Leaf Scars

With leaves falling from trees and swirling around our feet, today seemed like a good day to talk about leaf scars.

Horse chestnut stem showing seven bundle scars within a leaf scar (AnRo0002) Free Use

​When I say leaf scars, I am not referring to the ravages of battle or bad decisions. Instead, leaf scars occur naturally whenever a leaf falls from a tree. The information they ‘leave’ behind may surprise you. [Sorry, I couldn’t resist.]

What are leaf scars?

Leaf scars are the marks left behind on a twig after a leaf falls off. It is where the petiole, or leaf stem, was attached to the stem. As long as photosynthesis is taking place, the connection between leaf and stem is usually pretty strong. It has to be. Leaves flutter in the breeze, are battered by raindrops and send sugars produced through photosynthesis into the vascular bundle. But then things change.

Scar formation

At some point, a leaf becomes more of a burden than a sugar factory. In the world of plants, the process of getting rid of unwanted body parts is called abscission. Abscission can occur because of seasonal changes, or in response to conditions such as drought, shade tree decline, disease, or natural aging. That natural aging is called senescence.

Whatever the cause, plants try to pull as many resources from leaves as possible before dropping them. This is why leaves change color in autumn. Once a leaf has been sucked dry, the petiole softens, and a protective barrier starts forming between the petiole and the twig. This area is called the abscission layer. After the leaf falls, the wound is covered by a protective corky material, leaving us with a leaf scar.

White ash leaf scar (Michael Ellis) CC BY 4.0

Other scars

If you look closely at a leaf scar, you may see tiny holes arranged in a pattern. This area is called a bundle scar, and it is the torn vascular bundle. Buds often form just above leaf scars, and they leave scars of their own. They are called bud scars.

Leaf scars and buds on pecan stem (Paul Wray, Iowa State University, Bugwood.org) CC BY 3.0

Problems with leaf scars

In some cases, diseases can take hold in leaf scars. Olive knot is one of several bacterial diseases that start this way. European apple canker is a fungal disease that can enter through leaf scars. Any time there is an opening, there is a risk of disease. That being said, you need to resist the urge to seal up plant wounds artificially. In most cases, this results in moisture being trapped against the wound. This slows the natural healing process and increases the risk of disease and rot.

Papaya leaf scar (Ameena104) CC BY 4.0

Botanists and plant aficionados use leaf scars to learn more about plants. Plants with small leaf scars tend to have small leaves. Plants with large, curved leaf scars tend to have bigger leaves. This is because a curved attachment can support more weight. Leaf scars can be brown, green, or red, depending on the species. They can be flat or rounded. In some cases, hairs can be seen around the leaf scar.

If you have a mystery tree, you may be able to use its leaf scars to help identify its family ties.