Alder, or Alnus to use its Latin name, is a small genus of about 30 species of monoecious trees and shrubs. Most are deciduous with simple serrated leaves and catkins. Both the male and female catkins are found on each tree. Although Alder plants are mainly wind pollinated, they are occasionally visited by bees.
Nitrogen fixing properties of Alder trees
Alder trees are commonly planted for their nitrogen fixing properties. The root nodules of Alnus plants contain a nitrogen fixing bacterium which absorbs nitrogen from the air and makes it available to the plant. The alder plant then provides the bacterium with sugars which it produces through photosynthesis. This mutually beneficial relationship allows alder trees and shrubs to improve the quality of the surrounding soil. When used as a pioneer species (hardy plants used to colonise previously damaged areas) Alder provides additional nitrogen to the successional plants that follow.
The wood of Alder trees has been used for many years in different cultures:
If you're looking for a tree that is well suited to damp or wet sites, Alder is the tree for you! It's great for difficult planting sites and will provide you with an attractive tree, popular with wildlife. For more ornamental trees suited to damp sites, click here.