Trinity tree of the month - Black Walnut

Posted on: 21 June 2023

Black Walnut is native to North America and is one of the area’s most valuable trees; it has also long been grown in Europe.  The timber is especially valuable and prized by woodturners for its grain, colour and durability.

This tree has spectacular form, excellent architectural branching and is in great health.  The bark is grey with deep crossing fissures.  If you look up into the tree at this time of year, the fissured bark leads the eye up to intricate patterns in the canopy created by its large, divided leaves. When the tree has shed its leaves in the winter, the bark and structure really stand out.  

Black Walnut is monoecious, meaning separate male and female flowers are produced on the same tree. The flowers appear from late May to June and the species is wind-pollinated.  As with the Common Walnut, Black Walnut also produces a large fruit which encases the nut.  The fruit is held either singularly or in pairs on the tree and they have a very thick husk. The fruit ripens best in a warm climate, producing a sweet earthy flavour. It usually takes 15-20 years for the tree to first start producing large crops of nuts, but they can produce small amounts from a young age.

This tree can reach up to 40 metres in height and can live for a few hundred years. One fascinating fact about Black Walnut is that it can have an allelopathic effect on surrounding plants, meaning it produces a certain substance that can inhibit their growth.  The chemical produced by Black Walnut is known as juglone which can be found in all parts of tree, but especially the buds, fruit and roots which secrete it.  

We welcome you to come see the tree in the South Arboretum of Trinity College Botanic Gardens, where it is number seven on the tree trail.  (Accession number 19670047)  More information on the Botanic Gardens and its beautiful flora can be found on their website.

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