The European alder, or Alnus glutinosa, is also known as the black alder, common alder and European black alder. It is a large, fast-growing deciduous tree with a narrow crown. A mature European alder can reach heights of 30 to 50 feet. It often has a multi-stemmed trunk, and can be used as a specimen tree or as a screen in your landscape. Prune your European alder to remove dead, diseased, damaged and broken branches, to maintain its size within your landscape design, for safety reasons (walkways, structures, and roofs) and to thin the crown of the tree.
Remove any damaged, broken or diseased branches immediately, making a clean cut at the breaking point. (Depending upon where the break is, and where the branch is located, consider cutting off the entire branch.) Make clean cuts to ensure that the plant will heal properly.
Visually inspect your tree from all angles to determine if any branches need to be pruned to keep the tree from interfering with other plantings or structures.
Locate the branch collar (this is on the underside of the branch where it connects to the trunk) and the branch bark ridge (this is on the topside of the branch where it connects to the trunk) of the branch you are going to prune. Cut in front of the branch bark ridge and the branch collar. Do not leave a stub (stubs are an entry way for disease). Do not cut into the branch collar or the branch bark ridge; these areas should be left in tact to maintain the health of the tree. Prune when the tree is in its dormant state, in winter or early spring.
Examine the crown of your European Alder to determine if you need to have deadwood (twigs, branches, limbs) removed. This procedure is usually required on older mature trees, and is known as "thinning." If the tree is large, hire a tree service to thin out the tree.