First let us define “plant crown”. That area at the base of the shrub where the stems and branches come out of the ground is called the crown of the plant. Over winter it is allowable for the leaves and debris to cover your shrubs and the crowns of the shrubs but during the growing season allowing this debris to stay on the crown only leads to problems.
By allowing the mulch, leaves and organic debris to remain on the crown you create an environment that causes the wood to soften and lose its ability to compartmentalize and ward off attack by plant pathogens. This environment encourages fungal and bacterial growth that can easily penetrate the compromised woody tissue of the crown and may be harmful to the plant.
Excessive levels of organic buildup in the crown also can encourage adventitious root growth, especially on plants such as boxwood. These adventitious roots begin to suberize (turn woody) and actually can affect the plants ability to absorb nutrients and water correctly. This also can lead to plant deterioration.
If accumulated debris levels are very thick you may also have an increase of rodent activity encouraging gnawing on the crown stems.
The simplest way to prevent these problems is to clear the crowns of all debris (and vines such as ivy) each spring when you do your spring pruning and are refreshing your mulch. Maintain your mulch at a thickness of 3″ when installed. This will give you about 1″ of settled mulch. As the mulch rots it should be removed and replenished with fresh mulch. Do not cover the crowns with mulch.
The perception that the build up of rotting mulch (organic material) is beneficial is wrong, at least in regard to the area around your shrubs. Excessive layers of organic debris encourages roots to grow into these layers. Since there is no soil in these layers these roots do not function correctly and the plant can even change its physiology to compensate for the inefficient roots which results in an unhealthy plant. The tendency for these organic layers to dry quickly also causes problems for the roots.
The exception for allowing this organic debris build up is around your trees. As long as you are not excessively mulching or “volcano” mulching the natural buildup of organic below your tree canopy is actually healthy for the tree.« Back to Glossary Index