A tree’s root collar is the area where the roots join the main stem or trunk. This area is where the trunk flares out and enters the soil then transitions into the major roots.
The root collar is part of the tree’s trunk. Unlike roots, the trunk is not specialized to resist constant soil moisture. The movement of oxygen and carbon dioxide in and out of the phloem (inner bark) is inhibited by this water. Over a period of years the lack of gas exchange will kill phloem cells in consequence interfering with the downward movement of food (photosynthate) to the roots, eventually leading to root dieback and reduced water uptake. Root collars with declining phloem are more susceptible to infection and disease caused by certain pathogenic fungi, especially Phytophthora and Armillaria.
Often root collars are buried during landscaping projects or when they are planted they may settle in the planting hole or be set too deeply. Excessive mulch layers often lead to death of the root collar. Mulch layers should not exceed four inches in thickness and should never be placed against the root collar.
Symptoms of root collar disorders are often first evident as foliage yellowing, early leaf coloration and drop, dieback in the upper crown and reduced growth. Some trees will show no symptoms at all prior to their death during a drought. Secondary invaders such as canker fungi and insect borers often invade trees stressed by root collar problems. These cankers may cause sunken areas near the soil line.
First, as much soil and mulch must be removed from the root collar as possible. Root collar excavations can be performed by using small digging tools or a compressed air device such as the Air Spade. Whichever method is used great care should be used next to the tree to avoid more injury.