maple
2.0: Sugar Maple, White Birch, White Oak, Black Cherry
2.8: Dogwood, Ironwood, Redbud
For example, say a Silver Maple has a circumference of 50 cm (20 inches). The diameter (50 divided by 3.14) is 15.924. The diameter (15.924) x growth factor (1.2) = 19.108. The tree is approximately 19 years old.
These growth factor rates are typical for trees growing in a heavily wooded area. Trees in a landscaped or open setting grow more quickly and develop wider growth rings (and therefore a larger circumference). For trees growing in these type of conditions, the age estimates based on the growth factors in the chart would have to be adjusted down to compensate for this added growth.