Root crown

A root crown, also known as the root collar or root neck, is that part of a root system from which a stem arises. Since roots and stems have quite different vascular anatomies, major vascular changes take place at this point.
Root-crown temperature has been found to affect plant growth and physiology in a number of ways. Root crowns need to be exposed and 'breathe'; this is one way that some plants take in oxygen.
A number of pests and diseases affect specifically this part of the plant, including root-crown rot (or root-crown fungus) and a number of species of root-crown weevil.
The root crown area usually appears swollen, tapered, constricted or very thin - as well as a combination of these. The area of the root crown is usually located around or at the soil level and can be vaguely or clearly apparent.